On 03/15/2011 8:24 PM, Kirk Wallace wrote: > > Dooh (hits forehead). A transformer in series with a motor lead would be > a bad idea (unless designed to be there). Small toroidal's are easy and > plentiful, but another time sink. > There are things called current transformers. They are usually toroids wound from steel tape. You pass a motor lead through the middle, making a single-turn primary. They are rated with a nnn:1 marking, meaning the secondary is wound with nnn turns. So, a 100:1 transformer, when used with a single turn primary, will send 1/100 of the motor current through the secondary. If you want to read current, you put a 10 Ohm resistor as a "burden" or load on the secondary, and the voltage the scope sees would be (motor current / 100 ) * 10. You can vary the burden resistor to bring the scope signal to the right range.
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