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How to measure the current of a discrete op amp?
Old 19th January 2019
Lives for gear
๐ŸŽง 15 years
How to measure the current of a discrete op amp?

How does one go about measuring the current of an opamp?

I understand I need to change one of my multimeter leads to the Amperage setting . . . .

But where do I measure?

The supply rails? (+/-24V) So, just measure one leg first, and then the other or do I need two multimeters?

Or do I measure the opamp input/output?

It's just a comparative measurement, for current draw between one opamp and another.
Old 19th January 2019
Gear Guru
1 Review written
๐ŸŽง 15 years
Place the meter into current mode and put the test leads in series with the power supply to the opamp. You will need to do it for each rail or use two meters at one time.

Or, read the data sheets?
Old 19th January 2019
Lives for gear
nosebleedaudio's Avatar
๐ŸŽง 15 years
Most op amps draw the same current on each rail, some have a slight difference..
Old 19th January 2019
Lives for gear
To expand on what Jim said, you need the current you're measuring to flow through the meter, so you need to temporarily disconnect the voltage rail connection to the opamp circuit (with the power off, of course), and connect the meter in between the supply and the opamp, so the supply current has to flow through the meter. Likely for an opamp you'll need the meter set on a milliamp range, if that's separate from a higher current (several amp) range...sometimes it needs different lead connections at the meter, too, depending on the design.

Do not connect your meter across the voltage rails when it's on a current setting, or you'll promptly toast a protection fuse in the meter or worse. In current mode, the meter is ideally a short circuit to whatever you're testing, rather than an open circuit as for voltage mode. Similarly, many meters will be hurt if you apply voltage to the leads when it's in resistance mode, although some better ones are protected against such accidents.

(There are clamp on ammeters available that have a sort of jaws you place around the wire carrying the current you wish to measure. Many of these, though not all, only work for AC current, and in any case they are usually intended for use in measuring relatively large currents and probably wouldn't be any too accurate on the sorts of measurements you're making. Where they're suitable to use they are very convenient since you don't have to disconnect and reconnect things in the circuit.)
Old 19th January 2019
Lives for gear
๐ŸŽง 15 years
Okay, so the meter's "common" or black lead in this case wouldn't go to ground, it would be where the plus or minus power rail goes into the opamp, and then the red lead would be on the opamp's corresponding pin, with the meter in between.

Cool. (?)

Seems an easy way to toast an opamp, if one side of the bipolar rails gets disconnected for a bit.

Also,( with only 1 meter) inbetween 1 rail and the opamp, this doesn't represent any problems for the other rail?
Old 19th January 2019
Lives for gear
Yes, cool.

You don't put the meter in with the power supply on. You turn the power off, change the circuit, and then power it up to make your measurement...and afterwards, power it down before removing the meter. The meter is, in this case, pretty much just a magic piece of wire that shows how much current is going through it. Probably you would want to make the connections a little more solidly than just holding the probes in place with your hands (while simultaneously turning on the power to the circuit with your nose or something).

The meter does in fact add some resistance to the connection between the rail and the circuit, but probably not enough to worry about in this case. The net effect is having your one rail be a tiny bit undervoltage at the opamp compared to nominal. It would affect the headroom slightly, and maybe degrade performance in a few other ways a bit, but that's immaterial for checking the power consumption. Your meter manual should give the exact details on this; for instance, a Fluke Model 77 series II lists a burden voltage of 6 mV / mA for the millliamp current ranges. If the opamp uses, say, 50 mA (to pull a number out of thin air--I would suspect significantly less in practice) the voltage drop across the meter would be 0.3V...pretty much negligible for a 24V supply rail, but a valid consideration if it were instead something like 3.3V logic circuits.
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