Digital multimeter powered by adaptor?

[autir]

I own two cheapo (~10 Euro each) digital multimeters, and have been paying fortunes for 9V batteries. Can I supply the multimeters with power coming from a DC PSU or an adaptor/transformer? What are the cons of this? possible problems?

[Kevin Weddle]

I took my radioshack multimeter which came with two small batteries and replaced them with two D cells. It works fine. Unfortunately it works. Thoeretically if it were designed to perfection it would not work. But this is just another cheapo design.

[autir]

What is a D shell? :-[

And why "unfortunately"? Do you mean that a decent multimeter must accept voltage of a very specific and small range in order to provide the expected accuracy? What happens when we power a cheapo multimeter with a somewhat different voltage than that expected? Does it alter the values of the measurements we make?
If I create a circuit with a voltage regulator to make sure that the voltage of the multimeter will always be stable and 9V, will it function properly and accurately?

[ante]

Hi autir,

Refering to your question in

[Guest Yevgenip]

You know,
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AA
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D

Battary Sizes. D is HUGE!

[audioguru]

Hi Autir,
I have 2 multimeters so I can measure voltage and current at the same time. They have LCD screens (not high-current LEDs) so use very little current from their 9V alkaline batteries. Their batteries therefore last for many hours and years. I can hardly measure how little their accuracy changes when the battery runs down (only a couple millivolts on the 20V range).

I like them to be isolated from ground (even with capacitive isolation that a power supply wouldn't provide) so I can measure differential signals well.
With a power supply for a multimeter you might need a shielded cable to avoid interference (mostly mains hum) pickup, and wouldn't be able to measure differential signals 'cause the "0V" lead would have a high capacitance to ground.

[MP]

The power supply for multimeters is regulated. This is why you have the same accuracy on the day the battery is new as the day before the battery went dead.
Powering a multimeter from a 9 volt DC supply would be no different than plugging a 9 volt battery into it. This is always the case with a good multimeter. If you find there is a difference, there are adjustment pots inside multimeters which are there for fine tuning them.
For a cheap one, I cannot say. It would depend upon the unit. If you are eating lots of batteries, I would certainly try it. What can you lose?

MP

[ante]

Hi Audioguru,

How do they eliminate the phenomenon you describe in my bench top multimeter that is powered from 240VAC? Why wouldn

[Guest Alun]

Ante, I think Audioguru means that if a multimeter is power from a mains transformer it will be capacitively coupled to the mains by the winding capacitance of the transformer. I personally can't see how this could be a problem since multimeter's are normally used for measuring DC voltage and current. The AC range is of little use above 300Hz or so thus the small 100pf or so of inter-winding capacitance to mains won't make much difference.

[ante]

Yes, I think he does. I just checked my Rohde&Schwarz bench multimeter and it has a transformer PSU built in. I wonder how they deal with this problem? The ranges on this meter go way down on both AC and DC ranges.

[audioguru]

Hi Guys,
Here's my new Pentium4. I got a really good deal. ;D

My battery-powered multimeters can measure very small AC or DC voltages and fairly high frequencies (20KHz) differentially, so it doesn't even matter which lead is connected to common, without much interference pickup. If my meter was AC powered, the "ground" lead would be the shield and the positive lead would require shielding. If the AC powered meter has an "earth jumper" cut, it would still have an unbalance to ground for each lead and would pickup high frequency interference without a shielded cable, like my 'scope.

[ante]

Congratulations to your new computer! I can see the computer salesman cry when you left the shop! When he tells his boss about the deal he is going to be fired for sure, poor guy! $crooge has struck again! ;D ;D

Yes I know how you mean, but it

[audioguru]

Hi Ante,
My computer salesman was smiling, even though he didn't get any commission because he is paid salary only. When big computer stores buy in vast quantities they must make a huge profit, except for the sale price that I found. ;D

My workshop has flourescent lights and my soldering iron's transformer is nearby.
My 'scope picks-up lots of garbage with only about 5cm of "hot" exposed wire with alligator clip sticking out of its shielded cable, and it is isolated from earth (just mains transformer capacitance).
However, my battery-powered multimeters can be set for mV AC high-impedance input and with nothing connected to their unshieded wires (like antennas), they pickup very little, I suppose because their inputs are perfectly balanced. When I earth my meter's common wire with a 100pF cap, it picks-up lots of garbage from its unshielded "hot" wire.
Therefore I will never use an AC-powered meter in my workshop.

[autir]

Thank you all for your replies.
Audioguru, I realise that high-end multimeters (and equipment, in general) need quality input voltage in order to operate properly. Though most educative, your replies do not answer the original question regarding "el cheapo" equipment. From all replies I have read I assume that powering my multimeters via DC (and using a regulator, LM317 e.g.) would be fine.

[EnigmaOne]

Yes, it would be just fine.

In my home/garage shop, I'm powering several Fluke field DMMs (as well as a few DMMs that heretofore have no brand name--can't get any cheaper than that!) from a bunch of homebrew isolated/regulated power supplies that ended-up being built from surplus AC adapters that keep accumulating in my junk drawer.

If you add a connector to make hook-up easier, be sure to preserve any water proof/resistant (silicone sealer and various acrylic/vinyl glues are your friends here) qualities that the DMM case might possess, because condensation inside a meter really sucks--particularly where the higher voltage measurements are concerned.