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Registered Member #3450
Joined: Sun Nov 28 2010, 05:01AM
Location:
Posts: 61
I am attempting to build an oscilloscope current probe to measure primary current in my coil. So far I have this 100 turns of #30AWG magnet wire on an Electronic Goldmine ferrite core with a 1ohm burden resistor. When I try and measure primary current all I get is a garbled up mess on my scope screen. Has anyone got any contraction tips? *EDIT* Here is a scope shot.
Registered Member #1845
Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211
I'm almost certain your resistor is NOT wirewound. I have used resistors exactly like that before on the primary side of a GDT to damp ringing. (so if they were wirewound it would have messed up my GDT waveforms horribly)
One thing you can try is to put another 1 ohm terminator at the input to your scope. This will keep RF from reflecting back through the cable and messing up your waveforms.
When current surges through the main cable in your CT it will magnetize the core. The increase in magnetic flux through the core will induce an EMF across the 100 turns and the resistor you have on the core. The current will flow in that loop in such a way as to try and magnetize the core in the opposite direction.
So the peak current in the 100 turns you put on it will roughly be 100 times less than the current through the cable you are trying to measure. If you try to measure extremely high peak currents with it then the core will saturate, but with that many turns it probably won't saturate until you have hundreds of volts across it.
Say you are measuring a signal at 1000A peak with your transformer.
Then the peak current flowing through your CT will be roughly 10A (stepped down by 100) So if you have 10A through that loop then your burden resistor will have 10 Volts across it. (since E = IR)
All you really have to do is just apply ohms laws to find the voltage drop across the burden. Keep in mind that if you use a terminator then your voltage at the scope will be cut by a factor of two again.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Whats the time division on that scope shot? It looks like it may just be an error in your scope settings. What frequency do you have in your primary? Is it a SSTC or SGTC?
Registered Member #3450
Joined: Sun Nov 28 2010, 05:01AM
Location:
Posts: 61
Turkey9 wrote ...
Whats the time division on that scope shot? It looks like it may just be an error in your scope settings. What frequency do you have in your primary? Is it a SSTC or SGTC?
Time div was on mS. 11.5mS Off 2.4mS On at 71Hz. Fres of coil is around 400KHz with torroid. It's an SSTC.
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Oscilloscope current probe
