Testing Current Draw

railhead · Post by **railhead** » 31 Aug 2010, 19:44

Just picked-up a new DMM, and for the freaking life of me, I can't figure out how to test current draw. I have an old crappy needle meter and it's getting readings, so I know I have everything hooked-up properly, but no matter where I connect the leads or where I set the dial, I get nothing but -.02 on the display. The user manual is worthless, BTW.

The black lead goes to the common, and for mA, I would assume I should have the red hooked up to the mA jack. I get -.02. So I slap the red lead into the VohmF jack. -.02. This is an auto-range DMM, so there's no range dial -- and the Range button doesn't have any effect on things. Below is a pic of the DMM. Any help would be MUCH appreciated!

lolbou · Post by **lolbou** » 31 Aug 2010, 20:02

Do you have to set the AC/DC type of measurement with the button on the upper right or is it detected automatically too?

railhead · Post by **railhead** » 31 Aug 2010, 20:10

That just made it +.01 or +.02

lolbou · Post by **lolbou** » 31 Aug 2010, 20:59

what if you use the µA position? What's the circuit you're testing?

railhead · Post by **railhead** » 31 Aug 2010, 21:05

No joy there, either.

I'm just wanting to check the current draw of my effects pedals is all.

lolbou · Post by **lolbou** » 31 Aug 2010, 21:18

Is the battery of the DMM ok? If it reads voltages and other stuff right, then it may be damaged?

railhead · Post by **railhead** » 31 Aug 2010, 21:27

Yeah, it's reading everything else properly. Do you agree that I should have the red lead in the ,ACx jack and the MDD set to mA on the dial?

On my needle MM, I can set the range to 5, 50, or 500 — but I don't know which line/scale to read on the panel.

railhead · Post by **railhead** » 01 Sep 2010, 02:24

Okay, when I hook up the red lead to the A jack on the DMM, I can see that the pedal's LED comes on — so the circuit is being completed. But, no matter where I turn the dial, I get no reading other than -.02.

minnow · Post by **minnow** » 01 Sep 2010, 02:35

For current draw measurement, unplug the red probe from the "ohm" port.
Then connect the red probe to "mA". Black probe remains at "COMMON"

Now you should see the meter alive when taking current readings

railhead · Post by **railhead** » 01 Sep 2010, 02:50

minnow wrote:For current draw measurement, unplug the red probe from the "ohm" port.
Then connect the red probe to "mA". Black probe remains at "COMMON"

Now you should see the meter alive when taking current readings

When I hook up to that port, the LED doesn't come on — so I assume the circuit isn't being completed. I'm beginning to think my DMM may be messed up...?

minnow · Post by **minnow** » 01 Sep 2010, 03:13

Ok, how do you measure current draw?
This is what I do:

Hook up the pedal with the input jack connected (this will complete the circuit).
Un-snap one of the battery terminals (doesn't matter whether -ve or positive) leaving the other still connected to the battery (this will now "open" the circuit).
Use DMM, one probe on the battery (the one that is not connected to the snap) and the other probe on the snap (which is not connected to the battery).
Current draw should be displayed on DMM.
Current draw should differ when you turn on/off the effect.

Voltage reading - in parallel
Current reading - in series

If you're certain all the steps that you took are correct, the only way to be certain that your DMM is busted, is cross check with a working DMM.

Hope that helps...

railhead · Post by **railhead** » 01 Sep 2010, 03:17

Yes, that's exactly how I test it.

The only way the LED lights to show the circuit is complete is when I put the red probe int the "A" port. If the red probe is in any other port, the LED does not light-up — which to me means the circuit is not completed. With the probe in the A port, all I get is -.02.

minnow · Post by **minnow** » 01 Sep 2010, 03:30

But now you have the needle MM and the DMM both unable to measure current draw?

YuGi · Post by **YuGi** » 01 Sep 2010, 09:09

railhead wrote:The only way the LED lights to show the circuit is complete is when I put the red probe int the "A" port. If the red probe is in any other port, the LED does not light-up — which to me means the circuit is not completed.

I think the fuse in the mA port is broken.

railhead wrote:With the probe in the A port, all I get is -.02.

The sign minus is due to the inverted red-black probes, the .02 mean 20mA.. seems 0K to me for an effect, considering the rounding due to the scale in A (mA have more precision in this case).

railhead · Post by **railhead** » 01 Sep 2010, 12:54

minnow wrote:But now you have the needle MM and the DMM both unable to measure current draw?

No, the needle MM seems to be working fine.

railhead · Post by **railhead** » 01 Sep 2010, 13:02

SCORE!

The mA fuse was blown, so I'll be heading to Rat Shack to get a replacement.

YuGi wrote:
railhead wrote:The only way the LED lights to show the circuit is complete is when I put the red probe int the "A" port. If the red probe is in any other port, the LED does not light-up — which to me means the circuit is not completed.
I think the fuse in the mA port is broken.
railhead wrote:With the probe in the A port, all I get is -.02.
The sign minus is due to the inverted red-black probes, the .02 mean 20mA.. seems 0K to me for an effect, considering the rounding due to the scale in A (mA have more precision in this case).

minnow · Post by **minnow** » 01 Sep 2010, 13:09

railhead · Post by **railhead** » 01 Sep 2010, 13:10

With this bugger of an issue, I did finally learn to read the old needle gauge, though, so that's good.

railhead · Post by **railhead** » 01 Sep 2010, 22:47

I popped a new fuse into the sucker, and everything works perfectly. Man, I thought I was going insane!

YuGi · Post by **YuGi** » 02 Sep 2010, 08:16

railhead wrote:I popped a new fuse into the sucker, and everything works perfectly. Man, I thought I was going insane!