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Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
I'm building a simple board to throw onto lithium based batteries for overvoltage protection. It consists of a TLV3012 comparator with onboard reference, an voltage divider, load resistors, indicator LED and a transistor. It should turn on the transistor, and load the cell when its voltage is >4.25V
Ive attached a schematic. The diode represents the onboard reference in the comparator, eagle didnt have the device in its library.
After putting the circuit together today, and testing it out with a 5v supercapacitor instead of a battery, it discharges it to 3.xx volts when it should be discharging to 4.25 volts (after setting the trim pot...) After going though and checking all my components and re-checking my math, everything seems ok.
Then, I noticed the LED dims slightly at ~4.2 volts, and then goes out completely at 3.xx. Checking that out further, when the voltage of the cell is > 4.2 the Vbe of the transistor is .9v, when the LED goes dim, Vbe is ~.3v
Is the comparator getting somehow partially 'stuck' on from the lack of base resistor? I might be able to squeeze a 603 SMT resistor onto the board if that is a possible solution. I always thought that latch-up was from overvoltage of an input or other forward biasing of onboard protection diodes.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
The base of a transistor looks similar to a diode, and trying to hook it up to 5v will cause it to draw as much power as the supply (in this case the output of the opamp) can supply. The devise isn't latching up (in the formal sense), it is just shorting out...
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
... wrote ...
The base of a transistor looks similar to a diode, and trying to hook it up to 5v will cause it to draw as much power as the supply (in this case the output of the opamp) can supply. The devise isn't latching up (in the formal sense), it is just shorting out...
I knew its the same as a diode, but the comparator is rated for a continuous short to ground on its output, which is why I didnt expect it to be a problem.
Or does this mean that upon a short, it wont nuke the comparator, but it does need to be 'reset'?
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
Ok, I got them working, I added a .1uf capacitor between in+ and ground, and that solved all issues. I tried base resistors first, and even with a 1k base resistor it would still do the same, though I'm glad it actually wasnt that, Its a lot easier to add the capacitor onto the leads of the comparator then it would to add the resistor. Have them all trimmed for 4.23 volts now.
Time to get a scope though, I have a feeling if I had one I would have seen some oscilations there causing the trouble. Ive attached pics of the finished board.
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Current based latchup in a Comparator?
