Why not integrate Jaguars with Diodes?

[KrazyCarl92]

Most people in the FRC who have experience with Jaguars know how frustratingly fragile they are. If you hook up the positive lead from the PDB to the V- and the negative lead from the PDB to the V+ on the Jag accidentally, the thing is busted, blown, and destroyed. So I have to wonder, why wouldn't they integrate a diode into the Jaguar's circuitry so that current would only pass through the one direction? Seems like that would be 10 cents to fix a problem rather than a $90 fix for having to purchase an entirely new Jaguar.

Not asking if this would be FIRST legal (obviously you couldn't modify the internal circuitry of a motor controller), but just asking if this is something that could be integrated to make the product more durable, maybe by a manufacturer in the future. I know I'd be willing to pay an extra $10 for a product that wouldn't break when someone on the team unknowingly swaps the black and red screws...

[lemiant]

If they broke less people would buy less ;)

[Joe Ross]

I know I'd be willing to pay an extra $10 for a product that wouldn't break when someone on the team unknowingly swaps the black and red screws...

Would you pay $10 more for a speed controller that performed 6% worse?

[Jim Wilks]

Most people in the FRC who have experience with Jaguars know how frustratingly fragile they are. If you hook up the positive lead from the PDB to the V- and the negative lead from the PDB to the V+ on the Jag accidentally, the thing is busted, blown, and destroyed. So I have to wonder, why wouldn't they integrate a diode into the Jaguar's circuitry so that current would only pass through the one direction? Seems like that would be 10 cents to fix a problem rather than $90 fix for having to purchase an entirely new Jaguar.

Not asking if this would be FIRST legal (obviously you couldn't modify the internal circuitry of a motor controller), but just asking if this is something that could be integrated to make the product more durable, maybe by a manufacturer in the future. I know I'd be willing to pay an extra $10 for a product that wouldn't break when someone on the team unknowingly swaps the black and red screws...

A diode to handle the 60A or so that a Jaguar can handle would cost a lot more than 10 cents. As well. it would create a lot of heat that would somehow have to be dissipated. Finally, you would have a power reduction in the output due to the 0.7 volts lost in the diode. This would translate to something in the order of 6% at full output.

These all go together to make the "fix" you suggest a bit more difficult to accomplish.

[MichaelBick]

You can do a mofset instead, I'm pretty sure p-channel. Much more power effficient, and you can wire them in parallel to handle more amps.

[lemiant]

Your explanations of the possible problems make a diode implausible, but I feel it's nothing a little more complicated circuitry couldn't handle. The highly capable EEs who designed the jaguar should be able to come up with solutions better than a couple of kids who aren't yet our of college. For alternatives you could use Michael's solution or:

The diode wouldn't have to affect all the power, just the power going to the control circuitry. If the jaguar's internal computer didn't turn on, it would never send any signals to the internal relay (or whatever they use) and nothing bad would happen. The computing uses a lot less amperage so this is totally feasible.

[Matt Krass]

You can do a mofset instead, I'm pretty sure p-channel. Much more power effficient, and you can wire them in parallel to handle more amps.

I was contemplating this solution today, as my team destroyed a brand new Jag today with reversed wires. The simplest way to do that would be a P-channel FET with a grounded gate, power to Drain and output from Source. A suitable (set of) FET(s) would have to be as robust and efficient as the N-channel FETs in the H-bridge, which complicates things, it could be done, but my hunch is that it wouldn't be space, power or cost effective. Perhaps, using an N-channel FET or two in parallel with a diode protected driver IC with a charge pump might work better, but its still going to waste some power to heat, and add complexity and cost.

I think overall, despite the very frustrating problem of reverse polarity burnouts (today still irks me!) it wouldn't be worth it for such a reverse polarity protection scheme, I'd rather try to educate our students better so this doesn't happen again.

Matt

[Ether]

my team destroyed a brand new Jag today with reversed wires.

Can you share why that happened, and any measures you are contemplating as a result? There might be a lesson here that others could benefit from.

