How can I make a DIY AC probe for my DMM

phrarod · 2nd March 2011, 12:36 AM

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.

Matt.B.H. · 2nd March 2011, 01:09 AM

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.

phrarod · 2nd March 2011, 01:15 AM

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

tomchr · 2nd March 2011, 01:23 AM

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

Matt.B.H. · 2nd March 2011, 01:26 AM

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.

phrarod · 2nd March 2011, 01:32 AM

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.

phrarod · 2nd March 2011, 01:34 AM

Thanks Matt. Yes, trying to chase that 60/120Hz. Are there glasses I can buy to just see them.

Doz · 2nd March 2011, 07:39 AM

Quote:

Originally Posted by Matt.B.H.

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 ...

DF96 · 2nd March 2011, 11:13 AM

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.

Captn Dave · 02:34 PM

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.

2nd March 2011, 02:32 PM	#10
Captn Dave diyAudio Member Join Date: Jun 2008	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.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Headphone Line-Out Voltages - (Newbie DMM AC Voltage Reading)	trelin	Headphone Systems	7	9th May 2008 07:15 AM
diy differential probe?	zilog	Class D	7	18th April 2006 09:42 PM
DMM ac readout with signal	Ralph	Everything Else	1	18th August 2005 07:42 AM
DIY HV Probe thisway possible?	Kenshin	Power Supplies	5	9th August 2005 02:35 AM
DIY oscilloscope probe	Bricolo	Parts	2	25th September 2003 10:56 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

2nd March 2011, 01:09 AM	#2
Matt.B.H. diyAudio Member Join Date: Jul 2009 Location: Cheltenham	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.

2nd March 2011, 01:15 AM	#3
phrarod diyAudio Member Join Date: Aug 2005	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

2nd March 2011, 01:23 AM	#4
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	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

2nd March 2011, 01:26 AM	#5
Matt.B.H. diyAudio Member Join Date: Jul 2009 Location: Cheltenham	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.

2nd March 2011, 01:34 AM	#7
phrarod diyAudio Member Join Date: Aug 2005	Thanks Matt. Yes, trying to chase that 60/120Hz. Are there glasses I can buy to just see them.

2nd March 2011, 11:13 AM	#9
DF96 diyAudio Member Join Date: May 2007	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.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)