measuring amps on my computer's power supply with a multimeter

[q_q_anonymous@yahoo.co.uk]

I'd like to see how many amps my devices are pulling on each wire. To
calculate total amps demanded per rail.

It's my understanding that the PSU has a 12V,5V,3.3V rails and they
split into wires. Colour distinguishing the rail to whome it belongs.

I have a multimeter.
I read that Current must be measured in series but I don't want to cut
the PSU wires.

The 4 pin molex connectors on the PSU are male. So if I combine a M-F
F-M adaptor, it'll make a small extension cable . Maybe 1 inch of 3
inches. I can cut that and measure there. Will that wire extension
create too much of a voltage drop though?

is there are better way of measuring the current?

I dont think i'll be able to find a way like that to extend the 4 wire
plug with adaptors. That ATX12V 2x2 plug with the 2 yellow and 2 black
wires. They wouldn't make M-F and F-M adaptors for it.

I would sum the amps to help estimate PSU requirements, and perhaps
make sure that i'm not taxing a 300W PSU too much. And apparently
there's an issue (melting?!) of not wanting > 6A on any 12V rail!
though I suppose i'd know before that happens


thanks

 

[Chris]

I'd like to see how many amps my devices are pulling on each wire. To
calculate total amps demanded per rail.

It's my understanding that the PSU has a 12V,5V,3.3V rails and they
split into wires. Colour distinguishing the rail to whome it belongs.

I have a multimeter.
I read that Current must be measured in series but I don't want to cut
the PSU wires.

The 4 pin molex connectors on the PSU are male. So if I combine a M-F
F-M adaptor, it'll make a small extension cable . Maybe 1 inch of 3
inches. I can cut that and measure there. Will that wire extension
create too much of a voltage drop though?

is there are better way of measuring the current?

I dont think i'll be able to find a way like that to extend the 4 wire
plug with adaptors. That ATX12V 2x2 plug with the 2 yellow and 2 black
wires. They wouldn't make M-F and F-M adaptors for it.

I would sum the amps to help estimate PSU requirements, and perhaps
make sure that i'm not taxing a 300W PSU too much. And apparently
there's an issue (melting?!) of not wanting > 6A on any 12V rail!
though I suppose i'd know before that happens


thanks

Hi, q_q. Sorry, but you're heading in the wrong direction. If you use
a series resistor large enough to get a measurement, you'll cause the
power supply to sag, which will affect operation of the PC.

If you want to read DC current with a multimeter, you can either put
the meter on DC Amps, break the line and put the meter in series (won't
work for the same reason as above), or else use an AC/DC current clamp.
This device operates on a hall effect sensor, and outputs a voltage
proportional to the current through the wire being clamped.

Fluke and many other manufacturers make AC/DC current clamp accessories
for their DMMs, or you can buy a stand-alone unit.

Good luck
Chris

 

[q_q_anonymous@yahoo.co.uk]

Hi, q_q. Sorry, but you're heading in the wrong direction. If you use
a series resistor large enough to get a measurement, you'll cause the
power supply to sag, which will affect operation of the PC.

If you want to read DC current with a multimeter, you can either put
the meter on DC Amps, break the line and put the meter in series (won't
work for the same reason as above), or else use an AC/DC current clamp.
This device operates on a hall effect sensor, and outputs a voltage
proportional to the current through the wire being clamped.

Fluke and many other manufacturers make AC/DC current clamp accessories
for their DMMs, or you can buy a stand-alone unit.

Good luck
Chris

I just asked somebody else, they said that it's fine, here was their
argument in response to " If you use a series resistor large enough to
get a measurement,"


i'm not putting a resistor in there. But, even so,

if I will use a F-M extension cable, so, some short wires, and cut
them and put the multimeter in series, measuring the amps. So that will
add some resistance.

But the reverse of what you wrote is true, because, the lower the
resistance, the better the measurement.

And if I just cut the wire and put the multimeter in series, then it's
fine. A multimeter has an extremely low resistance, and in a low
voltage low current circuit, the resistance, or the effect, is almost
negligible.

 

[Chris]

I just asked somebody else, they said that it's fine, here was their
argument in response to " If you use a series resistor large enough to
get a measurement,"


i'm not putting a resistor in there. But, even so,

if I will use a F-M extension cable, so, some short wires, and cut
them and put the multimeter in series, measuring the amps. So that will
add some resistance.

But the reverse of what you wrote is true, because, the lower the
resistance, the better the measurement.

And if I just cut the wire and put the multimeter in series, then it's
fine. A multimeter has an extremely low resistance, and in a low
voltage low current circuit, the resistance, or the effect, is almost
negligible.

In which case, just try it. Worst case, the PC will malfunction due to
voltage sag (particularly on the 5V and 3.3V lines).

You have to ask yourself about your ability to measure a small enough
resistance to cause a negligible voltage drop. V = I * R, you know.
Actual values of resistance may vary significantly from wire tables,
you know. Not only that, but an standard multimeter accuracy is very
poor at the low end (e.g. a 1 milliohm shunt, 10 amps = 10mV).

Good luck with your project.

Chris

 

[Chris]

I just asked somebody else, they said that it's fine, here was their
argument in response to " If you use a series resistor large enough to
get a measurement,"


i'm not putting a resistor in there. But, even so,

if I will use a F-M extension cable, so, some short wires, and cut
them and put the multimeter in series, measuring the amps. So that will
add some resistance.

But the reverse of what you wrote is true, because, the lower the
resistance, the better the measurement.

And if I just cut the wire and put the multimeter in series, then it's
fine. A multimeter has an extremely low resistance, and in a low
voltage low current circuit, the resistance, or the effect, is almost
negligible.

In which case, just try it. Worst case, the PC will malfunction due to
voltage sag (particularly on the 5V and 3.3V lines).

You have to ask yourself about your ability to measure a small enough
resistance to cause a negligible voltage drop. V = I * R, you know.
Actual values of resistance may vary significantly from wire tables,
you know. Not only that, but an standard multimeter accuracy is very
poor at the low end (e.g. a 1 milliohm shunt, 10 amps = 10mV).

Good luck with your project.

Chris

 

[Bob Masta]

I'd like to see how many amps my devices are pulling on each wire. To
calculate total amps demanded per rail.

It's my understanding that the PSU has a 12V,5V,3.3V rails and they
split into wires. Colour distinguishing the rail to whome it belongs.

I have a multimeter.
I read that Current must be measured in series but I don't want to cut
the PSU wires.

I wonder how useful a multimeter reading will be. I would
imagine the current draw from a computer is anything but
constant, and the peak current draw may be in brief spikes
that the meter won't read. I'd guess a scope-based reading
may be better for this, but then you do get back to the
issue of inserting a dropping resistor.

Just a thought...


Bob Masta
dqatechATdaqartaDOTcom

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