[DonRotolo]

Or hey - here's a thought - let kids learn that if you hook it up backwards, it goes poof?

As far as training classes go, $85 plus shipping (http://www.chiefdelphi.com/forums/showthread.php?t=99701) isn't all that expensive.

Or maybe tatoo it onto their foreheads: "The world is not idiot-proof. Pay Attention." Backwards, so they can read it in a mirror.

[efoote868]

I was contemplating this solution today, as my team destroyed a brand new Jag today with reversed wires. The simplest way to do that would be a P-channel FET with a grounded gate, power to Drain and output from Source. A suitable (set of) FET(s) would have to be as robust and efficient as the N-channel FETs in the H-bridge, which complicates things, it could be done, but my hunch is that it wouldn't be space, power or cost effective. Perhaps, using an N-channel FET or two in parallel with a diode protected driver IC with a charge pump might work better, but its still going to waste some power to heat, and add complexity and cost.

I think overall, despite the very frustrating problem of reverse polarity burnouts (today still irks me!) it wouldn't be worth it for such a reverse polarity protection scheme, I'd rather try to educate our students better so this doesn't happen again.

Matt

Making a product like this fool-proof takes away an important $90 lesson, which can easily be split between 3-15 people. The other thing is that the lesson learned doesn't only apply to the $90 part, but potentially 1000 dollar or more parts.

When I was still in high school, one of the freshmen on the team fried the camera. If I recall correctly, that camera cost about $200. While they felt bad about it for a week, I'm certain they or anyone there never made that mistake again.

[Matt Krass]

Can you share why that happened, and any measures you are contemplating as a result? There might be a lesson here that others could benefit from.



We were rewiring the chassis (second pass for neatness, after testing drive systems) and under the gun of a ticking clock, a student connected the Jaguars power input quick connects backward to the main power connection routed for it and powered on the robot without having the wiring cross checked and the Jaguar bit it. In the aftermath we will be focusing on more stringent examination after any rewiring, and better education of team members so that they are more aware of the dangers.

Overall a lot of people (myself included) feel sheepish about this mistake, I think the lesson will propogate well on its own.

Matt

[EricVanWyk]

Yesterday I saw this post (http://www.chiefdelphi.com/forums/showpost.php?p=1116449&postcount=11) and smiled. There is nothing that makes me happier than knowing that a buck saved some student a couple hundred dollars of grief.

We evaluated every single power input connection for protecting against reversed inputs, and added it only where it made sense. In general, high power devices are quadratically more expensive to protect than low power devices. By expensive, I don't just mean raw dollars: I also include power loss and safety.

For example, the PD only reverse protects its power supplies, and does not protect the normal breaker outputs. The power supplies are ~50 Watts reasonable peak with mild pulse, and it cost a dollar. The entire robot is 2ish kiloWatts reasonable peak (with painfully higher pulse), and would have cost 40 dollars of FETs plus a heat sink plus a fan plus a significant drop in total robot performance.


There are two types of reverse input protection: Pass and Crowbar.

Pass solutions are things like series diodes or series FETs that have to handle (pass) the normal operating current. The control system components in blue plastic all use pass protection.

Crowbars are things like shunt diodes that short out the power input in an attempt to blow an upstream fuse before exploding. These make me nervous: It is a protection against incorrect wiring that assumes that the rest of the robot is correctly wired. In particularly badly wired robots, they can make the situation worse. I typically use crowbar protection in more controlled environments.

[Andrew Schreiber]

Making a product like this fool-proof takes away an important $90 lesson, which can easily be split between 3-15 people. The other thing is that the lesson learned doesn't only apply to the $90 part, but potentially 1000 dollar or more parts.

When I was still in high school, one of the freshmen on the team fried the camera. If I recall correctly, that camera cost about $200. While they felt bad about it for a week, I'm certain they or anyone there never made that mistake again.

So, what you're saying is "Measure twice, cut once" applies to things other than saws? Who'da thunk that being careful and thinking through what you were doing would have practical side effects?

