Looking for critique on this before having new boards made.
There will be 12 circuits on one board to drain 12s battery to lowest cell level. ie. 4.00v example

Pretty much standard circuit from tl431 data sheet except base resistor R5 added. Current thru reg. would occasionally jump from ~50ma to >200ma. without it. Overheating reg. ???

Found it helpful to add feedback R8 to latch on and off. Keeping pnp saturated.

R4 seems not needed. What is the purpose? Would like to keep stand by current down for using as balancing circuit during charge cycle.

R2 & R3 if needed for better fine tuning.
Testing has shown it to drain 47ah cells from 4.06 to 4.00 in a few hours, then turn off
No heatsink needed so far on Q1. So I may get away from isolated TO-220fp package to something with better thermal specs.

 

Pretty much standard circuit from tl431 data sheet except base resistor R5 added.
...
R4 seems not needed. What is the purpose?

I think you need R4 to avoid connecting TL431 directly to the upper rail, to limit current through the TL431 to under ~100mA. Once you've done that with R4, you probably don't need R5. Using both gives you the option of further lowering the base current to Q1 separately from that to TL431. So there's nothing wrong with it.

 

Seems to work without R4. D1 cathode to Q1 base only. With or without R5.
Got me thinking. Could omitting R4 overheat Reg?

 

Think I see now. In the data sheet circuit, there is current limit resistor in the high rail.
Moving load to collector circuit requires base resistor.

 

Will someone explain the need for R4 in post 1?

Stuffing the boards now.
It would be nice to keep current near zero when using as bms.

 

Will someone explain the need for R4 in post 1?

See post #2.

 

I think you need R4 to avoid connecting TL431 directly to the upper rail, to limit current through the TL431 to under ~100mA. Once you've done that with R4, you probably don't need R5. Using both gives you the option of further lowering the base current to Q1 separately from that to TL431. So there's nothing wrong with it.

Eliminating R4 does not connect TL431 to upper rail.
It removes connection to upper rail.
I'm looking for a reason to connect TL431 to upper rail at all.

 

I'm looking for a reason to connect TL431 to upper rail at all.

I don't see that you need to. However, R4 may well have been intended as a pull-up for Q1 base to prevent leakage currents from turning Q1 partly on in an uncontrolled way.

 

Oops, you beat me.
Good answer thank you.


Disregard this.............................

I'll try phrasing it another way.

If I were to ask why the capacitor on pin 5 of 555.
It might be answered. "So as not to connect directly to lower rail".
That's true. Should not be.

Getting back to the original question. There seem so be enough leakage current thru Q1 base to keep TL431 alive with no connection to top rail.

I have no problem using R4. I would like it to be largest practical value, however.
It may be in the data sheet. I guess I'm not smart enough to figure it out.

 

I don't see that you need to. However, R4 may well have been intended as a pull-up for Q1 base to prevent leakage currents from turning Q1 partly on in an uncontrolled way.

That makes sense. Then I will size it larger as a pull up.

 

As long as I have you thinking about this circuit.

I have intermittent problem overheating TL431.

It is possible that I did not have R4 in the circuit.

Adding base resistor R5 fixes problem.

Do you see any reason I should go back and eliminate R5 and reinsert 100 ohm R4 ?

Just because I have it working doesn't make it a proper design.

 

Your diagram differs from the data sheet application example in that there is no resistor in series with the main power in your diagram. So you need to somehow limit current to both the transistor base and the TL431. I think if you spend some time with a simulation of your circuit, you can probably accomplish that with one resistor. I can't simulate it well enough in my head to work it out, though. Familiar with LTspice?

 

I agree with wayneh, except that I don't see how just one resistor will achieve both those current limit functions.

 

"as testing"

 

Seems all is working ok.

Cell to cell accuracy is about 50mv.
I'm hoping for better.
Meter I was using only read to .01 volts.
Using my fluke now. .001v
Absolute voltage is not important.
Looking for the best way to set 12 regulators to the same value.

For a test, I removed feedback R8.

Any ideas how to improve circuit?

 

Here's what seems to work.
Connected each regulator to stiff power source. Ie. one of the 60ah cells.
Adjusted the pot until current rose to near top of linear range. 400ma
Maximum current when fully shunted is about 550ma.

This could be done very quickly and precisely using a high resolution analog meter.

For test I've reconnected to 12s pack. Leave it run overnight. See where each cell is in the AM.


This is before adjusting.
All shunt leds are now on.

 

For anyone using lithium packs, these little monitors are a great time saver.
Each will monitor, balance, discharge, and test up to 7 cells.
If only they would handle more current, there would be no need for my project.
It would take a month to balance a 100ah pack.
< $10 each.

 

I want one of those! ....goes to Google shopping with eyes wide

 

10 of them were $6.50 each.
No connectors with them though.

http://www.aliexpress.com/item/5-in...2637987266.html?spm=2114.13010608.0.64.UMbgvn

 

After overnight test, 3 cells to go, then check balance.

My ability to match the settings from 1 thru 12, may be the limit.

May need a design with one reference. Have no idea how to deal with series pack other than isolated devices.