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Old 2nd March 2011, 12:36 AM   #1
phrarod is offline phrarod  United States
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Default How can I make a DIY AC probe for my DMM

I'm trying to bust a 60 cycle hum. A friend told me how to make a homemade AC probe that can be used with a DMM.

I have a very good DMM for voltage but I must be doing it wrong. He said to wind it like an air coil overlapping back and forth.

Here's my version that didn't work. Can someone tell me the proper way to make one? Much appreciated.
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Old 2nd March 2011, 01:09 AM   #2
Matt BH is offline Matt BH  United Kingdom
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Not quite getting what you want?

looks like you are using a coil as a search coil for general nastyness in a power supply?

I have a good Fluke meter that wont do this. The input is >10MR. What is your "very good meter"?

Cheers Matt.
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Old 2nd March 2011, 01:15 AM   #3
phrarod is offline phrarod  United States
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Yes, I want to make a AC probe that will allow me to pick up stray AC fields in the power supply to try and see where the AC fields are building up and see if I can fix that and the hum. I have this one. Its their top of the line and quite good for voltages EX570 - 12 Function True RMS Industrial MultiMeter with IR Thermometer
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Old 2nd March 2011, 01:23 AM   #4
tomchr is offline tomchr  United States
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Looks like you're building a sniffer. Those work great with spectrum analyzers, oscilloscopes and the like that have a good, low-noise, high sensitivity input amp. They work wonders for finding sources of RF oscillations and the like. A sniffer for those kinds of applications tends to be a few (often just one) windings soldered to the end of a coax cable.

But I doubt your DMM has a low enough AC voltage range to make use of it. And it won't pick up much at 60 Hz anyway -- except for transformer leakage fields and such, which you don't want.

Your best bet is to trace the grounds and signals around. Eliminate any excessive loop areas in ground/signal routing. Are you sure it's 60 Hz hum? If it's 120 Hz, it's likely to be caused by supply ripple. Try doubling the reservoir cap temporarily and see if that reduces the hum. If it's caused by supply ripple, the hum should be reduced by half.

If you actually want to measure where the hum is injected (assuming it is capacitively or inductively coupled into the circuit) your best bet is probably to get a frequency selective voltmeter. For example an HP 3581. Those can be had for $100-ish on ePay. Pay a bit more and get one that works...

~Tom
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Old 2nd March 2011, 01:26 AM   #5
Matt BH is offline Matt BH  United Kingdom
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cool, pretty similar specs to most.

I think if you dont have a scope your best bet would be to design one

Take a look at the many op-amp primers.

Op-amp-rectifier-summing amp-moing coil meter will do the trick.

I have a very similar thing that detects up to 1Thz.

I guess you are watching for 60/120Hz hum?

Cheers Matt.
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Old 2nd March 2011, 01:32 AM   #6
phrarod is offline phrarod  United States
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Hi Tom. Maybe it is 120 Hz I guess I would need something to measure it. It doesn't sound like a ground loop hum as I've heard those in the past. The hum does not get louder with gain. I do have more ripple than in the prototype although that was a different psu circuit. Previously I had .045vac ripple on the old supply on a bread board. The Dual mono supplies are each generating .3vac which is a big leap. I can't find what's affecting it. The circuit is super simple. Here's schematics if it helps.
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Old 2nd March 2011, 01:34 AM   #7
phrarod is offline phrarod  United States
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Thanks Matt. Yes, trying to chase that 60/120Hz. Are there glasses I can buy to just see them.
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Old 2nd March 2011, 07:39 AM   #8
Doz is offline Doz  United Kingdom
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Quote:
Originally Posted by Matt.B.H. View Post
cool, pretty similar specs to most.

I think if you dont have a scope your best bet would be to design one

Take a look at the many op-amp primers.

Op-amp-rectifier-summing amp-moing coil meter will do the trick.

I have a very similar thing that detects up to 1Thz.

I guess you are watching for 60/120Hz hum?

Cheers Matt.
1THz? crumbs .... BTW I work in Tewkesbury ...
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"To err is human.. to make a real balls-up requires a computer"
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Old 2nd March 2011, 11:13 AM   #9
DF96 is offline DF96  England
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If you have a meter with a very sensitive AC volts range you could use an RF choke as a magnetic sniffer - say 10mH? This will be more sensitive than your homemade air-cored coil.

Your grounding scheme is wrong. You are injecting charging pulse noise into the PSU ground. C1 and C2 grounds should be connected together. From there a wire should connect to the secondary CT. A separate wire should go to C3 ground, then from there to the main star ground.
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Old 2nd March 2011, 02:32 PM   #10
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You didn't mention exactly what you are working on, but if you have AC heaters you don't want to overlook the possibility of heater-cathode leakage.

Last edited by Captn Dave; 2nd March 2011 at 02:34 PM.
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