[Matt Krass]

Crowbars are things like shunt diodes that short out the power input in an attempt to blow an upstream fuse before exploding. These make me nervous: It is a protection against incorrect wiring that assumes that the rest of the robot is correctly wired. In particularly badly wired robots, they can make the situation worse. I typically use crowbar protection in more controlled environments.

I was actually just toying with this solution last night in our lab, given the rather unnerving ratcheting click noise coming from the PD board, I thought it served as a fair indicator to turn the robot off immediately while saving the Jaguar, I also used a fairly beefy power rectifier to do it, so heat wasn't an immediate concern. I'm more concerned with the effects of repeatedly shorting the thing out rapidly.

Just curious, because I'm sure there's an angle I haven't thought of, what kind of incorrect wiring are you anticipating causing a crowbar to be an ineffective solution?

(I am mostly toying with idea out of build season curiosity, since I don't believe the current electrical rules would permit such a protection circuit in competition use).

Matt

[Sammy 3278]

We replaced the colored screws with all silver so that people won't assume black means negative and red means positive and will look at the molding on the speed control instead. Has been working so far. We had also toyed with the idea of using red and black nail polish on the speed control.

[EricVanWyk]

Just curious, because I'm sure there's an angle I haven't thought of, what kind of incorrect wiring are you anticipating causing a crowbar to be an ineffective solution?

The two factors to consider are the magnitude and duration of the current. Many fuses are rated in terms of amp squared seconds.

There are several common situations that accidentally bypass a circuit breaker OR uses the wrong circuit breaker. These allows the same magnitude but longer duration current pulse.

There are also weird ones where the wiring includes a pathologically bad amount of resistance. These allow a low magnitude but long duration current pulse. In this case, the crowbar might be able to protect its owner until it burns itself out.

[Matt Krass]

The two factors to consider are the magnitude and duration of the current. Many fuses are rated in terms of amp squared seconds.

There are several common situations that accidentally bypass a circuit breaker OR uses the wrong circuit breaker. These allows the same magnitude but longer duration current pulse.

There are also weird ones where the wiring includes a pathologically bad amount of resistance. These allow a low magnitude but long duration current pulse. In this case, the crowbar might be able to protect its owner until it burns itself out.

Ah, I did consider that, but I was under the impression that in this situation it wasn't relevant because in the event of a reverse power connection to the controller, my experience has been that it short circuits in the same manner as the crowbar and causes the same metronome of breaker popping. I admit I haven't (yet) taken the measurements, but my general hunch was that the crowbar solution was no worse on the system regarding violent current pulses of varying duration than the existing problem, but it would spare the controller. The only added concern I saw was buildup of heat on the diode, but I figured with a proper heatsink on a diode designed for 100A whacks, it's probably safer than the tiny components in the Jag absorbing the hit.

I appreciate your input, these types of discussions are always fun, do you have any more thoughts on my theory?

For the record, I was working with this guy for the test:
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00077607.pdf

Matt

[EricVanWyk]

That is a pretty diode! I just wish it was smaller than a spike :)

Compare figure 3 of that diode's datasheet against the figure on the 40Amp circuit breaker's datasheet (http://www.snapaction.net/pdf/MX5%20Spec%20Sheet.pdf). A 400Amp surge would blow the breaker in 30 to 60 milliseconds, and is on the edge of what the diode can handle.

For even more fun, take a look at the 120A circuit breaker's datasheet. At 600 Amps, it guarantees to blow within 3 seconds.

[Matt Krass]

That is a pretty diode! I just wish it was smaller than a spike :)

Compare figure 3 of that diode's datasheet against the figure on the 40Amp circuit breaker's datasheet (http://www.snapaction.net/pdf/MX5%20Spec%20Sheet.pdf). A 400Amp surge would blow the breaker in 30 to 60 milliseconds, and is on the edge of what the diode can handle.

For even more fun, take a look at the 120A circuit breaker's datasheet. At 600 Amps, it guarantees to blow within 3 seconds.

