(Earlier version of) Backup onboard charger not correct

[PhilBot]

What we really need here is somone at IFI to post a "Theory of Operation" for the charger.

I've read half a dozen "this is how it works" posts today, and none of them (spanning several years) fill me with confidence. It really does seem that a basic series steering doide would have been a good idea to prevent the battery from powering other devices on the 12V bus. So I wonder why they didn't/haven't add(ed) one.

It's unlike FIRST to insist we do something based on "faith", and that's the feeling I get with this charger.

Time for IFI to step-up and explain this circuit.
Or at least add some text to the schematic so it's funtion is clear.....

"An un-commented schematic is just as bad as un-commented code." Me.

Phil.

[dcbrown]

I wonder if just adding a switch into the circuit at the 12v + point would violate R56. At least then we'd have a manual method of disabling the circuit rather than physically unwiring it if there were issues we wanted to isolate.

As currently written, it would appear that adding a switch would be a violation of R56 though.

Bud

[Dad1279]

Originally Posted by dcbrown

I wonder if just adding a switch into the circuit at the 12v + point would violate R56. At least then we'd have a manual method of disabling the circuit rather than physically unwiring it if there were issues we wanted to isolate.

As currently written, it would appear that adding a switch would be a violation of R56 though.

Bud

You can switch the power (12v) with a spike. Then when the robot is shut down, the charger is off. Drain to the backup battery in this case would be through r3/r2, about 5ma.

[dcbrown]

You can switch the power (12v) with a spike. Then when the robot is shut down, the charger is off. Drain to the backup battery in this case would be through r3/r2, about 5ma.

Excellent idea. I'm sure there is spike already in the mix somewhere that is on if not on all the time, then most the time. The better idea would be a steering diode below the 'batt' point in the schematic... but the schematic isn't likely to change at this point so the spike at least addresses 1/2 the discharge/back power issue with the current charger design.

thanks!

[ptan]

Originally Posted by PhilBot

Time for IFI to step-up and explain this circuit.
Or at least add some text to the schematic so it's funtion is clear.....

"An un-commented schematic is just as bad as un-commented code." Me.

Phil.

Please, please, IFI, do explain.

I showed the circuit to another fellow Electrical engineer, and we both agree that it is either the MOST brilliant circuit ever designed, and it is using the components in ways we did not think about, or there is indeed something fishy in the circuit. Either way, I am most interested in finding out more about the way the circuit operates.


Paul C. Tan., P.Eng.,
Electrical Engineer / Mentor.

[SuperBK]

What a relief to find this thread. I thought I was losing it. The circuit doesn't seem right to me at all. I search on IFI's forums and didn't find anything. I'm going to post a question about it there.

[dcbrown]

There is another thread on the backup charger and I've been reading/bouncing back and forth in both.

http://www.chiefdelphi.com/forums/sh...ad.php?t=42591

I posted an attempt at operation theory - not that I know what I'm doing. Lots of questions that I'm hoping folks can help answer/correct.

http://www.chiefdelphi.com/forums/showthread.php?p=562476&highlight=charger#post5624 76

Thanks.

[SuperBK]

About the updated backup battery charger on IFI website:
http://www.ifirobotics.com/docs/firs...up-charger.pdf
Its better, the RC controller is connected right after the regulator now. The RC is still running off the 8.7V zener instead of the regulator though (although through the 43 ohm resistor). I think R1 could be removed. Let the battery charge off the output of the regulator. The Zener D2 could be left to protect the batery in case the regulator shorts outs. This would also take care of the discharge problem. when 12V is off.

I like how the manual highly recommends it, but IFI now says "use at your own risk".

Brian

[TubaMorg]

Ok after reading through this thread I've noted a number of objections to the backup charger design from the EE types. However a few people have actually built and used the circuit with no terrible ill effects. The chief complaint seems to be that when the main power is turned off, the backup feeds back into the 12v circuitry. And if I am understanding correctly, this problem doesn't seem to be taken care of with the new design. Other complaints seem to be concerning the charge rate; in other words the voltage supplied to the backup is barely more than what a fully charged backup battery would produce.

Have I summarized correctly?

So, I am thinking that the puny charge rate is fine, as the only real purpose of this circuit is to reduce draw down of the backup battery when using the camera. Other loads are pretty minimal and of negligable concern during competition.

