I'm using an LM324 op amp as part of a buffer/amplifier to measure the current output of a high voltage power supply. The input will be 0 to
-6.5 volts and the output (right now) is 0 to -12. How does the LM324 react when the output gets close to power supply rails? I need to power this with +/- 12v. Might have to put a 10:1 divider on the input.
From the Book of Perturbations, chapter 6 verse 13: "Readest thou thy datasheets with a gimlet eye that thou mayest tread the path of enlightenment and gain wisdom thereby."
-- Rich Webb Norfolk, VA
Rich, You made me laugh out loud! Congratulations! ...Jim Thompson
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As Rich Webb has pointed out, you can get this from the data sheet
The relevant curves are on the bottom pf page seven and the top of page eight.
The LM324 output doesn't source current very well when the output is less than 1.5V below the positive rail, and it doesn't sink current very well when the output is less the 0.5V above the negative rail. If you want to sink all the 10mA that output stage is guaranteed to be able to handle, you need to stay closer to 1V above the negative rail.
Getting the output right down to -12V with +/-12V supplies isn't practicable, but a 10:1 divider sounds like an over-kill.
-- Bill Sloman, Nijmegen
Well, for some reason, the power supply manufacturer designed the current monitor output, not only a negative voltage for a positive voltage output, but also designed it with a scaling factor. The output is 0 to 1.2mA, which turns out to be 0 to -6.55 volts. I'm going to use a digital panel meter so don't really care about range (within reason). Input into meter can just as easily be 0 to 1.2 volts as it can 0 to 12 volts.
The LM324 input voltage off-set specifications aren't marvellous, at
+/-3mV worst case.That's +/-0.25% in 1.2V or +/-0.025% in 12V - the former is significant (if small) on a three digit panel meter, which the latter isn't. I'd be tempted by +/-0.05% in a 6V range, or maybe +/-0.3% in a
10V range.-- Bill Sloman,
it gets a bit soft within 1V of the rails
I don't see any need for an input divider.
but to get -12 out you'll want -14 or so on the -ve power rail. the +ve power rail doesn't need to be +12 if your output is 0 to -12
2V (or 5V or 3.3 whatver's handy) should be enough.-- ?? 100% natural
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