Well, this may not be the perfect diode for the job, but I assure it was selected through a very rigorous "what can I get my hands on in the next five minutes" selection process. :) Also, if you were to parallel the two diodes per package it would give you some fairly substantial breathing room on those figures. If I get a chance soon I'd like to put my storage 'scope on the diode and capture the waveform and see exactly how effective it is.

Also, as I mentioned before, I'm not sure that the Jaguars failing is any gentler on the system, and I'm sure the components failing in that aren't nearly as good at handling the surging currents safely, I'm still feeling as if I'm in the lesser of two evils camp here. I concede this is mostly a hunch, after looking over the Jaguar schematic I suspect the failure mode is near a dead short in one of the logic regulators. I'm not sure that the opportunity to get hard data on the Jag failure is going to present itself anytime soon (I think my team would be a little unhappy if I sacrificed one of our remaining Jags to the cause :yikes:) though I am unsure if the blown out ones have all been disposed of yet.

What are your thoughts on the risk of the failing Jaguar blowing the breaker over and over vs a properly heatsinked crowbar diode (or two in parallel) rated for the actual load?

Matt

[philso]

It would be best if TI could add the reverse polarity protection in the Jaguar.

Adding protection devices externally is a distant second best since it is still possible to connect the wires from the protection circuit to the terminals of the Jaguar incorrectly.

It will always be necessary to check your wiring thoroughly. Even if the polaritiy is correct, the wire could be connected to the wrong point giving incorrect operation.

[techhelpbb]

Just a point.

The core of the Jaguar that handles the large current is the N-Channel MOSFET bridge.

That bridge has a high side driver to make the positive voltages above the input voltage required to actually turn on the high side MOSFETs (they aren't P-Channel so you need this trick).

If the bridge had P-channel high side MOSFETs you'd have more issues (I think I remember the older beige Jaguars being like this) because the match between the P-channel and N-channel MOSFETs is not great and P-Channel MOSFETs tend to be really easy to utterly destory under bad conditions.

The bridge itself could be rigged to survive inverting the positive and negative being reversed. Hence you wouldn't need a 100A or more diode (you best not use a 60A diode on a load that reaches 60A unless you like bad things happening).

The rest of the electronics, including the Stellaris could be powered from a simple bridge rectifier (which would make the positive and negative on the other side *always* the right polarity...if you don't mind loosing 1.4 or so volts DC on the input to the local power supply and I doubt based on the existing schematics it would).

In short I believe this could be done and NOT have to deal with high power parts to do it.

[Kevin Sevcik]

The diode wouldn't have to affect all the power, just the power going to the control circuitry. If the jaguar's internal computer didn't turn on, it would never send any signals to the internal relay (or whatever they use) and nothing bad would happen. The computing uses a lot less amperage so this is totally feasible.I'm a bit late coming to this, but I feel it necessary to point out that you have to protect the power circuitry as well as the logic in this application, which is why it's so complicated and expensive. The MOSFETs that make up the H-Bridge all contain a body diode as part of their construction. When properly powered, these diodes are reverse biased relative to the battery terminals. If you reverse the polarity on the battery terminals, you now have pretty much a dead short across two diodes and a 1mOhm current sense resistor. An FRC battery is going to dump enough amps into that circuit to melt something pretty quickly. (My money's on the current sense resistor, I've heard about them desoldering.)

Matt,
If you want to drastically reduce the chance of swapped quick connects frying something, keyed connections are the way to go. Anderson power pole or molex connectors will do this for you. Or you can do what we do with quick connects: Swap genders from one polarity to the other. So a positive out of your PDB gets a male QC, and a negative out of the PDB gets a female. As long as you're consistent across the whole robot, it doesn't hurt interchangeability, and you really reduce the chances of that particular failure. Or the chance of wiring a motor up backwards, etc.

[Newo95]

If they broke less people would buy less ;)

Planned obsolescence is everywhere.

