Lawrence Buja inquires:
> What can you tell me (an electrical novice) about the ability to
> measure typical automotive amperages with a DMM? Is it something that
> I want a special inductive pickup for? And are there amperages that I
> shouldn't try to measure with a DMM (i.e. cranking amps)?
Most DMM's will only directly measure currents in the milliamp range - often
up to 2000 mA, or even, in the case of the Fluke 70 series, may have a
seperate jack to measure up to 10 amps. But even with the latter, you can't
do much more than measure the current draw of light bulbs in a car. What
you need in this case is a current clamp that can measure DC as well as AC
current.
As a quick description of these, I'll include some info from a private email
discussion I had with Michael Sands, who is lucky enough to have a Fluke
current probe (*I* don't even have one!):
There are two basic types of clamps - current transformer type and current
sensor types. The current transformer types can only measure AC currents
(obviously), and usually have a "turn down" ratio of 1000:1 - that is,
for every ampre-turn of current through the jaws, 1 ma of current flows
out of the leads. Note that this type must be plugged into the *current*
measuring input on your DMM, not the voltage input.
The current sensing type typically uses a Hall-effect device to sense
the current density through the jaws, and typically converts each ampre-
turn of current into 1 mV of output. (Note *voltage* output.) The reason
for having this type of sensor is (typically) for the ability to measure DC
as well as AC currents. If the current you're measuring is DC, the output
will be a DC voltage; if AC current, the output will be AC voltage. This
type requires a couple of extra things that the current transformer type
doesn't - a power source and some method of zeroing the offset voltage of
the sensor.
Fluke, of course, sells various kinds and styles of these. The most
appropriate one for automotive use would be the 80i-410. This probe will
measure DC currents from 2A to 600A, enough for any measurements you'd
want to do on a car. It's accuracy isn't all that great, +/- 5% from 5A
to 400A, but again it's more than adequate for troubleshooting starting and
charging problems. Oh, and the bad news is it sells for $145.
> Actually, the list in general may be interested in the answer. At
> least it will be better than Yet Another SAAB 96 Horror Story (Of
> which I have a couple).
Ya don't have to *threaten* me, Lawrence! Sheesh! :^)
Pat Vilbrandt John Fluke Mfg. Co., Inc. Everett, Washington USA
UUCP: pwv@tc.fluke.COM or: { uunet, uw-beaver, sun, microsoft }!fluke!pwv
ARPA: fluke!pwv@uw-beaver.ARPA
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