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Power dissipation calculation uncertainty

If I have a power supply similar to a computer power supply where it's rated at 300W, what is the proper way to figure out what my actual power usage is when I have this system running under its maximum load?

Here's what I've done so far....

I have a power supply that provides 24.5 volts and 5.1 volts DC output, I find I can run a total current of 9 amps continuous, or 20 amps peak before tripping the internal protection.

When I put a current probe on the 24.5 and 5.1 volt lines, using a digital scope with math functions, I get 8 amps RMS on the 24 volt line and 1.15 amps RMS on the 5.1 volt line, in my worst case use of the machine.

I'm trying to decide what power dissipation I have...

With my scope, I saved 30,000 points of voltage and current data against the same time line, and I imported this V, I data to a spreadsheet....and created a new calculation where I multiplied V * I for each data point...for instantanious power per sample. I then squared each V * I data value, summed it, divided by 30,000 samples, and took the square root...hoping to find RMS power.

With 8 amps at 24.5 volts and 1.15 amps at 5.1 volts, I found about 200W power dissipation...

Does this make sense? I'm not experienced with "power" electronics to be confident about it so I wanted to seek advice.

Thanks.

Reply to
Roque
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WOW, talk about *overkill*! :) You don't need to calculate RMS when dealing with DC, you just multiply I by V. So (24.5 x 8) + (5.1 x 1.15 ) = 201.865W.

Reply to
A E

What's a guy who couldn't see that doing with an oscilloscope capable of doing what he did with it?

--
Then there's duct tape ... 
              (Garrison Keillor)
nofr@sbhevre.pbzchyvax.pb.hx
Reply to
Fred Abse

although the unit is rated at a total of 300W, don't you think that the wattage on each tap may be different? The 24V line may be rated up to 300W (12A) but I would be very suprised if the 5V line could deliver anything like the 60A that would be required to get 300W @ 5V.

I would suppose that the unit is designed to run some light electronics off the 5V line and some kind of heavier electric actuators off the 24V line. With these assumptions I would expect the

5V line to only be rated at a few Amps, while the 24V line would be rated much higher. You would only need to exceed the limit on the lowest rated line to trip the protection circuit (say, by drawing 2A on the 5V line that may only be rated for 1.5A)

- Jeff Dutky

Reply to
Jeffrey Dutky

V.

Yeah, so? The OP clearly stated the current on each rail, the numbers didn't trigger any alarms for me.

Reply to
A E

Bah, have you been in a university lately? The kids there have equipment that took a building full of engineers to design, and is used for the most trivial tasks. That's life, electronics is a commodity now.

Reply to
A E

If it's reality, then does it have to make sense?

"The difference between fiction and reality is that fiction has to make sense." Tom Clancey.

The 300W is probably the input power to the supply at the rated line voltage. Some goes to heat etc. If the supply is 75% efficient, then 225W will get to the loads. Zoramy

Reply to
Ghost Chip

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