[ratdude747]

We replaced the colored screws with all silver so that people won't assume black means negative and red means positive and will look at the molding on the speed control instead. Has been working so far. We had also toyed with the idea of using red and black nail polish on the speed control.

not a bad idea...

during my days as a student electronic lead I never had a jaguar die... in fact, I event was able to revive a few "dead" ones from the season before by opening them and cleaning out any aluminum swarf that had shorted them in the past... I guess I got REALLY lucky (now you see why I liked CAN so much).

also, we only used fork terminals for the Jags... if we had to use a ring I would cut part of it into a fork (until we could get more fork terminals). if the screws never come out, then problem solved

Even if you do use ring terminals, my advice would be to work one screw at a time. that also would eliminate the chance of mixing them up...

[EricVanWyk]

The bridge itself could be rigged to survive inverting the positive and negative being reversed. Hence you wouldn't need a 100A or more diode (you best not use a 60A diode on a load that reaches 60A unless you like bad things happening).

This does not match my experience. Could you further explain how to construct the bridge so that it can survive these conditions? When voltage is applied with the wrong polarity, the body diodes will conduct and soon destroy themselves.

[techhelpbb]

This does not match my experience. Could you further explain how to construct the bridge so that it can survive these conditions? When voltage is applied with the wrong polarity, the body diodes will conduct and soon destroy themselves.

Some ideas off hand...

1. Close a relay contact over the circuit and hope they really have a circuit breaker up the path of current to the battery. You can do this because you'd have power from the bridge rectifier on the logic side. Could be too slow, but remember these diodes do take large surges during field collapse anyway...you do have a little time.

2. Rig a capacitor circuit to charge it's way up and reduce the voltage differential (it's not AC it won't discharge). Just make sure you check with the Stellaris if the power is backwards before you start switching. Probably have to make this component exit from the circuit with a small relay as well in the proper polarity.

3. Use fast blow fuse not these thermal breakers and make it easy to replace.

No matter what the H-Bridge will not be operational with the power reversed but I think it should be possible to give it enough protection to survive a minor error.

[Kevin Sevcik]

Some ideas off hand...

1. Close a relay contact over the circuit and hope they really have a circuit breaker up the path of current to the battery. You can do this because you'd have power from the bridge rectifier on the logic side. Could be too slow, but remember these diodes do take large surges during field collapse anyway...you do have a little time.

2. Rig a capacitor circuit to charge it's way up and reduce the voltage differential (it's not AC it won't discharge). Just make sure you check with the Stellaris if the power is backwards before you start switching. Probably have to make this component exit from the circuit with a small relay as well in the proper polarity.

3. Use fast blow fuse not these thermal breakers and make it easy to replace.

No matter what the H-Bridge will not be operational with the power reversed but I think it should be possible to give it enough protection to survive a minor error.
You're over-complicating things. If you're already willing to go to the trouble and expense of tossing a relay into the circuit, then you simply use the relay to energize the power stage. Heck, with a double throw relay, you could have the relay swap your polarity on the power stage for you. Your only problem is that you need a relay that can handle 40A continuous. Those relays aren't small or cheap.

The better open would be a standard mosfet based pass-protection eric described, which I'm pretty sure is already too expensive for the product.

[techhelpbb]

You're over-complicating things. If you're already willing to go to the trouble and expense of tossing a relay into the circuit, then you simply use the relay to energize the power stage. Heck, with a double throw relay, you could have the relay swap your polarity on the power stage for you. Your only problem is that you need a relay that can handle 40A continuous. Those relays aren't small or cheap.

The better open would be a standard mosfet based pass-protection eric described, which I'm pretty sure is already too expensive for the product.

I agree if you were switching the actual power leads to the H-Bridge you'd need a relay of that size. That's exactly why I didn't suggest it. You can under-rate a relay if you're planning on operating it infrequently. The goal was to create a circuit with the relay that wasn't going to frequently operate (preferably not at all). If you can work the idea with the capacitor out the currents involved will reduce themselves.

