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Charger circuit advice needed

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studio45 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Topic: Charger circuit advice needed
    Posted: 12 May 2016 at 2:42pm
Now then, I've got a set of lithium polymer batteries made up into 4s packs, with charge controller boards on them. They basically just need a high-current 17 volt supply to charge from. So I built this circuit:
 

as it seemed to be the simplest possible variation on a number of configurations involving the LM317 and a large transistor. I used a TIP147. 26.5v DC unregulated input, about 6 amps maximum required output. Plenty of cooling for rectfier, regulator and transistor. On paper, all well within the specs of the devices.

It worked fine for about a week, during which time I charged the batteries twice for a total run time of about 8 hours. Then all of a sudden it stopped regulating, with both the LM317 and the TIP147 failing short-ciruit (26.5v appeared on the output). I replaced the components and checked the board for shorts, powered it up, and it worked for about 5 minutes then failed again.

Now I only have 1 more TIP147 in the box and it appears there is a serious issue with the circuit. As I said, it is the simplest design I could find - some of the others I looked at had a diode after the LM317, and more capacitors, and in some cases even a pre-driver transistor before the main power transistor. 

Has this circuit been over-simplified to the point of being susceptible to some subtle failure mode that I'm not aware of? Oscillation? Backfeeding from the batteries? No resistor to set a minimum output current?
Any help gratefully received :)
Cheers,
Matt
Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 12 May 2016 at 3:10pm
If you are using caps C2, C3, you really should have protection diodes over R1 and across the regulator input/output to provide a discharge path when the circuit is switched off.

Looks like the transistor will conduct at an input current flow of ~70mA.

Circuit seems fairly normal apart from that. What's your Vin, R1, R2 and expected Vout?

Edit: Are you sure the ICs are really well insulated, have you checked them for electrical isolation to the heatsink on each connector? The regulator will protect itself if it gets too hot as I recall.


Edited by odc04r - 12 May 2016 at 3:12pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 12 May 2016 at 4:56pm
You´re most probably pushing the limits of a single TIP147. Something like this would be better for sharing the load :






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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 12 May 2016 at 6:59pm
Might do the job, if you use that circuit plan to set your output voltage ~1.4v above the wanted value if using fixed resistors to account for the diode and then pass transistors Vbe drop


Edited by odc04r - 12 May 2016 at 7:08pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 13 May 2016 at 9:51am
Or stick in one of these to monitor exactly what`s going on:

http://www.ebay.com/itm/DC-100V-10A-Voltmeter-Ammeter-Blue-Red-LED-Dual-Digital-Volt-Amp-Meter-Gauge-/131577787046?hash=item1ea2a5b2a6:g:lFcAAOSwMmBVzF8~

When dealing with Li batteries, it`s useful to be able to control everything, even if you (think) the batteries have charge controller boards attached.   Wink





Edited by Earplug - 13 May 2016 at 9:52am
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