Input/Output Current Compare - Scope Measurement

Jun 22, 2017 6 Replies
R

I have a circuit that steps up a 30KHz sine wave from 12 to 240 volts. I would like to compare its input current with the maximum that can be drawn on the output. A variable ceramic resistor would be used as the load.



My idea was to make two of the circuits below, place one on the input, one on the output, and view both channels on an oscilloscope in subtraction mode.



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Is this a sound approach?



What is the preferred calculation for equating the secondary's higher voltage, lower current output with the input current?



Robert Miller


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F

I would presume you'd be more interested in the ratio of the input and output currents rather than the difference. Direct subtraction may also mislead if the phases are not identical.

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J

Just the turns ratio, which is eqial to the number of secondary turns if the primary is a simple pass-through.

CTs can be a pain to build. It's usually easier to use a shunt resistor and isolate the resulting signal with a wideband, signal-level transformer.

John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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T

Let's say I'm in the 3rd world, and have an assortment of unknown transformers from scrap electronics plus wire. If I'm lucky I may have a $2 multimeter. What would be my best choices to make a CT?

NT

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J

Some power transformer. Use it as-is, whatever turns ratio it has. Or remove some winding and add your own 1-turn primary.

John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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T

Yup. One can determine turns ratio by measuring offload output voltage.

NT

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