Strange voltage readings...

P

Paul Burridge

Guest
Hi,

What's the most likely cause for carrying out DC voltage checks on a
piece of equipment and getting stupid readings that don't make sense?
There's no RF energy involved here and the power source is just a
couple of 12 volt batteries in series. Any ideas? I've tried different
voltmeters and they all agree, so that possibility's eliminated.
 
Paul Burridge <redscar@waitrose.notformail.com> writes:

What's the most likely cause for carrying out DC voltage checks on a
piece of equipment and getting stupid readings that don't make sense?
There's no RF energy involved here and the power source is just a
couple of 12 volt batteries in series. Any ideas? I've tried different
voltmeters and they all agree, so that possibility's eliminated.
Click to expand...
One cause is something stupid like referencing the readings to the wrong
place due to confusing a couple of traces.

Would need more details on the circuit to really provide any meaningful
suggestions. What kind of stupid readings?

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Could there be a constant /current/ circuit involved?

I once saw a circuit board with 4 AA cells in series, and half a dozen
3.5 V filament lamps which could be put in series, parallel, or
series-parallel.

The audience were invited to predict which lamps would be brightest in
various circuits.

/All/ of us, from 1st-year students to retired Physics professors, got
most of the predictions wrong.

UNTIL we spotted that one of the AA cells was actually an
AA-cell-dummy, with a constant-current 2-transistor circuit hidden
inside.

So, ALL predictions became "nonsense".

BTW you could have a circuit or component fault, which has
inadvertently created a constant-current combination.

BillJ,
Edinburgh

(Another thought: I once found a faulty Ni-Fe cell which was acting as
a negative resistance. Never found out why.)


On Thu, 17 Jul 2003 15:55:08 +0100, Paul Burridge
<redscar@waitrose.notformail.com> wrote:

Hi,sa

What's the most likely cause for carrying out DC voltage checks on a
piece of equipment and getting stupid readings that don't make sense?
Click to expand...
 
I have a computer subwoofer which (I) blew out trying to measure the output
voltage at the external DC outlet. When I opened it up I was getting 15vdc
readings all over the board. Eventually, I discovered that the two pico
fuses (two small devices which look 1/4 watt resistors) were both blown. I
think I got taken in by the sensitivity of my meter; measuring stray
voltages which had no current to back them up.

Sometimes I think I should get an old VOM....

jak

"Paul Burridge" <redscar@waitrose.notformail.com> wrote in message
news:g4ddhvs7bn0qp03v1srsdg5p8gqllpg03p@4ax.com...
Hi,

What's the most likely cause for carrying out DC voltage checks on a
piece of equipment and getting stupid readings that don't make sense?
There's no RF energy involved here and the power source is just a
couple of 12 volt batteries in series. Any ideas? I've tried different
voltmeters and they all agree, so that possibility's eliminated.
Click to expand...
 
"Paul Burridge" <redscar@waitrose.notformail.com> wrote in message
news:g4ddhvs7bn0qp03v1srsdg5p8gqllpg03p@4ax.com...
Hi,

What's the most likely cause for carrying out DC voltage checks on a
piece of equipment and getting stupid readings that don't make sense?
Click to expand...

Are you sure you're using the right reference point for "ground"?

Lots of newer equipment has the main circuitry isolated from the power
busses
thru a DC-DC converter, so it's pointless to use the outer chassis or minus
lead
as your ground reference. Find something that is almost certainly a ground
bus,
like a thick PC board trace that goes to pin 7 on a lot of IC's (assuming
there's
digital IC's in there). Or find the negative end of the biggest
electrolytic capacitor.

Sometimes a DC-DC inverter will put out lots of noise or radiated magnetic
field,
which can amount to almost a volt of noise.

I've also been fooled by clipping the ground clip onto a chassis rail that
happened to be anodized. Aluminum oxide is a very good insulator!
 
jak:
In addition to a sometimes wanted lack of sensitivity when doing voltage
readings, I also find myself reaching for the old analog VOM to do a basic
check of diodes and transistors and other semiconductors...... not only can
it be faster, but it is easier to find leakage.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -


"jakdedert" <jdedert@bellsouth.net> wrote in message
think I got taken in by the sensitivity of my meter; measuring stray
voltages which had no current to back them up.

Sometimes I think I should get an old VOM....

jak
Click to expand...
 
On 17 Jul 2003 13:06:50 -0400 Sam Goldwasser <sam@saul.cis.upenn.edu>
wrote:

Paul Burridge <redscar@waitrose.notformail.com> writes:

What's the most likely cause for carrying out DC voltage checks on a
piece of equipment and getting stupid readings that don't make sense?
There's no RF energy involved here and the power source is just a
couple of 12 volt batteries in series. Any ideas? I've tried different
voltmeters and they all agree, so that possibility's eliminated.

One cause is something stupid like referencing the readings to the wrong
place due to confusing a couple of traces.
Click to expand...
Another possibiliby is that there are extrememly high resistances in
your circuit, so that the (10MOhm?) impedance of your meter can't be
ignored.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------
 
Ricardo Matos Abreu said:
"Sofie" <sofie@olypen.com> wrote in message
news:vhggsq52b4gi02@corp.supernews.com...
jak:
That is exactly why I still do a lot of initial troubleshooting with
an old (analog) VOM....... and not the "newer" FET versions either.

snip


"jakdedert" <jdedert@bellsouth.net> wrote in message
news:v2XRa.1224which look 1/4 watt resistors) were both blown. I
think I got taken in by the sensitivity of my meter; measuring stray
voltages which had no current to back them up.

Sometimes I think I should get an old VOM....


I'm missing something here...
How can the fuses blow because a load (VOM in this case) that barely
draws current is connected?

Ricardo
Click to expand...
I don't see anywhere a mention a VOM blew a fuse.
jakdedert was reading some voltages on a circuit that had fuseable resistors
blown, but those voltages would probably be very high impedance (through the
failed resistors that are almost open). An analog VOM that needs to draw
some current to measure that voltage would have proved no voltage at all on
the same test nodes.
 
"Jeroni Paul" <JERONI.PAUL@terra.es> wrote in message
news:bfhlso$3aj$1@nsnmpen2-gest.nuria.telefonica-data.net...
Ricardo Matos Abreu said:
"Sofie" <sofie@olypen.com> wrote in message
news:vhggsq52b4gi02@corp.supernews.com...
jak:
That is exactly why I still do a lot of initial troubleshooting with
an old (analog) VOM....... and not the "newer" FET versions either.

snip


"jakdedert" <jdedert@bellsouth.net> wrote in message
news:v2XRa.1224which look 1/4 watt resistors) were both blown. I
think I got taken in by the sensitivity of my meter; measuring stray
voltages which had no current to back them up.

Sometimes I think I should get an old VOM....


I'm missing something here...
How can the fuses blow because a load (VOM in this case) that barely
draws current is connected?

Ricardo

I don't see anywhere a mention a VOM blew a fuse.
jakdedert was reading some voltages on a circuit that had fuseable
resistors
blown, but those voltages would probably be very high impedance (through
the
failed resistors that are almost open). An analog VOM that needs to draw
some current to measure that voltage would have proved no voltage at all
on
the same test nodes.
Click to expand...
That makes sense. I misinterpreted the post.
Thank you,

Ricardo
 
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