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Old 10th January 2012, 08:37 PM   #1
tvrgeek is offline tvrgeek  United States
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Default Current probe for scope

Any advice on an affordable/used current probe for a scope or for that matter, one that could feed a sound card in? AF range, for investigating amplifier anomalies. It need not be precisely calibrated.
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Old 10th January 2012, 09:32 PM   #2
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virtually every meter measures current by measuring the voltage across a resistor. So depending on the amount of current you wish to measure and the sensitivity of the scope etc you can calculate the value of the resistor you need. ie 1ma through a 1kohm resistor will give you 1 volt. through a 100 ohm resistor 0.1 volt, with a 10ohm resistor .010 volt or 10 milli volt. So a current meter with a 10ohm resistor has a sensitivity of 10 millivolts per mili amp. A sound card cannot measure dc as they normally have a blocking capacitor on the input.

knowing what you want to measure would be helpful.
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Old 11th January 2012, 03:02 AM   #3
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Allegro makes some interesting linear HALL devices that might be adapted. No such thing as a cheap clip on current probe that does audio bandwidth.
Easiest is probably a 10 Ohm power resistor in series with line.
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Last edited by thaumaturge; 11th January 2012 at 03:07 AM.
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Old 13th January 2012, 11:48 AM   #4
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only good current probe for full Audio bandwidth - and a fair bit more so you can see HF things - that I know of and is affordable is the Tektronix P6042. This covers DC to 50MHz with a 50ohm output to match a 'scope or DMM; so should work with a soundcard input but with limited bandwidth.

These are now quite old but can be found for reasonable cost in working condition.
Mik

Last edited by audiomik; 13th January 2012 at 11:49 AM. Reason: remove duplicated comment
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Old 13th January 2012, 12:56 PM   #5
AndrewT is online now AndrewT  Scotland
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If we stay with the added sensing resistor method, can we measure the two ends of the resistor when both are not at mains earth potential?
Somebody said use both scope channels in subtract mode. I have tried that, very poor results.
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Old 13th January 2012, 01:10 PM   #6
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Hello guy´s. I did a current probe with an old clamp meter, removing all electronics, only the coil and mechanical components. I placed a 1R 1W resistor between coil terminals and solder a coax wire from an broken oscilloscope probe. This is not so accurate, but for search phase differences in variable speed industrial equipment is very effective. I´m talking about tenths of amperes, in quasi sinusoidal waves. The scale factor in this case is of less importance than the quality of the wave to be viewed in a 20MHz scope. Perhaps you can emulate this best oriented for audio applications.

Good luck!!!
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Old 13th January 2012, 01:26 PM   #7
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Hello guy´s. I did a current probe with an old clamp meter, removing all electronics, only the coil and mechanical components. I placed a 1R 1W resistor between coil terminals and solder a coax wire from an broken oscilloscope probe.....
Good idea! I had thought of trying the same but hadn't gotten to it yet. It would of course only work on AC. I repaired lots of the old TEK probes. They used custom HALL IC's mounted in a flux transformer head. The surface of the trap door lid had to be glass smoothe or they wouldn't work right.
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Old 13th January 2012, 01:30 PM   #8
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[QUOTE=thaumaturge;2859892]"It would of course only work on AC. "

OK, yes, at the output of a variable speed controller always is there AC currents, except when DC injection for fast braking.

Good luck.
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Old 14th January 2012, 07:40 AM   #9
Frex is offline Frex  France
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Hello tvrgeek,

If you want an easy to do and non intrusive solution that measure AC and DC current for a very reasonable cost, you can look the LEM current sensor like this one.
Unlike a shunt solution, your are not bothered by the common mode issue and you have also no additionnal loss.
The output signal are a current, that is it better if you use long coax wire to go on your scope input.
These type of sensors can measure high or low current level accurately. Bandwidth are not exeptionnal but probably sufficient for audio investigation (~200kHz).
You can easily get higher sensitivity with more turns on the current sensor(sensitivity are multiplied by the number of turns).
So, many LEM (or other brand) sale many type of these sensors withs differents nominal sensitivity and with voltage or output mode, you have the choice !
Regards.

FRex.
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Old 14th January 2012, 09:32 AM   #10
Elvee is offline Elvee  Belgium
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One possibility is a Rogowski probe.
Here is an home-made example. Its slim form factor and small dimensions mean the LF response is limited, but with a suitable conditioning circuit, it is still capable of resolving a few mA down to 30Hz.
It was made for power supply tests, not audio.

The construction is of gimped wire wound on a silicone and PTFE core.
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