# Power Supply Question

Discussion in 'The Projects Forum' started by mikejp56, Aug 18, 2015.

1. ### mikejp56 Thread Starter Member

Jun 14, 2015
58
4
Hi All,
I am starting to build a bipolar power supply with dual volt meters. If I was only interested in using a single current meter and put it in the ground leg, wouldn't it read incorrectly "the sum of positive and negative currents" drawn from this supply? In other words, if I drew 1 amp from the V+ supply and 1 amp from the V- supply, wouldn't the meter read 0 amps?
If so, so would you use a single current meter to measure total current draw from a bipolar supply?
Regards,
mikejp56

2. ### Lestraveled Well-Known Member

May 19, 2014
1,957
1,225
Correct. The current would sum together at the return connection. You would have to use a switch to separately connect to each output.

Jan 29, 2010
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4. ### mikejp56 Thread Starter Member

Jun 14, 2015
58
4
Hi Lestraveled and ericgibbs,
Thanks for your quick responses. That was what I kind of figured.
If I really wanted to use a single current meter, I guess I would have to use the current shunt resistor method to read the voltage drop across a .1 ohm resistor in each power supply hot leg; V+ and V-; and then sum and scale the voltages in an op amp, and read them on a voltmeter calibrated in amps.

5. ### Lestraveled Well-Known Member

May 19, 2014
1,957
1,225
If you used .1999 Volt (full scale) digital panel meters you wouldn't need an op-amp. With a .1 ohm sense resistor your display would read 1999 at 1.999 amps. Put the decimal point in the right place and you are there. You could use a 0.01 ohm sense resistor for a 19.99 amp display.

If you do used digital panel meters, be careful, some REQUIRE a floating power supply.

6. ### AnalogKid AAC Fanatic!

Aug 1, 2013
5,649
1,595
If you want to read the non-algebraic sum of the two currents, that is, positive one amp plus negative one amp equals a displayed value of two, you will need some kind of invert and add circuit.

ak

7. ### crutschow Expert

Mar 14, 2008
16,518
4,449
Many years ago I made a dual voltage bench supply with a single analog voltmeter and ammeter.
The voltmeter was simple to switch between the two voltages, of course, with a DPDT switch (the two supplies had isolated outputs).
The ammeter was a little more difficult.
Since the full-scale voltage drop across the ammeter was a few tenths of a volt, I put a silicon diode in series with each output and connected the ammeter to a DPDT switch that would switch the ammeter across either of the two diodes. That way I could switch the ammeter to monitor either current without any significant glitches in either output voltage.
The diode would then carry the current for the side not connected to the ammeter.
The ammeter would carry essentially all the current when connected across the diode since its voltage drop was much less than the diode's.

8. ### mikejp56 Thread Starter Member

Jun 14, 2015
58
4
Hi Lestraveled,
I want to stay away from DPMs just because I like the old school look; I am an old school kind of guy.

Hi AnalogKid,
That is what I said in my post #4 with using an op amp; great minds think alike, eh!

Hi Crutschow,
That is interesting; I'll look into trying that.