Looking for a reference voltage to check voltmeter?

B

bluedragon

Well-Known Member
Joined
7 Apr 2004
Messages
1,773
Location
Cardiff Bay
Visit site
I've got two multimeters that give differing voltage readings. As I'm trying to modify the battery charging system to maximise charge efficiency, the difference (around 0.1 - 0.2V) is enough to be a problem. Does anyone know of a simple way of getting a reference voltage (preferably around 12-14V) that I can use to check the accuracy of the meters?
 
Get hold of a 12v Zener diode for a few bob fom Maplin along with a 1 k resistor. Wire them in series and connect across a suitable power supply. Using the diode reference documents correct for current and temperature.

Otherwise its off to a calibration lab for one of your meters.
 
A very soft 'knee' on a zener - the volts you will see across it are very dependent upon the current through it...

Better off to find a couple of other peeps with multimeters, and gain a consensus (I know there are risks in this, but it is supposed to be ok to do this to elect a government...)
 
I am not quite sure that I fully understand what you want to do but is not a car battery off load 13.8 V? Lift the bonnet and check both meters readings. What they are actually reading is less important that Knowing the differance?

jeremy
 
"(I know there are risks in this, but it is supposed to be ok to do this to elect a government...) "

And look what a bloody disaster that caused.

Not sure how accurate you are hoping to be. Or why or what your going to do if you are a bit out.

An ordinary dry cell gives 1.5V - use a new one and its going to be very close say +/- 0.2 V

If thats not near enough Its find someone with a calibrated meter and compare.
 
Tangofour's suggestion is not a bad one. Thing to watch is tolerance and temperature coeficient, both can cause you problems if you want absolute accuracy. Most zeners are designed to give their rated voltage (+/- 5%) at a standard current, usually 5mA, so if you set your power supply and resistor to achieve this you won't go far wrong. But 5% at 12V is a possible error of 0.6V in either direction, but at least it won't change between reading with different meters. However, in my experience, they are usually pretty close to the center of the allowed range of values. Providing you avoid extremes of temperature the temperature coeficient shouldn't cause you any problems either. As AlanPound says, zeners do have a "soft knee" but setting up for 5mA avoids any problems due to this (Assuming that the voltmeter is very high impedance - it is a digital one isn't it?).

Usually, digital voltmeters are very accurate, unless the battery is failing - did you check this first? (Sorry if you did and it was too obvious to mention)
 
I know nothing about this, but I would probably charge my ship's batteries fully and then switch the charger off for 24 hours and the voltage should be close to 12.8. As I say, this may be naive.
 
How about using a calibration company such as:-
A-T Instruments Ltd
Alan Thompson
1 Banc Yr Afon
Cardiff
Cardiff
CF15 9TU
029 2081 0597

I found this with a quick google search under Calibration Services Cardiff. There are others.

No idea what it would cost but it shouldn't take long so if they charge by the hour it ought to be cheap.
 
Re: comments about zener diodes & stuff -

This is stoneage technology /forums/images/graemlins/wink.gif - all done with laser trimmed chips now (I wondered how they got them crinkle cut...)

Try the AD587 (about £6.50 + vat from RS components) 10.000v ±5mv out for 15-36v in; 10ppm/°C accuracy. datasheet

Personally, I think one of your meters is duff, and checking against someone elses' meter should tell you which one. If you insist on a voltage reference, however.....

Andy
 
Thanks for all these ideas. It's probable I have a duff meter (unfortunately it's likely to be the one I've done all my battery and alternator output measurements with /forums/images/graemlins/frown.gif). Although I must admit (gulp!)...that NO I didn't check the battery in the meter. I'll do this and see if I can borrow some others to check as a first shot. As I'm trying to assess battery SoC and voltage regulator output, then a -0.2V error could make all the difference between leaving things as they are and spending some serious money!
 
I have an extremely accurate meter.

