<VV> Electrical question, now measuring drain current

[FrankCB]

Craig,
     What you say makes a lot of sense.  I like your method better than mine.
Thanks for the good advice.
     Frank Burkhard

-----Original Message----- 

From: craig nicol <nicolcs at aol.com> 

To: virtualvairs at corvair.org 

Sent: Sat, 2 May 2009 9:20 am 

Subject: Re: <VV> Electrical question, now measuring drain current 
To help Bob figure out his car's electrical drain, someone suggested 
removing the battery cable and putting a DMM in series to connect the 
battery post to the cable. While this seems like an obvious solution, in 
practice this is problematic. Most often this will result in a blown fuse in 
the DMM and a zero reading. Common DMMs don't autorange for current, they 
just have a fuse or two.  The high-current range, usually 10 amps, is OK for 
a gross drain but since the typical drain is say, 200 ma against a 30 ma max 
spec, it takes a much lower scale, typically 1A or 500ma. 

The slightest mistake, say opening the door, will blow the meter fuse on the 
lower setting. You can't just pull the fuse on the dome lamp circuit because 
90% on the drains (IME) are in the courtesy-clock circuit. You also have to 
consider inrush current as the wiring is connected - you know, the sparks 
that occur when the battery cable is first connected. That requires a little 
dance of having the DMM already connected to the *connected* battery cable 
and then separating it.  You see the problem? 

Here's the solution: Purchase a 10W or 25W 1-ohm wire-wound resistor and 
install medium-sized alligator clips on both leads.  Insert the resistor in 
series with the battery terminal and cable. Use your handy-dandy DMM to read 
*volts* or *millivolts* across the 1-ohm resistor. Using the 1-ohm resistor, 
measured voltage drop IS the current flowing in the circuit. 1V = 1A, 1mv = 
1ma. With this setup, you can accidentally turn something ON, change scales 
at will, and manage inrush without frying your DMM. 



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