I also agree that Eric's suggestion is perfectly reasonable of course the cost of the MOSFET is the issue. Then again, if you can work it out such that the parts in question are the same as those in the H-Bridge perhaps the cost advantages of quantity would kick in and trump any of the other ideas I've provided.

[EricVanWyk]

Some ideas off hand...

1. Close a relay contact over the circuit and hope they really have a circuit breaker up the path of current to the battery. You can do this because you'd have power from the bridge rectifier on the logic side. Could be too slow, but remember these diodes do take large surges during field collapse anyway...you do have a little time.

2. Rig a capacitor circuit to charge it's way up and reduce the voltage differential (it's not AC it won't discharge). Just make sure you check with the Stellaris if the power is backwards before you start switching. Probably have to make this component exit from the circuit with a small relay as well in the proper polarity.

3. Use fast blow fuse not these thermal breakers and make it easy to replace.

No matter what the H-Bridge will not be operational with the power reversed but I think it should be possible to give it enough protection to survive a minor error.

My apologies, I must have misunderstood your earlier post. Can you sketch a schematic of what you are suggesting?

The confident tone led me to believe you had a somewhat detailed suggestion in mind with a compelling cost/benefit ratio.

[techhelpbb]

My apologies, I must have misunderstood your earlier post. Can you sketch a schematic of what you are suggesting?

The confident tone led me to believe you had a somewhat detailed suggestion in mind with a compelling cost/benefit ratio.

Not now I can't. I am sitting in a McDonalds in downtown Manhattan eating my dinner. If I have time this weekend I'll draw and test what I have in mind (it's my dime right....if I nuke my MOSFETs you've got nothing to loose....in the meantime I'll spare you the lengthy written descripition).

That being said, the other way to look at the cost/benefit ratio of this particular issue is that you don't have to make all the Jaguars the same. You could make room in the housing (or use room in the existing housing) and essentially make an additional module or a different PCB for the reverse polarity protected version.

Then, if someone desires, they could pay extra for this feature without penalizing all the teams more confident they'll avoid this pitfall.

I really don't see the harm in that idea do you?

[Ether]

make an additional module or a different PCB for the reverse polarity protected version.

I really don't see the harm in that idea do you?

It would require more thought to ascertain whether it makes business sense. There are NRE and fixed costs involved.

[techhelpbb]

Just wanted to add this...when I get a chance I'll test my other idea...not that what I am about to post here is anything magical as it's basically similar to what's on the robot already...but something to consider as it would seem faster and designed for precisely the sort of protection we are talking about.

http://circuitprotection.ru/upload/pdffiles/Auto04-Hbridge.pdf

Costs about $5.50 in small quantity:
http://www.mouser.com/ProductDetail/TE-Connectivity/MHP30-36-T/?qs=HSSPLe7ToFFbhY9dFfZLyw%3d%3d

[jp1247]

Something kind of interesting. Today, while flashing tan jags, I had connected the first black jag backwards, turned it on, and the breaker immediately popped back and forth rapidly. I smelled burning electronics. I turned it off, and switched the cables. It still works, and all is well, perhaps I just lucked out?

[Ether]

I had connected the first black jag backwards ... perhaps I just lucked out?

The question you should ask yourself is, what caused you to connect it backwards, and what could/should be done in the future to prevent that from happening again.

[Matt Krass]

Something kind of interesting. Today, while flashing tan jags, I had connected the first black jag backwards, turned it on, and the breaker immediately popped back and forth rapidly. I smelled burning electronics. I turned it off, and switched the cables. It still works, and all is well, perhaps I just lucked out?

I suppose if it was really quick, the body diodes might not get destroyed. Though I would have expected the reverse voltage to travel down through the regulators and take out the control logic too, that's what seems to be happening on my end of things...

