Hi all. I hope you can help, this is driving me nuts.

Attached is the DXF (zipped) of my circuit.


The problem is the potentiometer isnt allowing me to properly adjust the charge point. Normally it is set near full clockwise, which sets it to the lowest possible charge threshold, around 12.5V

When a battery below this is connected, the Fast Charge LED lights and the battery begins charging.

So currently, I cant seem to set the threshold properly which is most likely a resistor value issue. Some values are slightly off. I corrected one resistor which seemed to fix the threshold setting but the charger never seems to want to cut off at all once it starts!

My questions are:
What mode are the op-amps running in?
What tells the charger to stop charging? My guess is over-current protection
What is the best way to troubleshoot this circuit?

What is the best way to set and test this circuit?
How much is Xanax?

Arggh. Thanks for any help.
Cloud

Attached Files

BAT C.zip (14.0 KB, 42 views)

 

Can you convert it to a .gif or .png

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Attached now includes a WMF (windows MetaFile) which can be viewed with Windows Fax/Picture viewer.

Attached Files

BAT C.zip (37.6 KB, 49 views)

 

So in researching I see that the OP amps are set up as Comparators. Makes sense so far. I am still abit confused on what exactly tells the charger when to stop charging.

See the voltages listed above the nodes near the input of the op amps? Are those correctly calculated based on the VREF of 6.8V?

Also, is it normal that my CR6 6.8V Zener ref should actually show 6.5V. A normal diode I would expect this but I expect this one should always be 6.8V.

Regardless I can see how the VRef being off, or resistor tolerances being an issue here.

 

I don't see how that can work. Can you point to a web site, or otherwise provide a description of how it is supposed to work?

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Ron

 

Are you referring to the schematic? If so, I agree, how can that work? Lol, we've been using this same board for 7 years so it does work, unfortunately I just dont understand how.

If you arent refering to the schematic could you clarify please.

Thanks, Cloud

 

I am referring to the schematic. I had missed R6, the current sense resistor, on my first perusal. I'll look at it some more.

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Ron

 

Additionally I am baffled as to why I have 13+V on every pin of Q4 regardless of charge state or charge threshold. Peculiar.

 

Zener diodes have a tolerance, the A series is nominal voltage + or - 10%,
the B series are + or - 5%. In this case it is normal to be 6.8 + 0r - 0.34 volts.
So yours in within tolerance.

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I had made the assumption that Q3 and Q4 were a sort of poorly-designed linear regulator. After staring at the schematic for awhile, it dawned on me that they are in a positive feedback loop. The TIP145 is a switch. It's either on, or it's off. That's probably why Q4 is saturated.
EDIT: I tried to simulate this with not much luck, but I'll try a little bit longer.

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Ron

 

Thanks for the replies both of you.


I agree with you observations. I can actually see a change in the output current by changing the fast-charge potentiometer as well in addition to it affecting the cutoff voltage. Does it seem correct that Q4 (TIP145) has 13V (or more) on every pin? What are some conditions that would cause Q4 to be always on?

Also how critical are the diodes (except the zeners) to the operation? I mean what purpose does each serve?

 

I tried to manually force the charger to stop charging by shorting Q3 but what happens is the Power LED is cutoff but the Fast Charge stays illuminated.

Ps - I believe at least 1 or 2 of those diodes are used for clamping.

 

Here is an observation. The overcurrent protection is a negative feedback loop.
None of the diodes are for clamping.

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Ron

 

Thanks again for your input Ron.

With knowing that, I disconnect C3 which creates the negative feedback loop and what I see is that the chargers cutoff and turn on values are the same. The charge light blinks on and off when it is triggered (by adj the pot).

So that is pointing to a problem in the fast charge negative feedback loop for some reason. I checked the capacitor (which is not polarized). Do you have any more insight with that being said?

 

I said the overcurrent protection loop, not the fast charge loop.
The fast charge loop looks to me like positive feedback, and removing C3 just speeds up the output of its op amp. I don't understand what that circuit is supposed to do, but I don't see how it could do much, due to the fact that R1 is only 51 ohms.
The overcurrent loop is definitely negative feedback. Current above the threshold starts to turn on Q2, which steals current from Q3 (and in turn, from Q4). This should tend to limit the current to about 4 amps (400mv/0.1 ohms). Simulation bears this out.

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Ron