reliably testing diodes in circuit ?

R

robb

Guest
while checking diodes **in circuit** with diode setting on DMM

i ran across a unexpected results where diode test at (.567 V) in
one direction then swap probes around and expect (0 V) but i
get unexpected (1.897 V) reading.

are there conditions where this is a valid reading that does
not indicate a circuit error or fault with diode?

thanks,
robb
 
"robb" <some@where.on.net> wrote in message
news:13jonqj250u8t9a@corp.supernews.com...
while checking diodes **in circuit** with diode setting on DMM

i ran across a unexpected results where diode test at (.567 V) in
one direction then swap probes around and expect (0 V) but i
get unexpected (1.897 V) reading.

are there conditions where this is a valid reading that does
not indicate a circuit error or fault with diode?
Click to expand...
Certainly. Your meter can't tell the electrons to *only* to through the
diode. They will go through whatever else is in the vicinity.

A diode by itself is not .567 one way and 0 the other. It is .567 (or so)
one way and infinity ("overload") the other way, indicating that there is no
path for the electrons.

In this case the electrons are finding a path through other components.
 
robb wrote:

while checking diodes **in circuit** with diode setting on DMM

i ran across a unexpected results where diode test at (.567 V) in
one direction then swap probes around and expect (0 V)
Click to expand...
Why would you expect 0V ? That would indicate a short !

0.567V is about the right forward voltage for a diode check.


but i get unexpected (1.897 V) reading.
Click to expand...
That's the non-conducting direction for the diode. The 1.897 V will come from
other components in parallel with it. It's hardly unexpected !


are there conditions where this is a valid reading that does
not indicate a circuit error or fault with diode?
Click to expand...
I think you need to practice measuring some diodes before drawing incorrect
conclusions as at present.

Graham
 
On Thu, 15 Nov 2007 10:02:56 -0500, the renowned "robb"
<some@where.on.net> wrote:

while checking diodes **in circuit** with diode setting on DMM

i ran across a unexpected results where diode test at (.567 V) in
one direction then swap probes around and expect (0 V) but i
get unexpected (1.897 V) reading.

are there conditions where this is a valid reading that does
not indicate a circuit error or fault with diode?

thanks,
robb
Click to expand...
Yes.



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
Spehro Pefhany wrote:

the renowned "robb" <some@where.on.net> wrote:

while checking diodes **in circuit** with diode setting on DMM

i ran across a unexpected results where diode test at (.567 V) in
one direction then swap probes around and expect (0 V) but i
get unexpected (1.897 V) reading.

are there conditions where this is a valid reading that does
not indicate a circuit error or fault with diode?


Yes.
Click to expand...
LOL !

That's an unusually terse response from you there Spehro.

Graham
 
"robb"
while checking diodes **in circuit** with diode setting on DMM

i ran across a unexpected results where diode test at (.567 V) in
one direction then swap probes around and expect (0 V)
Click to expand...

** You meant the meter shows over-range, not " 0.000 " - OK ?

but i get unexpected (1.897 V) reading.
Click to expand...

** As it will with a resistor of about 10 - 20 kohms

are there conditions where this is a valid reading that does
not indicate a circuit error or fault with diode?
Click to expand...

** It is generally not possible to make "reliable " tests on diodes that are
still in circuit - certainly not with a DMM set to " diode test".

I find an old fashioned analogue multimeter to be far more useful ( since
its has a test current of circa 50mA), but even it will not reliably find
leaky diodes in circuit.



....... Phil
 
"mc" <look@www.ai.uga.edu.for.address> wrote in message
news:fLZ_i.7445$A71.1260@bignews9.bellsouth.net...
"robb" <some@where.on.net> wrote in message
news:13jonqj250u8t9a@corp.supernews.com...
while checking diodes **in circuit** with diode setting on
DMM

i ran across a unexpected results where diode test at (.567
V) in
one direction then swap probes around and expect (0 V) but
i
get unexpected (1.897 V) reading.

are there conditions where this is a valid reading that does
not indicate a circuit error or fault with diode?

Certainly. Your meter can't tell the electrons to *only* to
through the
diode. They will go through whatever else is in the vicinity.