Matt

[Kevin Sevcik]

Just wanted to add this...when I get a chance I'll test my other idea...not that what I am about to post here is anything magical as it's basically similar to what's on the robot already...but something to consider as it would seem faster and designed for precisely the sort of protection we are talking about.

http://circuitprotection.ru/upload/pdffiles/Auto04-Hbridge.pdf

Costs about $5.50 in small quantity:
http://www.mouser.com/ProductDetail/TE-Connectivity/MHP30-36-T/?qs=HSSPLe7ToFFbhY9dFfZLyw%3d%3d
Your mouser link is to a 36V, 30A PTC. If you'd bothered to look at the data sheet, you'd notice that that particular device has a trip current of 50A. That's potentially a recipe for annoyance with the way we use Jags. Not to mention that the time to trip at 100A is a blistering (literally) 4.5 seconds. I regret to inform you that the 40A breaker you already have to have in circuit is going to trip faster than that PTC, in 0.8-1.8 seconds.

[sand500]

My team broke a jag because we switched the polarity.

[Ether]

My team broke a jag because we switched the polarity.

The question your team should ask itself is, what caused you to switch the polarity1, and what could/should be done in the future to prevent that from happening again2.


1 in industry this is sometimes called "root cause analysis"

2 in industry this is sometimes called "lessons learned" or "continuous improvement" or "corporate learning"

[techhelpbb]

Your mouser link is to a 36V, 30A PTC. If you'd bothered to look at the data sheet, you'd notice that that particular device has a trip current of 50A. That's potentially a recipe for annoyance with the way we use Jags. Not to mention that the time to trip at 100A is a blistering (literally) 4.5 seconds. I regret to inform you that the 40A breaker you already have to have in circuit is going to trip faster than that PTC, in 0.8-1.8 seconds.

This is the datasheet for the Snap Action breaker in the 2012 KOP.
http://www.snapaction.net/pdf/MX5%20Spec%20Sheet.pdf

150 % overload = 3.9 - 47 seconds
175 % overload = 2.2 - 9.2 seconds
200 % overload = 1.5 - 3.9 seconds
250 % overload = 0.8 - 1.8 seconds
300 % overload = 0.5 - 1.1 seconds
400 % overload = 0.3 - 0.6 seconds
500 % overload = 0.2 - 0.3 seconds

Correct?

[Joe Ross]

You could always buy one of these instead: http://www.vexrobotics.com/victor-speed-controller.html.

You'll blow up a victor just as fast as a jaguar if you hook it up backwards.

[AlecMataloni]

Really? I'm sorry to say that I didn't know that. Thank you for informing me. I've only seen people discussing the issue with Jaguars, and I incorrectly assumed that the Victors didn't have that same issue.

[techhelpbb]

So I'll post this in the form of a question:

Other than the consumption of say 1.75W of battery power for each Jaguar, and the space it consumes...is there any reason that someone couldn't put a a single pole relay normally open contact in series with the H-bridge and then put a diode in series with that relay's coil so that if the power is backwards the coil simply will not energize and connect the H-bridge at all? Would this not completely elimate the risks to the body diodes? Couldn't you perhaps reduce the energy required to power that coil using the control electronics if you wanted to do it a more complicated way (considering you'll only close that relay contact when the polarity is correct and your controller here can easily pump out PWM?)

If so what rating would you all think is fair for those contacts?

How about 70A like these:
Mouser, 70A, 14VDC relays (http://www.mouser.com/Electromechanical/Relays-I-O-Modules/General-Purpose-Industrial-Relays/_/N-5g36?P=1z0x0yf&Keyword=relay&FS=True&Ns=Pricing|0)

[Kevin Sevcik]

That would provisionally work, yes. The issues I'd want thoroughly addressed before I'd use it:
1. How reliable is it under FRC service conditions? We're talking pretty exciting bumps and crashes, and available voltage dipping into the 9V range under heavy current draws. And you have that diode dropping .7V of your available voltage for energizing your coil. It'd be unfortunate if the coil dropped out under those conditions when an unmodified Jag would hum merrily along.
2. I don't have the Jag schematics up at the moment, so I can't say that one of these will work. You may need two to completely disconnect the H-Bridge to actually protect everything important.
3. You do realize you're proposing to slap a 1" cube into the Jaguar's innards? The Horribly Oversized and Oppressively Bulky Jaguar? If this turned out to be the solution, I think I'd take a leave of absence from CD for a year to save myself the howls of outrage.

