measuring current draw with a multimeter

[rhinson]

hello there,   what is the easiest/most accurate way to measure current draw (say in a mosfet based device like jack orman's single mosfet booster) using the current function on a multimeter?  where do you put the probes?  or should you measure voltage from some point to another point and calculate the current draw?  i know it's simple but thanks very much for your help.
rh

[Craig V]

I want to know something similar, current draw from a pedal.  

There was a website that had some pedals current draw listed, maybe someone has a link.  I've been looking for it for about a week.  

Thanks.

[aron]

Set the multimeter to measure current and put it in series with the battery.

But it's more detailed here:  

http://www.diystompboxes.com/cnews/FAQ.html

[rhinson]

hello aron,   i tried this method with a couple of different boxes and got no readings each time.   either this doesn't work or i'm missing something.  is there another way to do this?    thanks.      rh

[Jason Stout]

The method described in the FAQ is correct. Do you get any reading?

This link may help.

http://mechatronics.mech.northwestern.edu/mechatronics/design_ref/tools/multimeter.html

[Nasse]

Might depend the resolution or range of your multimeter, just a quess.

But here´s my useful advice for those who measure current with cheapish multimeter, where the current measurement function is not fused or auto protected, and probe leads are put in other sockets when measuring current:

Remeber to put probe leads back to normal sockets after the current measurement. Otherwise you may zap your meter when you measure voltage next time, and probes are in wrong sockets...

You quess how I learned this... I was lucky my meter works but probes just melted, what a smell and smoke  :oops:

[Jim Jones]

You could always use the method a lot of tube amp guys use when biasing.  Put a 1 ohm resistor in between one of your battery terminals and the corresponding battery snap terminal.

Measure the voltage drop across the resistor and use Ohm's Law to solve for current.  Although it's simplified by using a 1 ohm  resistor.  If you're using a 1 ohm, then the voltage drop in volts is the same as current in mA.  

Jim

[niftydog]

Set the multimeter to measure current and put it in series with the battery.

..but make sure you put the probes into the correct sockets for measuring current!  Also, make sure the "common" socket is connected to the more negative side.

If you're using a 1 ohm, then the voltage drop in volts is the same as current in mA.

If I were you I'd revist ohms law!

Current (amps) = votage (volts) / resistance (ohms).

mA = V / kohms
or
mA = mV / ohms

[hawkeye2u]

Here's a simple  ex OHM's law

U=R x I
U=Voltage
R=Resistance
I=Current

as an example
U=8
R=4
I=2
8=4 x 2
4 =8/2
2=8/4
apply this simple eq( with the above notes from other people )

The example of a 1 ohm resistor in SERIES with the load maks a lot of sence!!!

I e a voltage drop of 2 v ACROSS the SERIES resitor ( 1 ohm) ( just a crude ex)
2= 1 x X
X= 2/1
X= 2A current draw
Now if the voltage across the resistor is say300mV or 0.3v
0.3=1 x X
X= 0.3/1
X= 0.33 or 330mA
Simple huh!!!

[smoguzbenjamin]

If your DMM is not reading any current draw and won't measure current at all, cheack if you DMM's fuse might have blown. I had this problem a while ago, it really sucked