I'd like to measure AC current from a driver with a scope.
It's 12v driver, and desired measuring range is 10mA to about 750mA.
I guess I'd need a current probe, or a current transformer.
My question is in selecting a current transformer.I see many options on Digikey or Mouser, and could use a pointer as to what to select for what I'd like to do.
Can anyone lend any info?
thanks!
How much have you got to spend?
You can spend more on a current probe than your scope costs.
For a casual measure you can just measure the volts drop across a low value resistor, that is often good enough.
A metal film 1 ohm 0.5W resistor would work as a convenient shunt (could get
pretty hot though).
If you are measuring at high frequency remember to avoid wire-wound shunt resistors
which have inductance to confound the measurement.
I see that current probes are expensive. CTs are cheap though; any disadvantage?
These AC drivers have weird waveforms, so when I try to measure current the normal way with a DMM, I get weird results.
I wonder if this odd AC driver means my voltage drop is jumping around. If I use a shunt, will the sample rate determine the accuracy of the current measurement? Is a CT more accurate?
No a current transformer is not more accurate than a shunt resistor. Especially if you wind the scope up to maximum sensitivity allowing you to use a very small rsistor value.
Current transformers will tend to Intergrate the signal and you will not see a true AC waveform.
I guess with a scope and shunt though I need to be sure the circuits are isolated, no?
The scope does not have battery power.
Yes that is correct if you are measuring something from the top side.
An old engineers trick is to remove the ground wire from the scope and leave it sticking out of the plug so you know it is removed. In these days of sealed plugs that can not always be done.
However most laptops and power supplies float, with no ground connection so that means you don't have to bother about where the mains ground is.
Note the two systems do not have to be isolated just that one must be floating with respect to the other.
Hi, what is the driver that you are using?
Tom...... 
It's an LED driver.
It's an LED driver.
Use a series resistor of, say, 1 Ohm (each mA would give 1 mV). 1 A current would heat the resistor 1 Watt (and stole 1 Volt). You can have easily a resistor like this (carbon film up to, say, 1/2 - 1 W. Use ceramic ones for bigger currents).
As others have mentioned, if you go the shunt resistor method, don't blow up your scope in the process. A good video on the subject: EEVblog #279 - How NOT To Blow Up Your Oscilloscope! - YouTube
I ordered a 5w 0.1ohm current sense resistor. With scope, hopefully I can see the real current..
I ordered a 5w 0.1ohm current sense resistor. With scope, hopefully I can see the real current..
By the way, resistance varies with temperature. If your current is too high and resistor gets hot (you have to have a very high current with 0.1 Ohm resistor for this to happen), resistance could increase a bit. Have this into account (the best is to "calibrate" the whole thing if you want a good measure; don't worry if you need +- 5% accuracy)
Regards
But wirewound resistors use constantan or similar wire with minimal temp-co.
If you want to measure high frequency currents don't go for a wire-wound as their inductance can
distort readings.