[techhelpbb]

That would provisionally work, yes. The issues I'd want thoroughly addressed before I'd use it:
1. How reliable is it under FRC service conditions? We're talking pretty exciting bumps and crashes, and available voltage dipping into the 9V range under heavy current draws. And you have that diode dropping .7V of your available voltage for energizing your coil. It'd be unfortunate if the coil dropped out under those conditions when an unmodified Jag would hum merrily along.
2. I don't have the Jag schematics up at the moment, so I can't say that one of these will work. You may need two to completely disconnect the H-Bridge to actually protect everything important.
3. You do realize you're proposing to slap a 1" cube into the Jaguar's innards? The Horribly Oversized and Oppressively Bulky Jaguar? If this turned out to be the solution, I think I'd take a leave of absence from CD for a year to save myself the howls of outrage.

1. No doubt...testing is absolutely critical. Some of the Panasonic 40A relays seem to have drop out voltages well into the 7VDC and below range. Not sure that's really a good idea as a contact rating, but there are plenty of choices.

2. At some point either on the positive supply side, or negative supply side of the H-bridge you should only need to disconnect one contact to break the circuit. Course there is a current monitor resistance in that circuit, but breaking the circuit should protect that as well.

3. As I pointed out above, the idea in my mind was to consider this as a purchasing option. Frankly, with reservations, I agree with other mentors here that damaging something by powering up wrong can be a good lesson. However, I could easily see how it could be a painful lesson for poorly funded teams (even a show stopper). I don't think we should all pay for mistakes we might not make (in power, in size, in weight). Then again if someone has concerns like this perhaps there is value in considering the possibilities if they are willing to pay for it.

Additionally, I'm not sure you can escape the whole extra cubic inch issue. Even if you use a MOSFET(s) in the existing available space in the Jaguar the space involved seems to approach that size. Perhaps it could be built as a module outside the existing Jaguar and attached by someone that won't get it backwards? In that manner the Jaguar itself is the same...but sadly you'll not be able to power up the Jaguar at all like that (I had entertained the idea of having the Jaguar being able to even communicate on the CAN bus that the power is reversed).

Perhaps a connector could be made available on the outside of the Jaguar. Designed so that what plugs in there can't be plugged in with the wrong polarity. At that connector you could either plug in a 'jumper' which is merely a piece of sufficent gauge wire, or you could plug in the reverse voltage protection module which itself is designed to mate that connector with guarenteed polarity. The idea being that a team that has this concern could buy the modules if they want them, have them attached to the Jaguars, and then remove them later and put the 'jumpers' in their place (after they are sure they haven't made a serious error).

[Al Skierkiewicz]

Something kind of interesting. Today, while flashing tan jags, I had connected the first black jag backwards, turned it on, and the breaker immediately popped back and forth rapidly. I smelled burning electronics. I turned it off, and switched the cables. It still works, and all is well, perhaps I just lucked out?

JP,
I would consider that Jag dead. When you stress components to the point where they smell like they were burning, they probably were. It may be working now but it will come back and haunt you when you need it the most.

Relays of the type needed for our service will likely be susceptible to mechanical shock due to the need for large contacts. There is no substitute for checking and rechecking before applying power. I highly recommend more than two people check all wiring and someone should never check their own work.

[techhelpbb]

I should have written my last paragraph above differently.

If you place the relay and the circuitry for reverse voltage protection on a removable module (per my last paragraph above). Then remove that module after confirming you've gotten the polarity correct. The mechanical characteristics of the relay would not be an issue for the robot in motion because the relay would not be present in the circuit at that time. All that would be there is the 'jumper' you put in it's place.

In the end Al is absolutely correct that with the current circumstances it is absolutely essential to confirm the polarity yourselves because there are likely no second chances.