Replacing battery in digital multimeter with AC/DC power adapter

[mariush]

You could use a digital power isolator, like Adum5010 or Adum5000 for example : http://www.analog.com/media/en/technical-documentation/data-sheets/ADuM5000.pdf

Adum5010 : https://www.digikey.com/product-detail/en/analog-devices-inc/ADUM5010ARSZ-RL7/ADUM5010ARSZ-RL7CT-ND/3821204

It converts 5v to 5v or 3.3v with around 30% efficiency, but considering a multimeter only uses a few mA you don't really care.  If the multimeter normally runs on 9v battery and shows low battery with just 5v, you could have a tiny boost regulator on a circuit board to boost the voltage to around 6-7v, or whatever the threshold is for low battery.

Or you could have something like this isolated 5v->5v converter which easily fits in the 9v battery compartment : https://www.digikey.com/product-detail/en/murata-power-solutions-inc/NXE1S0505MC-R7/811-3040-1-ND/5047366
 
And if you want to go a step forward you could add a super capacitor after the digital isolator to charge up and keep the boost regulator powered for tens of minutes after you disconnect the power adapter (but note that it may take a couple of minutes for super capacitor to charge enough to use the multimeter and then a few more minutes to charge to 100% while you can use the multimeter still plugged in the adapter).

dc in (5v ) ---> isolated converter / digital isolator 5v-> 5v -> boost regulator to 5v .. 9v

dc in (5v) ---> isolated converter / digital isolator 5v --> lithium charging IC ( MCP73811 for example) --> tiny 100-400mAh lithium battery -> boost regulator to 5v..9v

dc in (5v) ---> isolated converter /  digital isolator 5v or 3.3v -> ldo to 2.5v -> super capacitor (10F+ 2.75v) --> boost regulator 

[lordvader88]

Money is the problem, the 9V battery died and I don't have $10. I have 3 other meters, + multi-tester, and a little LCD voltmeter, so at least I'm not suck, but I only have 1 9V that barely works in the little 1, and the 3V 1's dial is shorting somewhere so its a hassle change dial setting. So for now I have to use a combo rather than just my main DMM

Also I already breadboarded an adjustable LM723 for another transformer, so its ready