Measuring Current For Dummies

[Lizard King]

So I get an el-cheapo power supply off flea-bay....it claims to be 1A.  How do I measure the current output?  I don't think I can just put an ampmeter across the jack.  I'd have to plug it into something and measure the output once it is driving the pedal?

Is there a better way?  Put a big-ass ceramic resistor across the output & measure the current across the resistor?


I've never been motivated to measure current before.

[blackieNYC]

The meter needs to be inserted in series. Connect it's plus to the plus and its- to the -.
It's a very good idea to see what your pedal draws.  You can figure out how long a battery will last fairly accurately, and you can keep an eye on what you are asking your pedal supply to do. 

[EBK]

Two things:
1) Remember, current is a measurement through a device, not across it.
2) Depending on your meter, you may have to change how your leads are plugged in.

One more thing:
That 1A is not going to flow out of that supply except under specific load conditions.

[Lizard King]

Cool...thanks to both....makes much sense.

[antonis]

Roughly, you have to find a load (simple resistor) which can stand your PS maximum power..

i.e. for a 12V/1A it's 12Watts (better oversize it a little bit..)

So, for a resistor of 12R/15W your PS should give a current of 1A with a voltage drop of 12V across the resistor..

If you decrease the resistor value (Ohms), your PS should give a geater current but with lower voltage..
(don't overcome more than, say, 30% of the rated current..)

Again, very roughly, your maximum current specification is that for which you have the rated output voltage... 

P.S.
Because of DMM fuses are rather expensive, measure the voltage drop across a 0R1 resistor (IN SERIES WITH THE LOAD of course - or else you'll short-circuit your PS..!!!!!) in mV scale and multiply it by 10 to find the current in Amperes...

edit: Do you want to know if your PS is capable of giving an 1A output or if it's OK to feed your pedal(s)..??

[duck_arse]

thanks antonis, now I don't have to type anything. what he said.

[anotherjim]

I did type it, but I just wiped it out
Thanks Antonis (grrrr...)

[antonis]

thanks antonis, now I don't have to type anything.

Your wellcome duckie, but now I've a more worn keyboard comparing to your's... 

Sorry Jim.. - I can't claim the same for you.. 

P.S.
There should be a way of real-time information about the content of simultaneous typers.. 

[merlinb]

Because of DMM fuses are rather expensive, measure the voltage drop across a 0R1 resistor

Or... just measure the voltage across the load resistor

[antonis]

Because of DMM fuses are rather expensive, measure the voltage drop across a 0R1 resistor

Or... just measure the voltage across the load resistor

That may require complex maths if the resistor isn't of 0.01% tolerance... 

Of course you're right Merlin..!! - by suggestion had a wider option on measuring current with a voltmeter..
(my FLUKE has a forgotten bown-out ammeter fuse and a pair of leads with permanently mounted 0R1 wire-wound resistor..)

[merlinb]

That may require complex maths if the resistor isn't of 0.01% tolerance

[antonis]

That may require complex maths if the resistor isn't of 0.01% tolerance

I could quote some formulas for stability of value, temperature coefficient & noise level calculations but this would give aron a good reason for ban... 

[merlinb]

I could quote some formulas for stability of value, temperature coefficient & noise level calculations but this would give aron a good reason for ban... 

I'm talking about loading the PSU with a 9 ohm resistor (for example). If you measure 9V across it, you know you have 1 Amp. No need for fancy tolerances.

[antonis]

I was just joking, Merlin... 

[blackieNYC]

I don't think he's testing the maximum output of his supply.
If I'd had a current meter in line for the breadboard power supply, I'd have a few more healthy chips in the drawer.

[anotherjim]

Antonis

There should be a way of real-time information about the content of simultaneous typers..

Well, I think some of us may already have that ability

Dummy Loads. Not much said about this. In my hazy memory, there were designs and ideas for these found in hobby magazines. I used to have an old wire-wound electric fire element fitted with Jubilee/hose clips to use as a variable rheostat - don't know where it is now. Also possible are heater wires from toasters, hairdriers etc. Or filament lamps - a 24W/12V lamp takes 2 amps for example. Many old engineers, a lamp in a holder with 2 wires WAS their test equipment!
There are also active load designs for those who find the foregoing just too simple & easy.
Proper, professional laboratory loads can be surprisingly expensive.

What's your dummy load solution?

[PRR]

Lamps are untrustworthy dummy loads. They vary 10:1 from hot to cold.

Resistors, even large wattage, are not expensive. For rock-n-polka audio work, 10% accuracy is more than we deserve, but 5%-better is widely sold.

[anotherjim]

Lamps are untrustworthy dummy loads. They vary 10:1 from hot to cold.

Power on surge simulation built in!

[karis12]

I've tried multiple times to measure my pedals' current draw with the aforementioned method, but my multimeter won't seem to register anything, even at microampere settings. Other than that, my multimeter works perfectly for measuring resistances and voltage. Am I doing something wrong?

[merlinb]

I've tried multiple times to measure my pedals' current draw with the aforementioned method, but my multimeter won't seem to register anything, even at microampere settings. Other than that, my multimeter works perfectly for measuring resistances and voltage. Am I doing something wrong?

1) Fuse blown
2) Leads plugging into the wrong sockets on the meter