[DMM] Different reading on reverse polarity

canvas · 28th January 2011, 06:39 AM

I tried to measure a 7915 regulator on my HP3478A 5.5 DMM. With correct polarity (Lo=-15V & Hi=GND), the reading is -15.82xx volt, but if I reverse the test lead (Lo=GND & Hi=-15V) the reading becomes +15.63xx volt! According to service manual, there is no calibration on negative DC, and all calibrations on DC is only based on positive reference. So, I don't think a full calibration will solve this problem. Is there anyone having this problem?

Speedskater · 28th January 2011, 06:06 PM

Why not measure some other DC voltage sources like your automobile battery.

AndrewT · 29th January 2011, 11:20 AM

One of the tests for a correctly working voltmeter is to check it reads exactly -Value = +Value when reversed.

tomchr · 18th February 2011, 03:08 AM

It sounds like your meter has developed an offset. What do you get when you short the inputs to the meter?

bob91343 · 18th February 2011, 04:15 AM

I don't know if that's a bench type or a hand held. If the former, it could be leakage to the power line. Does it have a ground lead or are both leads floating?

mrfeedback · 18th February 2011, 04:35 AM

Perfectly normal behaviour for handheld DMM's in my experience.........don't worry about.

Eric.

canvas · 08:38 AM

It's a bench type DMM. When both leads are shorted, it is about -0.0014mv ~ -0.0019mv. I will try a 12V car battery next time.

Mooly · 18th February 2011, 10:25 AM

Any battery will do... 1.5 or 9 volt etc.

It's possible the error measuring on the reg is down to some noise etc on the rails or AC "noise" on the chassis (ground), so reading a battery will prove beyond doubt. I would guess with a meter like yours it should be equal to within the last digit or so.

28th January 2011, 06:39 AM	#1
canvas diyAudio Member Join Date: Nov 2003 Location: Taiwan	[DMM] Different reading on reverse polarity I tried to measure a 7915 regulator on my HP3478A 5.5 DMM. With correct polarity (Lo=-15V & Hi=GND), the reading is -15.82xx volt, but if I reverse the test lead (Lo=GND & Hi=-15V) the reading becomes +15.63xx volt! According to service manual, there is no calibration on negative DC, and all calibrations on DC is only based on positive reference. So, I don't think a full calibration will solve this problem. Is there anyone having this problem?

28th January 2011, 06:06 PM	#2
Speedskater diyAudio Member Join Date: Sep 2002 Location: Lakewood, Ohio	Why not measure some other DC voltage sources like your automobile battery. __________________ Kevin

29th January 2011, 11:20 AM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	One of the tests for a correctly working voltmeter is to check it reads exactly -Value = +Value when reversed. __________________ regards Andrew T.

18th February 2011, 04:35 AM	#6
mrfeedback diyAudio Member Join Date: Feb 2002 Location: Perth, Australia.	Perfectly normal behaviour for handheld DMM's in my experience.........don't worry about. Eric. __________________ I believe not to believe in any fixed belief system.

18th February 2011, 08:34 AM	#7
canvas diyAudio Member Join Date: Nov 2003 Location: Taiwan	It's a bench type DMM. When both leads are shorted, it is about -0.0014mv ~ -0.0019mv. I will try a 12V car battery next time. Last edited by canvas; 18th February 2011 at 08:38 AM.


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

18th February 2011, 03:08 AM	#4
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	It sounds like your meter has developed an offset. What do you get when you short the inputs to the meter?

18th February 2011, 04:15 AM	#5
bob91343 diyAudio Member Join Date: Mar 2010	I don't know if that's a bench type or a hand held. If the former, it could be leakage to the power line. Does it have a ground lead or are both leads floating?

18th February 2011, 10:25 AM	#8
Mooly diyAudio Member Join Date: Sep 2007	Any battery will do... 1.5 or 9 volt etc. It's possible the error measuring on the reg is down to some noise etc on the rails or AC "noise" on the chassis (ground), so reading a battery will prove beyond doubt. I would guess with a meter like yours it should be equal to within the last digit or so. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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

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
reverse polarity of the chinese goods?	PreSapian	Tubes / Valves	36	13th June 2010 08:46 PM
Headphone Line-Out Voltages - (Newbie DMM AC Voltage Reading)	trelin	Headphone Systems	7	9th May 2008 07:15 AM
Reverse Polarity	icecoolwas	Car Audio	2	28th April 2008 08:30 PM
Reverse polarity on Power inputs	spooney	Car Audio	5	20th December 2007 04:38 AM
subwoofer polarity reverse	kdlsrinu	Subwoofers	12	22nd May 2006 08:39 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