# Modified a computer power supply into a lab power supply

1. Sep 11, 2006

### Squall

I recently modified a computer power supply into a lab power supply. I would like to check the amperes bieng output, but where can I get an ammeter?
Or can i make one and how much does one cost.

Heres a link to a pic of my PSU
http://i106.photobucket.com/albums/m261/Squall_11/PSU002.jpg

2. Sep 12, 2006

### Danger

Most any hardware or electronics supply store carries them. Here, a cheap analogue unit costs about $15 (Canadian). I just got a digital one that's normally$40 on sale for \$17. Those are multi-meters, by the way... volts, amps, resistance, and my new one has a specific battery tester function as well.

3. Sep 13, 2006

### denni89627

Don't the multimeter types usually only measure in milliamps? I've had a few digital flukes and none of them went beyond milliamps. Only the clamp type amprobes (or similar flukes) did. But they are upwards of 100 bucks. I've never used the analog ones because the only one I had lasted a week. I blew the fuse, repaired with tin foil, blew up the meter.

4. Sep 13, 2006

### Danger

To tell you the truth, I really don't know about the amp range; I've never used it for that. I just measure voltage and check for circuit continuity with it. I can't look at it now because it's at home and I'm at work.

5. Sep 13, 2006

### Staff: Mentor

Most meters will have a milliamps input and range (with appropriate fuse), and a 10A input and range (with a higher-rated fuse). 10A is about the max I've seen for handheld meters, though. If the OP needs to go higher, he could just use power resistors and measure the voltage across the resistors to see when the supply output is starting to droop below output specs.

6. Sep 13, 2006

### Squall

yes i need to be able to measure up to at least 35A and digital meters only go up to ten. i was wondering what about the Ammeters used in cars can i use those.

7. Sep 13, 2006

### chroot

Staff Emeritus
The clamp-type current probes will measure hundreds of amperes with no problem, but they are expensive. Your best bet for a cheap measurment is simply to use a low-valued power resistor, as berkeman mentioned.

I wasn't aware that cars had ammeters. Could you provide more information?

- Warren

8. Sep 13, 2006

### Danger

Ammeters are usually only an aftermarket add-on in North American cars. Quite often they're included in a 'trio' panel along with the oil pressure and coolant temperature. They're just to show your alternator function, or to indicate a problem with the battery or charging system, but they're also handy for discovering current drains. Same idea as the idiot light, but more informative.

9. Sep 13, 2006

### chroot

Staff Emeritus
Well, I'm pretty sure that such ammeters are just using shunt resistors. There's really no more economical way.

- Warren

10. Sep 13, 2006

### Squall

so how exactly would you use a resistor to measure current. Would you get a 33A resistor and see if the voltage is what is supposed to be in this case it should be 5. Is that basically it

11. Sep 13, 2006

### chroot

Staff Emeritus
What on earth is a 33A resistor?

If you assume your ammeter has essentially no resistance, you can use two power resistors of known ratio.

Here's the idea. Say you buy two resistors, one of 0.1 ohm resistance, and one of 0.01 ohm resistance. You put the 0.1 ohm resistor in series with your (essentially resistance-less), and put the 0.01 ohm resistance in parallel with the meter and the 0.1 ohm resistor. The result is that 90% of the current flows through the 0.01 ohm resistor, while only 10% flows through your meter. You have thus increased your ammeter's range ten-fold.

The trouble comes in actually obtaining such small, known resistances.

- Warren

12. Sep 13, 2006

### Squall

thanks that clears it up and sorry for my stupid mistake, but this is my fist time dealing with such electronics.

13. Sep 13, 2006

### Staff: Mentor

And since meters use some real shunt resistance for measuring the current (surprisingly large resistors for the mA setting!), you can use a separate meter to measure the shunt resistor of your meter's 10A input, and then make an external resistor that is 1/10 as big, as chroot suggests. I measure about 0.1 Ohm for the 10A setting on my Fluke DVM. (Be sure to subtract out the probe resistance on your extra meter when making this measurement.)

You can make that external resistor by paralleling up a bunch of more standard resistors. Like 100 1 Ohm resistors in parallel for my case. Quiz question -- what power rating should those 1 Ohm resistors have?

14. Sep 13, 2006

### NoTime

Yes. You could use one of these.
They are reasonably accurate from my experience.

15. Sep 13, 2006

### Danger

I'm home now, and just looked at mine. I was mistaken in two regards. The stupid thing doesn't have an amp setting at all, and it has a diode test function, not a battery one.

16. Sep 15, 2006

### david90

Just find a halls effect type amp sensor. Allegro has them but they are used to measure +50 amps. It's too big for your application. It outputs analog signal proportional to the amp being measured. I guess you would need additional electronics to display the amp value.

17. Sep 15, 2006

### david90

can't he just use one low value high wattage resistor and put in series and measure the voltage drop across it to determine the amperage?

18. Sep 15, 2006

### chroot

Staff Emeritus
Sure, david90, in theory. It can be difficult to get such small resistors, however.

- Warren

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