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

[Newo95]

Originally Posted by lemiant

If they broke less people would buy less

Planned obsolescence is everywhere.

[Joe Ross]

Originally Posted by KrazyCarl92

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]

Originally Posted by 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 $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.

[Matt Krass]

Originally Posted by MICHAELABICK

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]

Originally Posted by Matt Krass

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.

[Matt Krass]

Originally Posted by Ether

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 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.

[Matt Krass]

Originally Posted by EricVanWyk

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.

[ratdude747]

Originally Posted by 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.

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]

Originally Posted by Matt Krass

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]

Originally Posted by EricVanWyk

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/TECHN...CD00077607.pdf

Matt