Oddity with maplins DMM/mains voltage - UK Vintage Radio Repair and Restoration Discussion Forum

Pete_kaye · 30th May 2008, 1:06 pm

Can anyone explain this.When testing a 9V(or any other value) battery on Dc range,I get a good reading which changes sign if the leads are reversed.When I switch to 200VAC range, it reads approx 210% higher(eg 19.6v for a 9.2v battery) and 0VAC if leads are reversed .Is this normal for this DMM? my other one does it too.
I first noticed this when checking the output from a smoothing 50uf cap which showed the same oddity ie reading 190AC but 0 when leads back to front.
Also does the mains supply vary during the day

My meter gave 223VaC at 10 am then 233 later the same day.

mastermanx2001 · 30th May 2008, 1:51 pm

Just the second bit of your query, Mains voltage can vary quite a lot. I was setting up the output transistor bias current in a Hi Fi amp and wondered why it was not stable over a period of time. It was because it went up and down with the mains voltage. Watching the bias current and switching on a fan heater connected to a different! 13A! (actually not even on the same ring main) the amps bias current could be seen to take a dive.

BGmidsUK · 30th May 2008, 2:13 pm

223v is rather low! At my old place it was always around 245-250v, last night I thought I'd check the voltage whilst having a look at my Sobell TV in case the jumper needed changing and it was spot on 240v. It has measured as low as 233v on occasion

As for reading DC voltages using the AC scale, mine does the same thing, which does call into question the accuracy of DMMs in some applications (such as if a DC bias were present) so the trusty Scope fulfils this purpose

BG

ALANS ANITAS · 30th May 2008, 3:19 pm

According to the "Times" newspaper, on Tuesday 27th June, because of a large number of power station failures (I think was nine, including both Sizewell stations and Hunterston B) voltages had to be reduced for a number of hours, until other stations could be bought on line. Some half million consumers lost their supplies.

ALAN

dseymo1 · 30th May 2008, 3:29 pm

Now you know how your meter behaves, it gives you the useful option of AC or DC 'coupling' by reversing the leads, depending on what you want to measure!

BGmidsUK · 30th May 2008, 11:18 pm

I remember ages ago measuring the voltage across a small fluorescent tube, driven by a battery-powered inverter - there was quite a difference in readings depending on which way the leads were connected. This was with an analogue multimeter, so I assume it had the same characteristics as these DMMs with AC readings. These meters measure with reference to zero volts and are presumably calibrated with the assumption the waveform will be symmetrical

.. thing is, never assume

BG

John Robson · 31st May 2008, 9:35 am

You pay for what you get with multimeters.

The differences between various types of DMM are explained in this Agilent
report.

In the end if you can afford it, buy a True RMS DMM.

http://cp.literature.agilent.com/lit...988-5513EN.pdf

30th May 2008, 1:06 pm	#1
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Oddity with maplins DMM/mains voltage Can anyone explain this.When testing a 9V(or any other value) battery on Dc range,I get a good reading which changes sign if the leads are reversed.When I switch to 200VAC range, it reads approx 210% higher(eg 19.6v for a 9.2v battery) and 0VAC if leads are reversed .Is this normal for this DMM? my other one does it too. I first noticed this when checking the output from a smoothing 50uf cap which showed the same oddity ie reading 190AC but 0 when leads back to front. Also does the mains supply vary during the day My meter gave 223VaC at 10 am then 233 later the same day.

30th May 2008, 1:51 pm	#2
mastermanx2001 Retired Dormant Member Join Date: Jun 2007 Location: London, UK. Posts: 366	Re: Oddity with maplins DMM/mains voltage Just the second bit of your query, Mains voltage can vary quite a lot. I was setting up the output transistor bias current in a Hi Fi amp and wondered why it was not stable over a period of time. It was because it went up and down with the mains voltage. Watching the bias current and switching on a fan heater connected to a different! 13A! (actually not even on the same ring main) the amps bias current could be seen to take a dive.

30th May 2008, 2:13 pm	#3
BGmidsUK Retired Dormant Member Join Date: Oct 2007 Location: Leicester, UK. Posts: 809	Re: Oddity with maplins DMM/mains voltage 223v is rather low! At my old place it was always around 245-250v, last night I thought I'd check the voltage whilst having a look at my Sobell TV in case the jumper needed changing and it was spot on 240v. It has measured as low as 233v on occasion As for reading DC voltages using the AC scale, mine does the same thing, which does call into question the accuracy of DMMs in some applications (such as if a DC bias were present) so the trusty Scope fulfils this purpose BG

30th May 2008, 3:19 pm	#4
ALANS ANITAS Rest in Peace Join Date: Jun 2006 Location: North West Northamptonshire, UK. Posts: 346	Re: Oddity with maplins DMM/mains voltage According to the "Times" newspaper, on Tuesday 27th June, because of a large number of power station failures (I think was nine, including both Sizewell stations and Hunterston B) voltages had to be reduced for a number of hours, until other stations could be bought on line. Some half million consumers lost their supplies. ALAN

30th May 2008, 3:29 pm	#5
dseymo1 Retired Dormant Member Join Date: Dec 2007 Location: Shropshire, UK. Posts: 3,051	Re: Oddity with maplins DMM/mains voltage Now you know how your meter behaves, it gives you the useful option of AC or DC 'coupling' by reversing the leads, depending on what you want to measure!

30th May 2008, 11:18 pm	#6
BGmidsUK Retired Dormant Member Join Date: Oct 2007 Location: Leicester, UK. Posts: 809	Re: Oddity with maplins DMM/mains voltage I remember ages ago measuring the voltage across a small fluorescent tube, driven by a battery-powered inverter - there was quite a difference in readings depending on which way the leads were connected. This was with an analogue multimeter, so I assume it had the same characteristics as these DMMs with AC readings. These meters measure with reference to zero volts and are presumably calibrated with the assumption the waveform will be symmetrical .. thing is, never assume BG

31st May 2008, 9:35 am	#7
John Robson Retired Dormant Member Join Date: Apr 2008 Location: Hertfordshire, UK. Posts: 96	Re: Oddity with maplins DMM/mains voltage You pay for what you get with multimeters. The differences between various types of DMM are explained in this Agilent report. In the end if you can afford it, buy a True RMS DMM. http://cp.literature.agilent.com/lit...988-5513EN.pdf Last edited by John Robson; 31st May 2008 at 9:45 am.