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Sep 09, 2004, 10:48 AM
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Larry3215's Avatar
OK, you win.... What's your favorite beer

Larry
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Sep 09, 2004, 11:10 AM
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Larry3215's Avatar
OOps - you forgot the fuse:
652-MFR185 for $.58 each

A couple of questions:
1) do we only need to heat sink the TIP137 or do we also need one for the KSP2907?

2) Are the heats sinks conected to the circuit electricaly in any way (its been many years since I did this sort of thing)?

I am planing bread board construction on these and then heat shrink them to prevent shorts between the boards. I was going to leave the sinks exposed, but dont want to chance a short between units while they are bounceing around on the bench or in the charging fire box.

Thanks again!

Larry
Sep 09, 2004, 11:59 AM
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Dan Baldwin's Avatar
My favorite beer is free beer. Dang. I thought I had the MFR185 in the first list. That brings the total to $3.69.

Only the TIP137 needs a heat sink. If you don't insulate the heat sink, it will end up connected to the collector of the TIP137, so it would have to be treated as a live connection. You would be better off to insulate them. The case of the transistor, however will still be live, so you need to be careful that they don't get together. You could use TO-220 mounting kits (mouser #532-4880M) at $.68 ea in qty of 10, but that would mean that the total would be over $4.00, and you would win, so I suggest using a mica insulator (mouser #532-77-11) at $.10 in qty of 10 and shoulder washers (mouser #532-7721-7PPS) for .15 in qty of 10, then use your own #4-40 screws and nuts. That way the total is $3.94/cell, and I still win.

I can almost taste it now

Dan
Sep 09, 2004, 01:13 PM
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Larry3215's Avatar
Don, I will get that beer on the way

Two other questions just occured to me.

1) What is the max input over voltage these can handle/shunt away from the cell?

In other worrds - will they protect from selecting the wrong cell count on the charger?

I was just looking at my DVM and noticed it has a listed accuracy of 1%.

I assume that is % of full scale range. My options for range are 2 volts and 20 volts. 1% of 20 volts is .2 volts. That does NOT sound accurate enough to set the balancers.

2) Short of buying a very expensive Fluke meter (you would loose for sure then), how can I accurately set the balancer voltage?

You are doing us all a BIG service with this circuit and I am makeing you earn your free beer

Larry
Sep 09, 2004, 02:59 PM
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Dan Baldwin's Avatar
Boy, the questions keep getting trickier. Next you'll ask if I stopped beating my wife.

1) What is the max input over voltage these can handle/shunt away from the cell?
In other worrds - will they protect from selecting the wrong cell count on the charger?

Although this circuit is intended to balance packs, it is not intended to protect against setting the wrong cell count,yes, they will protect from selecting the wrong cell count but;
you would have to hold the charge current to under 2 amps
you would need larger heat sinks than those I suggested

2) Short of buying a very expensive Fluke meter (you would loose for sure then), how can I accurately set the balancer voltage?

I have no intentions of loosing by that much, so I'll see if I can help you with calibration. There are several ways that you can calibrate the balancers. You could do something as simple as setting them 1% low, so you know your safe even if the meter is wrong. One problem with that is that your CCCV charger (you are using a CCCV charger aren't you?) would never stop charging because it would never see full voltage.

You could start by setting the balancers on the low side, and gradually turn them up until the charger will show that a pack is fully charged. Keep in mind that if the balancers are set low, they will be sinking the whole charge current once the batteries get to full charge, so they will get hot. If you do this, keep the charge current low. Depending on what kind of charger you have, you might even be able to hook up the balancers without the batteries in circuit to set them.

You could go into the local electronics store with a 9 volt battery in your pocket and ask for a demo of their best high resolution meter, measure your battery voltage (on the volts scale, not on the battery test setting), write down that number, and check the same battery on your DVM at home. That will give you a correction factor for your meter.

I think that there are some voltage references available that are much tighter than 1%. I'll look and post the results if I find them. I want to be able to check calibration on my meters as well.

I never thought I'd have to work so hard for a beer

Dan
Sep 09, 2004, 03:32 PM
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Dan Baldwin's Avatar
Digi-Key has a precision 5 volt reference (LM4040AIZ-5.0-ND) that is rated for .1% initial accuracy. That should be close enough. You could use that to check calibration on your DVM.

Dan
Sep 09, 2004, 11:40 PM
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Larry3215's Avatar
This is from the Data sheet for the voltage reference. I want to be sure I understand this correctly.

