# How to measure mA draw?

Discussion in 'Electronic Basics' started by AB, Feb 22, 2007.

1. ### ABGuest

Hi all,
Have a newbie question for you. I have this burglar alarm that has
some very specific current draw requirements. It works on 12VDC. One
set of warning horns cannot exceed 120mA current draw from a
particular set of output terminals. I have a collection of small
piezo type horns, but no specs for them, so I'm not sure how much
current they draw at 12VDC. I have a small multimeter. Can anyone give
me a primer on how to go about measuring current draw in mA for a
particular device? Thanks.
AB

2. ### Homer J SimpsonGuest

Connect to a 12 V source and measure the current. BTW, piezo devices don't
draw much.

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3. ### Jonathan KirwanGuest

The usual connection for current measurement (and 120mA should be fine
with your multimeter, as most of them seem to support that much) is to
connect your meter in series with the device.

Without the meter, you have two connections like this:

(+) ------ (+)piezo horn(-) ------ (-)

I've labeled the polarity of the horn above and the remaining (+) is
the plus side of your 12V battery and the (-) is the other side.

To test the current draw, insert your meter this way:

(+) ------ (+)meter(-) ------ (+)piezo horn(-) ------ (-)

or else this way:

(+) ------ (+)piezo horn(-) ------ (+)meter(-) ------ (-)

And make sure you connect up the leads to the A or ampere connections
and set the meter to measure current with the range appropriate for
120mA, at least. (There may be smaller ranges and you don't want to
injure the unit, so make sure the range you use supports at least
120mA or more.) Also, verify the meter's polarity as well as the
horn's.

Jon

4. ### chuckGuest

The multimeter needs to be in series with the siren.

One lead of the siren to the positive 12 volt source. The other lead of
the siren to one lead of the multimeter. The other lead of the
multimeter to the negative 12 volt source.

Make sure the multimeter is set up for measurement of current. If it is
digital, it will not matter which multimeter lead (black or red) goes to
the negative 12 volt source. With an analog meter, it does matter and it

When measuring unkown values, it is good practice to begin your
measurement at the highest range on the meter, which in this case may be
20 amperes. A piezoelectric siren will probably not show any indication
when the meter is on the 20 ampere scale. You can move to more sensitive
current ranges until you get a decent reading. The idea is to avoid
damaging the meter by passing too much current through it on any
particular range.

Chuck

5. ### Anthony FremontGuest

And never ever ever connect it to a voltage source when in current measuring
mode, the meter is basically a short circuit. Just replaced a fuse in my
meter yesterday. :-(

6. ### Jonathan KirwanGuest

Well.... that's right! Though the OP may not know what you mean.

To the OP, Anthony means don't connect your meter, when switched into
current measuring mode, directly across the 12V terminals of the
battery. Make sure you have your horn or other current limiting load
inserted in series with it, as well. That's because without the load
added in series, there is very little to otherwise limit the current
flowing and the result is that your meter looks about like a wire to
the battery and a LOT of current starts to flow, with the result that
your meter will either be damaged or else will pop a protective fuse.

If you've ever made the mistake of touching a jumper cable across the
two terminals of your car battery, you will be able to understand the

Jon

7. ### Homer J SimpsonGuest

Or ohms modes.

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8. ### Anthony FremontGuest

For some reason that doesn't seem to bother my old Micronta. I'm not sure
how they do that though. My first meters sure didn't like you doing that.
:-O Usually the application of 120VAC in Ohms mode is what did most of them
in.

9. ### ChrisGuest

your burglar alarm drive a small relay. If you put a back-biased
diode across the relay coil, you should be able to turn on a relay
that can switch several amps with no problem.

Look for a 5V relay with a coil whose resistance is greater than 42
ohms. Here's one from Radio Shack that will do the job:
5VDC/1A SPDT Micro Relay \$4.49 USD Model/Catalog #: : 275-240

This relay can switch up to 1 amp.

Cheers
Chris

10. ### ABGuest

Thanks to all for the great info. I feel confident going forward now.