YB27VA Dual Digital voltmeter/Ammeter in Dual Rail PSU?

Thread Starter


Joined Jun 21, 2013
Hi Forum,

Ive made myself a nice adjustable voltage dual rail lab style Power Supply Unit (PSU); just a standard design using adjustable Linear Voltage Regulators (LM317 & LM 337 for the positive (+ve) & negative (-ve) voltage rails).

These regulators are rated at up to about an amp & will give me about +1.4v to +30v for the +ve rail, & -1.4v to -30v for the -ve. No adjustable current limit control, apart from the built in short circuit & over temperature protection.

Obviously it's not as good as something I could buy, but will be adequate for my circuits. Maybe a few extra decoupling capacitors would improve it further.

My question is about fitting volt meters & ammeters to it. It's easy to fit an analogue voltmeter & an ammeter to the +ve & -ve supply rails, but these 4 meters will take a lot of space.
I found these twin-colour digital modules (YB27VA seems to be the part), each showing voltage in red & current in blue.

I can see how easy it would be to put one in my +ve supply rail, but am not sure if I can ever put one in the -ve supply rail, as wouldn't I need to connect it 'backwards' to work? But would it work or get damaged?

Does anyone have any experience in these modules? It took a while to work out what to do with the red, black & yellow wires & the badly translated specification & weird trio of connection diagrams!!!. It seems the module derives its power form the test voltage, but only if its between 4.5v & 30v, & who knows how the one in the -ve rail derives its power from!

If anyone has any advice or suggestions, please let me know.

If I get it working properly, I shall put it in the Projects Section of the website.

Many thanks,

Simon Bond.

Sent from my iPad2:-
Simon.D.Bond MEng, BSc(Hons), IEng, MRAeS.


Joined Feb 19, 2010
From looking around, it does not do negative values, it only does positive.

From a bit of reading, some of these don't have connector to external power supply. There is a mod you can do to allow connection to external power supply.

Thread Starter


Joined Jun 21, 2013
Thanks for your help.

I bought a pair & wrongly assumed I could measure current in the negative supply - back to analogue ammeters then!

Please can you tell me about the modification for external supply? I'd like my supply to go down below 4.5v, which is the minimum supply I think.




Joined Mar 14, 2008
What is the source of the voltages to the regulators? Can their commons be isolated?

You can power the voltmeter from the input voltage to your regulator. You will need to use a resistor and zener to reduce the voltage to below the 30V required by the voltmeter.

Thread Starter


Joined Jun 21, 2013
Hi again,

As suggested, I have abandoned using the ammeter for my negative rail.

I was trying to get the positive side working, & the voltmeter was straight forward, but I could not get a reading on the ammeter. I was using a decade resistance box as a load, & used the yellow wire which is the ammeter supply labelled IN+.

But according to the spec, these modules have a resolution of 0.1A or 100mA, so neither is suitable for use in my lab PSU as current drawn will me minimal.

So definitely back to analogue ammeters!

Thanks again for your help.

Simon B.