multimeter loading circuit?

Mikelo · 29th January 2005, 11:42 PM

Following the manual I tested voltage between two test
points with my multimeter in a CD transport circuit (CDM 4/19)
and smoked a resistor that is used as a fuse in the power supply.
I double-checked with folks familiar with the circuit and it seems I
didn't do anything wrong in where I placed the probes etc.

My hypothesis is this. My multimeter (Sperry DM-4100A)
has a relatively low impedance (1M) and although this has
not been an issue in the tube equipment and speakers I
usually work on, it was too low for this CD player circuit and
so I loaded the circuit too much with this meter.

Is this a plausible explanation?

BlackDog · 29th January 2005, 11:52 PM

This may sound stupid, but are you sure you didn't have your meter switched to measure current by accident? - This equates to a dead short.

One of the meters I have measures current (upto 200mA) using the same probe inputs as voltage & resistance. In the past I have picked the meter up and forgotten to check the settings....if you're lucky it just blows the fuse in the meter.

Steve

Mikelo · 30th January 2005, 12:35 AM

To measure current you have to change the probes on
my meter and I hardly ever measure current directly.
It was definitely set for VDC in the lowest range.
(I was looking for 50mV.)

AndrewT · 30th January 2005, 05:29 PM

Hi,
your multimeter is high impedance cf. the moving coil type that were struggling to get above 20kohms/v.
More likely you shorted between 2 tracks.
regards Andrew T.

Mikelo · 30th January 2005, 05:57 PM

You must be right, but I am still
struggling to find out how I did it.
I was careful and it was easy to avoid
other contacts. The only thing I did
that was different was hold the probes
there longer than usual -- maybe 5 or 6
seconds.

Thanks for your patience!

29th January 2005, 11:42 PM	#1
Mikelo diyAudio Member Join Date: Feb 2004 Location: Colorado	multimeter loading circuit? Following the manual I tested voltage between two test points with my multimeter in a CD transport circuit (CDM 4/19) and smoked a resistor that is used as a fuse in the power supply. I double-checked with folks familiar with the circuit and it seems I didn't do anything wrong in where I placed the probes etc. My hypothesis is this. My multimeter (Sperry DM-4100A) has a relatively low impedance (1M) and although this has not been an issue in the tube equipment and speakers I usually work on, it was too low for this CD player circuit and so I loaded the circuit too much with this meter. Is this a plausible explanation?

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29th January 2005, 11:52 PM	#2
BlackDog diyAudio Member Join Date: Feb 2004 Location: Wellington, New Zealand	This may sound stupid, but are you sure you didn't have your meter switched to measure current by accident? - This equates to a dead short. One of the meters I have measures current (upto 200mA) using the same probe inputs as voltage & resistance. In the past I have picked the meter up and forgotten to check the settings....if you're lucky it just blows the fuse in the meter. Steve

30th January 2005, 12:35 AM	#3
Mikelo diyAudio Member Join Date: Feb 2004 Location: Colorado	To measure current you have to change the probes on my meter and I hardly ever measure current directly. It was definitely set for VDC in the lowest range. (I was looking for 50mV.)

30th January 2005, 05:29 PM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, your multimeter is high impedance cf. the moving coil type that were struggling to get above 20kohms/v. More likely you shorted between 2 tracks. regards Andrew T.

30th January 2005, 05:57 PM	#5
Mikelo diyAudio Member Join Date: Feb 2004 Location: Colorado	You must be right, but I am still struggling to find out how I did it. I was careful and it was easy to avoid other contacts. The only thing I did that was different was hold the probes there longer than usual -- maybe 5 or 6 seconds. Thanks for your patience!

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