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Old 3rd June 2011, 06:32 PM   #1
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Default How do I measure amperage?

I want to build a simple amp for my computer so I can test speakers. I found the Sure Class-D Audio Amp Board at Parts Express for $26. I tore apart an old VCR in the hopes that I could remove the bulk of the innards, and retain it's power supply for my audio amp. I'll jury-rig a mounting for the amp inside the VCR case.

The Power Supply board is completely separate from the VCR board. I can tap into the power supply and find several outputs with 12v. But I have no idea if the power being supplied is actually appropriate for the Class-D amp. There's also an ouput that measures 35v, and I'm hoping I can add phantom power for a electret measurement mic (a separate project).

My initial question is How do I measure the amperage from the Power Supply 12v output? I thought I saw a spec that said the Audio Board needs 2000 mAmps (but I could easily be mistaken). I have a cheap multimeter with an amps setting. When I set the meter to measure mAmps, and touch the same outputs, I get readings that match the volts (i.e., the 35v output is 35 mAmps, the 5v output is 5 mAmps). I suspect I'm doing something wrong.
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Old 3rd June 2011, 06:58 PM   #2
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Well OK - You should find out for sure what the amp needs, but lets asume that it is 2000 ma or 2 amps. When measuring volts you measure accross the output of the power supply. When measuring amps, or current you measure in series with the leads, but the power supply has to me loaded. By that I mean that you need to put a resistor accross the output and the meter set to current mode in series with the positive lead from the power supply. I suggest a 6 ohm 10 watt resistor. This will tell you if the power supply can put out 2 amps. That seeems like more than the VCR power supply will be able to put out but it worth a try. Dont put the meter in current mode and then accross the power supply. You would probally blow a fuse in the meter. What kind of meter is it? brand and model.
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Last edited by firechief; 3rd June 2011 at 07:00 PM.
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Old 3rd June 2011, 07:02 PM   #3
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Randallm View Post
I suspect I'm doing something wrong.
I see firechief beat me to it Was going to say you could well damage the meter.

VCR PSU's might be tricky (it was the day job )
2 amps is more than most would supply... and many don't run as you expect unless certain of the (multiple) rails are loaded. It's perhaps not an ideal choice tbh.

Edit... I'm assuming the PSU is a switched mode type... it may not be of course.
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Old 3rd June 2011, 07:04 PM   #4
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Ditto above. Your amp may work on 12 volts, but will the supply have enough current? Although most of these amplifiers draw very little at idle or low sound levels, loud passages and bass can really tax the power supply.

As firechief says, there are ways of finding out how much the VCR PSU can actually supply. If it can do a solid 2A, you should be fine. That seems like a lot to ask of a VCR power supply, but you're about to find out!
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Old 3rd June 2011, 07:16 PM   #5
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yes you probably did, to measure current almost every meter measures a voltage across a known resistor. That resistor is normally a low value. example a 200mv full scale meter on the current scale would use a resistor such that the voltage developed across that resistor would equal 200 millivolts. A 1 ohm resistor in the meter would give you a full scale reading of 0.2mv/1ohm=.2amps or 200 ma full scale. The resistor is inserted in series with the load. You did not mention what the load was when you measured your power supply. Did you put the meter in current mode and used the correct input connections on your meter and put the probes directly between the + and - of your power supply. If you did you are effecively putting a 1 ohm load on your power supply. It will try to put 35 amps into the one ohm load and poof goes your meter or just the fuse if you are lucky. To read higher or lower currents the measuring resistor is changed up or down. using the previous example of 1 ohm for 200 ma we would use 0.1 ohm for 2 amp and 0.01 for 20 amp scale for 20 ma 10 ohm etc. Another way to measure your power supply would be to put different loads on the output of the power supply and watch what happens to the voltage. Some of the voltages in the vcr would be regulated some not. A 1000 ohm resistor would draw 1 ma per volt of power supply ie with the 35 volt on your vcr power supply it would draw 35 ma. A 100 ohm resistor would draw 350 ma, a 10 ohm resistor 3.5 amps. remember I=E/R look at the fuse ratings if any in the vcr powersupply to give you a rough figure of the total max current the power supply can deliver.
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Old 3rd June 2011, 07:17 PM   #6
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I type too slow
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Old 3rd June 2011, 07:56 PM   #7
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Default Thanks for the help

I just love this place. I can ask the dumbest question, and just get a helpful, straight-forward answer.

I'm going to go buy a resistor so I can measure the amperage with a load. But given that the fuse in the Power Supply is 1.6 amps, I'm guessing I won't be able to use the power supply. At least I can use the case.

Thanks guys.
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Old 4th June 2011, 12:42 AM   #8
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I found a left over resistor from another project. It is 8 ohm, 20 Watts, 5% tolerance. With the ground to one end of the resistor, the black lead from the meter to the other end of the resistor, and the red lead to the 14.5 volt supply line, I get 50 mAmps. That strikes me as a miniscule amount of currrent. Am I interpreting this correctly?

It's a cheap Triplett 2030 meter. It was $20 at Frys.
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Old 4th June 2011, 06:16 AM   #9
Mooly is offline Mooly  United Kingdom
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Can you post a picture of the PSU ?

If the 1.6 amp fuse is in the primary side of the PSU then that has no direct bearing on the current you can draw from the output. 1.6 amps at 120v ? (120 where you are) is nearly 200 watts. Thats could in theory allow 16 amps to be drawn at 12 volts...

If the 1.6 amp fuse were on the 12 volt rail then it's as it is... 1.6 amp max.

You need to confirm your meter is OK. I just googled it... couldn't find much on it but it looks to small to measure anything over a few hundred ma ! What is its maximum current range ?

50 milliamps is miniscule... that may or may not be down to the meter. Remember I said some VCR PSU's don't behave as you might always expect They sometimes need other inputs into them and other rails to be loaded... sometimes.
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Old 4th June 2011, 06:38 AM   #10
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It's often easier to just measure the voltage across the load and use I=V/R instead of using a series current measurement. 50mA through 8 ohms isn't even a volt.
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