"In a conventional shunt regulator application (Figure 1) , an
external series resistor (RS) is connected between the supply
voltage and the LM4040. RS determines the current that
flows through the load (IL) and the LM4040 (IQ). Since load
current and supply voltage may vary, RS should be small
enough to supply at least the minimum acceptable IQ to the
LM4040 even when the supply voltage is at its minimum and
the load current is at its maximum value. When the supply
voltage is at its maximum and IL is at its minimum, RS should
be large enough so that the current flowing through the
LM4040 is less than 15 mA.
RS is determined by the supply voltage, (VS), the load and
operating current, (IL and IQ), and the LM4040ís reverse
breakdown voltage, VR."

The formula they give and a circuit is shown in the attached pics.

I assume Vs is a 12 volt source ( I have one handy).
I dont know how much my DVM draws when measureing voltages, but lets assume 1 ma?
Also allow 1 ma for Iq

So Rs should be 3.5 ohms?

Does that sound rite?

Larry
Sep 09, 2004, 11:42 PM
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Larry3215's Avatar
Dan, I owe you some of that really expensive free beer

Larry
Sep 10, 2004, 09:10 AM
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Dan Baldwin's Avatar
No, I'm afraid you have the decimal point off by a few places. A 3.5 ohm resister would yield a current draw of 2 amps! That won't work. The IL and IQ in the formula are in amps, you used MA. A 3.5K would work just fine. I would probably use a 1K resister because that would yield a current draw of 7MA which is about in the middle of the allowable range. The current drawn by your DVM will probably be less than 10 microamps, or less than 1/100th of 1 MA, so you really don't have to take that into account. If you order an LM4040 make sure it is the "A" type because it is the .1% reference.

Dan
Sep 10, 2004, 10:02 AM
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Larry3215's Avatar
Oops.

Well, whats a decimal place or two among drinking buddies

Thanks Dan!

Larry
Sep 11, 2004, 02:17 AM
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Larry3215's Avatar
Well darn.

I built the first balancer and it does not work

I suspect I got the pinout of one (or all) of the transistors wrong.

I'm too tire to go back and chck now tho.

Here is how I thought it went?
(its been a long time)

Larry
Sep 11, 2004, 07:48 PM
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Dan Baldwin's Avatar
Larry

It doesn't work because I didn't get my beer yet.

Your pinouts of the TIP137 and the 2N3906 look right if I assume that the picture you drew of the 2N3906 is a top view, but the TL431 doesn't have an emitter, base and collector, so the pinout you show can't be right. The schematic I posted shows a pictorial view of the TL431 from the top, so you need to hook it up as shown on the schematic. with the flat side of the TL431 facing you with the leads on the bottom, the left lead goes to the wiper of the voltage pot, the middle lead goes to - or ground, and the right lead goes to the resistors that go to the transistors. What does the balancer do when you hook it up to a 12 volt power supply through a resister. Open circuit, with about 12 volts across the balancer, or low voltage across the balancer. Does the LED come on? If you post a couple of pictures I can take a look and see if I can spot the problem, but I think it's the beer.

Dan
Sep 11, 2004, 11:48 PM
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Larry3215's Avatar
It goes low voltage - .8 or so - and the lite stays off no matter what the pot setting.

I will go thru and double check all the conections. I did it on a breadboard and it is really ugly I could easily have gotten something in the wrong place.

Larry
Sep 12, 2004, 12:04 AM
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Larry3215's Avatar
OOps - one discrepance I just noticed - I have 2N2907 instead of 2N3906. Could that be the problem?

P.S. I have written a strong letter of protest to the beer shipper I used. I suspect he may have side tracked the shipment before it got to you

Larry
Sep 12, 2004, 11:04 AM
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dowd's Avatar
Since Dan hasn't had a chance to answer yet, I'll emerge from my normal lurking mode, and take a stab here. But if Dan tells you something else, believe him since I've never built this circuit.

Since the 2n2907 is also a pnp with a reasonable voltage and current rating it should be fine to drive the led. So I don't think that is your problem.

It sounds like the TIP might be full on. Since it is on a breadboard, try lifting the wire connected to the Base of the TIP and see if the voltage across the circuit rises from 0.8 volts. If it does then the TIP is full on when it shouldn't be. Take a look at the voltage on the base of the TIP. Something is pulling it low, which turns it on.

Take a really close look at the way you wired the 431; it controls the TIP. Take another look at Dan's schematic and keep in mind that he shows the top view of the 431. Also confirm the values of the resisters.

And did you add in the additional components in Dan's updated schematic?

Another thought, what value resister did you put in series with the 12 volt supply to drive this circuit in place of the battery while you test it?

Good luck,

Dowd
Last edited by dowd; Sep 12, 2004 at 11:12 AM.


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