|
Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
|
Thread Tools |
30th May 2008, 1:06 pm | #1 |
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 |
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 |
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 |
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 |
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 |
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 |
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. |