I have just measured a Durcell AAA cell at 1.5505 volts in the house at 22 degrees. Unfortunately I don't have a 9 volt non rechargable battery which I would have prefered to use.

It will give you a reference near enough 1.55 volts.

I realise that your voltmeter will probably be on another range when checking your boat supplies. But it would be good check for the basic accuracy of your meter.

If you PM your address I will put the battery I have measured in the post tomorrow.

This will then be a check for your two meters.

Cheers

Iain
 
[ QUOTE ]

An ordinary dry cell gives 1.5V - use a new one and its going to be very close say +/- 0.2 V


[/ QUOTE ]
How will +/- 0.2 V tell him which meter is correct? He's looking for greater than 0.1v accuracy, as his two meters differ by 0.1v.
 
sorry, I thought you were replying to my post.

I was offering to send him a battery which I have measured to have a fairly precise voltage of 1.55 volts. This would then give him a voltage reference.

I was not relying on the rather random voltage available from a new battery.

A lot really depends on the resolution of his volt meters.

Iain
 
[ QUOTE ]
I am not quite sure that I fully understand what you want to do but is not a car battery off load 13.8 V? Lift the bonnet and check both meters readings

[/ QUOTE ] After resting for 12 hours it could be about or 12.7 or 12.8 if charged with a mains charger My alternator regulates at only 13.8 when charging (which is the bottom end of its spec) so once off charge the volts is is much lower.

You could expect a car battery to read anything from 12.5 up to maybe 13.something depending on its age, the actual regulated volts from the alternator and the time it has been resting.
 
[ QUOTE ]
If you PM your address I will put the battery I have measured in the post tomorrow.

[/ QUOTE ]

That's really very kind of you! First let me check some new AA and AAA cells I have here and see if the two meters are showing a readable difference at these lower voltages. If so I'll PM you.
 
You are quite right of course

0.2 V is not close enough

I would recommend that he go to someone like Glanford electronics

They will calibrate his meters to a much tighter tolerance.

Only cost him £37.00 each

Plus VAT
 
[ QUOTE ]
Re: comments about zener diodes & stuff -

This is stoneage technology /forums/images/graemlins/wink.gif - all done with laser trimmed chips now (I wondered how they got them crinkle cut...)

Try the AD587 (about £6.50 + vat from RS components) 10.000v ±5mv out for 15-36v in; 10ppm/°C accuracy. datasheet

Personally, I think one of your meters is duff, and checking against someone elses' meter should tell you which one. If you insist on a voltage reference, however.....

Andy

[/ QUOTE ]

I'll try this idea and the zener diodes out of interest. I need a new meter anyway I think and most seem to have +/- 5% on the DC voltage specs. They are probably much better than this in reality, but having a close tolerance reference voltage is never a bad idea. Thanks.
 
If you have a friend who maintains electronic equipment ( like your local IBM/B.T. engineer) he will have a calibrated meter in his bag. This is part of BS5750/ISO9000 or whatever it's called these days. Just compare the values against the same source.
 
[ QUOTE ]
I am not quite sure that I fully understand what you want to do but is not a car battery off load 13.8 V? Lift the bonnet and check both meters readings. What they are actually reading is less important that Knowing the differance?

jeremy

[/ QUOTE ]Sorry but you are wrong on both counts.

Car battery off load is NOT 13.8 volts. The voltage depends on the state of charge of the battery and could vary from 11.7 (or less) to 12.7 volts.

The difference is NOT the most important thing in this application. The accurate voltage is very important.

In answer to the original post, the simplest answer would be to check against a few meters from your friends/aquaintances.

If you were in the West Country you could check it against my (calibrated) AVO.
 
Open up your PC. Stick the probes into the motherboard somewhere and it might be 3.3V, or 5v, or summat:-)

More seriously the output from the PSU is pretty regulated and there will be a data panel telling you what this is and between which wires..

At own risk...
 
You must log in or register to reply here.

Share this page

Top