As for the back feed issue:
I believe the problem can be solved by introducing a switch that disconnects the backup from the RC when main power is removed. Best scenario would be to route the pigtail through a relay that disconnects the backup from the RC when main power is turned off. I don't think this is feasible with a Spike, so next best solution is a manual switch. Orrrr, just unplug the backup as part of the robot power down procedure!

[Dad1279]

Circuit makes more sense now. We have not scrutinized the rules with this in mind, but I think it can and should get power through a breaker & spike.

I have one built, and will power it up and take some measurements soon.

[dcbrown]

There should be enough EE types hanging around here to agree on an alternate backup charger design. Once there is a consensus we can at least offer the design to Ifi - if not for this year, then maybe next.

Attached is a straw design, let the darts fly. I purposely removed the values of most of the resistors to start.

Theory of Operation - abbreviated.

1. output voltage should be set to 8.5V, there will be a .2v(?) drop across the forward biased steering diode D1 presenting 8.3v to battery to be charged.
2. D1 prevents discharge when 12v is disconnected.
3. R1 provides a non-charging current to the backup battery input (100ma)
4. R2 provides charging current to backup battery (45ma)
5. D2 prevents the non-charging current available from R1 from mixing with or contributing to the charging of the backup battery
6. D3 is zener crow-bar to prevent voltage runaway due to the charger failing in some manner
7. D4 shunts any current spikes around LM chip to prevent failure.
8. Change C2 to 1uf
9. R2 and R3 selected per Vout = 1.25v * (1+R3/R2)

[Al Skierkiewicz]

Bud,
I think you are getting closer. I would make the input cap 500mfd and add a fuse or breaker to the input side. The diodes will have a 0.6 volt drop if you use standard 1n4xxx diodes. Since the output cap is very small the diode across the regulator is not needed but wouldn't hurt. It might be a good idea to make the diode in series with the backup a Schottky to gain an additional 0.2 volts of available power supply. If you add a diode in series with the charging reistor then the series resistor to the load is not needed. That way the diode in series with the battery will only conduct when the regulator drops out or goes below about 6.8 volts.

[Hieb]

OK, after much speculation (and a couple posts to Q&A) Update #6 clarifies the wording to match the interpretation that if you chose to use a charger on the robot, it must be the IFI provided circuit.

[dcbrown]

Updated circuit as follows:

. selected 0.1% resistor values available from mouser.com that yeilds 8.5v
. use germanium diode (or small power transitor w/Vce of 0.3v) to block reverse current and drop voltage to 8.2v through charging resistor which has a C/16 trickle charge current target (45ma).
. use 1N4xxx diodes to drop voltage to 7.1v (or less, say down to 6.8v min), load current through R1 provides up to ~100ma of helper current if battery backup voltage drops due to extra load or loss of backup battery capacity.
. dropped steering diode between battery and R1 - shouldn't be needed on normal battery operation at 7.2v, but this new circuit will add charging current via R1 as battery voltage drops below 7.1v
. changed .1uf cap to 100uf on input. this is quite large for most regulators, shouldn't have to go bigger -- would need to see the charger circuit input at regulator in use with oscilliscope to determine if there were additional problems that would be addressed by larger capacitor.

Still not happy with diode selection, I'd almost rather swap out the germanium diode with a small power switching transistor that had a typical Vce of 0.3v - it should do the same thing as the diode, namely block discharge current when 12v battery not present.

User could add slow blow fuse into circuit as optional component in front of regulator. Say 250ma fuse. Another optional component would be a switch to disable the whole thing. Another optional component set would be a LED and series resistor at point in front of D1 to ground to show when circuit was active.

Looking for help on D1/D2 selection to achieve design voltages.... or someone to redesign the whole thing. Its easy to get carried away, for example I thought of using a 2nd LM317 in load support branch. It would be in constant current mode at 7.1v. This would current limit that branch more effectively but changes all the resistors and output voltage target of the first LM317. This would replace the D1/D1/R1 components. But goal of the design is to just be a basic/simple on-board charging circuit to prevent loss of voltage from the backup battery due to camera loading. The backup batteries main purpose is simply to prevent a brown out interrupt on the PICs which would result in processor resets.

1N4xxx diodes list Vf as 1.0v, but this is at maximum load. 1N58xx schottky diodes list Vf as .55-.6v range. Just need to get to a 1.4-1.5v drop across a couple diodes or transistors but don't know which would be best choice.

I'm looking to order some components with another order something in next day or two so I can breadboard and try this circuit out/take some measurements.

[oddjob]

Where are you all buying the 1n5345 8.7V 5W zener? Digikey sell every 5W zener EXCEPT 8.7V