# determining current use of circuit

Discussion in 'Electronic Basics' started by tempus fugit, May 2, 2004.

1. ### tempus fugitGuest

Hey all;

This something I've wondered about for some time now. If I want to check the
DC current consumption of a circuit powered by, say, +15v, all I need do is
insert an ammeter in series with the circuit. Easy enough. How do I
determine what the consumption of a dual supply circuit is though (i.e.,
+/-15v)? Do I insert the ammeter at a positive point in the circuit and at
a - point in the circuit and add the 2? Or can I just insert an AC ammeter
in between the xformer 2ndary and the rectifier circuit?
On that subject, If I have a xformer rated at 200mA, I assume that is an AC
current rating. Will measuring the AC current draw (by inserting the ammeter
before the rectifier circuit) give me the total current load on the xformer,
or do I need to include the DC current as well?

Thanks

2. ### Robert C MonsenGuest

Consider a circuit with +15, 0, and -15 rails. Depending on the load between
the rails, the current will take different paths. For example, if you have a
15 ohm load between +15 and 0, and no load between the 0 and -15 rails, then
the current will travel out of the +15 terminal of your PS, back into the 0
terminal.

If, on the other hand, you now add a 15 ohm load between the 0 and -15 rail,
then the current will now travel out of the +15 back into the -15, and no
current will pass through the 0 terminal.

Thus, the total DC current is really the maximum of the 15 to 0 and the 0
to -15 current, not the sum.

Regards,
Bob Monsen

3. ### tempus fugitGuest

Thanks Bob.

OK..... so how do I measure that?

4. ### JamieGuest

sine you have 2 sept. outputs on your power supply it should be the
combined current.

5. ### CFoley1064Guest

Subject: determining current use of circuit
True -- except the voltage burden of the ammeter may change the current draw.
Circuits are specified for current draw on the + and the - separately. When
choosing a power supply for your application, choose one with a current
capability that's more than the greater of the two. For instance, if your
circuit uses 0.85A @ +15V and 0.01A at -15V, get a +/-15V power supply rated at
more than 0.85A.
If you're using a full-wave rectifier circuit like this (view in fixed font or

Full Wave Rectifier Power Supply ____
| |
.-----o-----|78XX|----o-----o+
.-------. | | |____| |
-. ,-------o~ +o--' --- | ---
)|( | | --- | ---
)|( | | | | |
) ,---. | | o-------o-------o-----oGND
)|( | | | | | |
)|( GND | | --- | ---
-' '-------o~ -o--. --- _|__ ---
| | | | | | |
'-------' '-----o-----|79XX|----o-----o -
|____|

Buy a transformer with an AC current rating 1.8 times the larger of the two DC
current outputs.

Good luck
Chris