A diode by itself is not .567 one way and 0 the other. It is
..567 (or so)
one way and infinity ("overload") the other way, indicating
that there is no
path for the electrons.

In this case the electrons are finding a path through other
components.

Thanks for help and explanation.
Click to expand...

regards,
robb
 
"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in
message news:473CD8D9.3237FF4C@hotmail.com...
robb wrote:

while checking diodes **in circuit** with diode setting on
DMM

i ran across a unexpected results where diode test at (.567
V) in
one direction then swap probes around and expect (0 V)

Why would you expect 0V ? That would indicate a short !
Click to expand...
apology, i mistyped. i meant to type 0L, at least that is what
the meter displays when i use diode test on the DMM

Thanks for help.
 
"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in
message news:82ppj3lmp6cbarpelnu6k5liji9o9bafor@4ax.com...
On Thu, 15 Nov 2007 10:02:56 -0500, the renowned "robb"
some@where.on.net> wrote:
in circuit diode measures (.567 V) in one direction then
(1.897 V) the other .

are there conditions where this is a valid reading that does
not indicate a circuit error or fault with diode?

Yes.

thanks Spehro.
Click to expand...
 
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:5q490nFuaookU1@mid.individual.net...
"robb"
while checking diodes **in circuit** with diode setting on
DMM
i ran across a unexpected results where diode test at (.567
V) in
one direction then swap probes around and expect (0 V)

** You meant the meter shows over-range, not " 0.000 " -
OK ?
Click to expand...
apology, yes, 0L on DMM display not 0V .

but i get unexpected (1.897 V) reading.

** As it will with a resistor of about 10 - 20 kohms

are there conditions where this is a valid reading that does
not indicate a circuit error or fault with diode?

** It is generally not possible to make "reliable " tests on
diodes that are
still in circuit - certainly not with a DMM set to " diode
test".

I find an old fashioned analogue multimeter to be far more
useful ( since
its has a test current of circa 50mA), but even it will not
reliably find
leaky diodes in circuit.
Click to expand...
thanks for helpful explanation Phil.
 
while checking diodes **in circuit** with diode setting on DMM

i ran across a unexpected results where diode test at (.567 V) in
one direction then swap probes around and expect (0 V) but i
get unexpected (1.897 V) reading.

are there conditions where this is a valid reading that does
not indicate a circuit error or fault with diode?
Click to expand...
I ran into a situation like that when trying to figure out what a
little four-leaded device was. Turns out it's a diode ring - four
diodes connected anode-to-cathode - intended for use in a balance
mixer/modulator. For any two adjacent leads, it would read about .5
volts with the leads in one orientation and three times that voltage
in the other direction... I was actually reading the forward voltage
drop of either one or three diodes.

In your case, I expect that you're probably reading the forward
voltage drop of the diode in one direction, and reading the effect of
other components (diodes with the opposite orientation, shunt
resistors, etc.) in the other. Since you're reading in-circuit, you
cannot be certain that either reading is due solely to the diode you
think you're measuring, and the actual number displayed is going to
depend somewhat on the measurement technique used by the meter.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
 
On Nov 15, 9:02 am, "robb" <s...@where.on.net> wrote:
while checking diodes **in circuit** with diode setting on DMM

i ran across a unexpected results where diode test at (.567 V) in
one direction then swap probes around and expect (0 V) but i
get unexpected (1.897 V) reading.

are there conditions where this is a valid reading that does
not indicate a circuit error or fault with diode?

thanks,
robb
Click to expand...
Hi, Robb. When on diode check, the DMM typically puts out a current
of a couple of mA, and then measures the voltage across the probes.

As mentioned elsewhere, doing a diode check on a diode in-circuit also
measures everything else in parallel with the diode. The readings may
have little or no meaning.

One exception -- if you're measuring a diode's forward voltage drop in-
circuit, the reading has to be less than or equal to the diode drop
out of circuit. If the reading is greater, that reliably indicates a
problem with the diode. Since diodes do sometimes fail open, that
data point can be of some use.

Cheers
Chris
 
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