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From: exujet@exu.ericsson.se (Jerry Ethridge)
Subject: Re: Multimeter modification...
Message-ID: <1994Oct19.000604.11793@exu.ericsson.se>
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Reply-To: exujet@exu.ericsson.se
Organization: Ericsson North America Inc.
References: <Cxvs0C.A1y@nyongwa.montreal.qc.ca>
Date: Wed, 19 Oct 1994 00:06:04 GMT
X-Disclaimer: This article was posted by a user at Ericsson.
              Any opinions expressed are strictly those of the
              user and not necessarily those of Ericsson.
Lines: 56

In article A1y@nyongwa.montreal.qc.ca, matrix@nyongwa.montreal.qc.ca (Albert Yale) writes:
>Albert Yale (matrix@nyongwa.montreal.qc.ca) wrote:
>:  
>: I have a multimeter that i keep on blowing the fuse, so i'd like to replace
>: the fuse by a resetable fuse. How can i do that? Do i simply need a special
>: piece of hardware or a whole circuit? (email prefered) tnx...
>
>btw, i blew it 2 times, once testing the amps of a 9v battery and once
>testing a 300mA 9v power-supply. But the fuse says 400mA 125v! I don't see 
>why it blew anyway. :( (both times i was on the maximum amps scale i.e.
>0.25mA... maybe that's why, but the fuse can whistand 125v at 400mA! gr...)
>
>btw, anyone can give me the amps for a regular 9v battery? tnx...
>
>'Matrix'


If you want the short circuit current of the 9v battery, then you were
doing your test correctly, however a 9v battery will supply several amps
of current and the NiCd 9v batteries (really 7.2 volts) will deliver in
excess of 10 amps.

A 300mA 9v power supply will deliver approx .3A x 9V = 27 Watts
If you were testing the power supply by connecting a multimeter on
the current setting directly to the output of the power supply, then
the power supply cannot reach 9 volts and will supply more current to
a limit of 27 watts (within reason). That means if the voltage were only 
allowed to reach .1 volts 
( similar to being shorted by a multimeter on the current setting )
then it could deliver up to 27 Watts / .1 V = 270 amps depending on the 
power supply. Now I'm sure your power supply cannot supply 270 Amps but
it will deliver more than 400mA and pop your fuse.

Point is, this is not how you test your power supply, and it is not how you
test a battery. A much better way is to monitor the supply voltage
while slowly increasing the load with a potentiometer ( that can handle the
power of course) until the voltage dips to an acceptable level. Then measure
the current draw at that point. 

Example:

Load the 9v battery until voltage dips to 8 volts and measure the current at
that point. For the power supply, load it down until the regulator cannot
maintain 9 volts and the current draw at this point is the limit for the 
regulator to maintain proper regulation.


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Jerry Ethridge		     Richardson, Tx.                 |
ERICSSON NETWORK SYSTEMS                                    /O\
exujet@exu.ericsson.se                            \_______[|(.)|]_______/
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