# Measuring negative voltage

Discussion in 'The Projects Forum' started by Regular_hobbyist, Oct 22, 2015.

1. ### Regular_hobbyist Thread Starter New Member

Mar 9, 2015
9
0
I have a power supply built on veroboard. It's based off this design. The transformer is 36VCT. I would like to measure the voltage of both rails. I have two voltmeters like this.

As the schematic depicts, I attempted to measure the negative rail voltage by powering the meter off an lm7805 and connecting the Voltage sensing lead to the output of the lm337. This gives me the difference between the negative output and the unregulated DC negative input. How would I get the voltmeter to measure the difference between ground and the negative output?

I'm a lurker and this is my first time posting. I don't have much electronics knowledge. Is there any other information you would need?

Also, to improve current handling capacity, I tacked down 22 awg wire on the input and output strips.

File size:
18.8 KB
Views:
14
2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
The positive side is easy. The negative side depends on if the meter can display a minus sign in front of the reading?

3. ### Regular_hobbyist Thread Starter New Member

Mar 9, 2015
9
0
Can you elaborate further?

The meters I have won't measure voltage if the polarity is wrong. The meters were cheap. Should I get different meters?

4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
With the voltmeters you chose, I cannot see a way to hook one up to read the negative side and have it work for voltages less than -4V. Is that ok?

I'm assuming that you are using a pot for R4, and you intend to adjust the voltage from -1.25V to ~ 20V?

To make the voltmeter read the neg supply, connect red and white together to the Gnd (0V) output terminal. Connect the black wire to the -VDC output terminal. Since the voltmeter is powered off the supply output, it will not read below 4V.

To make the voltmeter read the positive supply, connect the red wire to Vin of U1, the white wire to the +VDC output terminal, and the black wire to Gnd (0V) output terminal. This side should read from 1.25V to ~20V...

Last edited: Oct 23, 2015
5. ### Regular_hobbyist Thread Starter New Member

Mar 9, 2015
9
0
R2 and R4 are potentiometers.

Yes, that is the voltage I'm getting. I just grabbed a random transformer out of my spare parts to make it work.

Is there a way to measure the full voltage range of the negative side?

Thanks for helping out with this. It's late where I'm at. I'll post back tomorrow.

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Not with the voltmeter you bought. You would need one that has four wires; two for power, and two for the measurement input. It would also have to have an appropriate common-mode range to work with your set up.

7. ### Regular_hobbyist Thread Starter New Member

Mar 9, 2015
9
0
Do you have any recommendations for meters that would work?

8. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
No, I would have to do a search, and wade through all of the Crappy Chinglish on Banggood or EBay

9. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,281
224
There should be some decent application notes here: http://www.murata-ps.com

They are currently offline at the moment.

10. ### ScottWang Moderator

Aug 23, 2012
4,933
777
To buy two of these 3-30V voltage meters, the input voltage also provided the power for voltage meter, they only can measuring the voltage from 3 to 30V, it is for the most applications.

11. ### GS3 Senior Member

Sep 21, 2007
408
35
No problem. I have several like that and they work fine.

Join red and white wires and the device will measure from 3 V (minimum required for power) to 30 V (max supply voltage).

If you need to measure under 3 V then connect black to negative output, red to ref point, and supply between 3 to 30 Volts to white which is no problem as you have plenty of positive places to get it from.

12. ### Regular_hobbyist Thread Starter New Member

Mar 9, 2015
9
0
The positive side meter successfully measures the positive output. I put in the LM7805 so I could measure below 3 volts.

How would I get a meter to measure the entire negative side?

I understand I'll need a voltmeter with separate power and voltage sense wires to measure the negative output. Where can I find one of these meters? Would it be possible to modify my current voltmeter to do this?

13. ### ISB123 Well-Known Member

May 21, 2014
1,240
531
I used PM-438 ,works fine the only pain in ass problem is decimal point placement which has to be set using jumpers.

14. ### Regular_hobbyist Thread Starter New Member

Mar 9, 2015
9
0
I realized what I really wanted was to find the search terms on how to find these meters.

Upon reading the PM-438 data sheet, I realized that most voltmeters with separate wires for power and voltage sense also have settable decimal points. Therefore, these voltmeters say they read 200mV.

Sorry for not asking for the correct question.

Also, thanks guys!

15. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
I had a similar problem reading different voltages that did not share a common point. Also, the meters I used did not allow their power source to have a common with the measured voltage. I solved it by using battery power for the meters with the battery power being "switched" on and off with opto isolators that were powered by one of the supplies I wanted to measure. The batteries lasted for years.