[R-390] OT: Diode Matching

[Cecil Acuff]

Hi Jim,

        I have to believe they are interested in the resistance of the 
junction with the diode forward biased.  Your meter biases the diode when in 
the diode test mode and shows the resulting resistance as a voltage drop 
across the junction.  When you switch to the resistance mode you no longer 
have enough voltage to forward bias the junction thus the 1.7 meg reading. 
It should actually be infinity on a healthy junction...you may be reading 
your hands if you are holding the diode to the test leads with them.

The intermittent reading is an anomaly of your meter....probably because of 
switching between modes with the leads connected to the diode.

I would just pick a pair of diodes that give you the exact same voltage drop 
reading in the diode test function....should be close enough.  Use clip 
leads and make sure you have a good solid connection at all points.  There 
are other more complicated ways to measure the forward biased resistance but 
I'm not sure it would result in any better selection of diodes.  A regulated 
power supply with a 1% tolerance resistor of a determined value in series 
with the diode and using the meter to either measure series current or 
voltage drop across the resistor or the diode junction are a few that come 
to mind.

Cecil...
----- Original Message ----- 
From: "Jim Temple" <jetemp at insightbb.com>
To: <R-390 at mailman.qth.net>
Sent: Wednesday, April 20, 2005 2:49 PM
Subject: [R-390] OT: Diode Matching


>I am trying to match a set of diodes for a tube tester.  They are 1n540's.
> The schematic says "matched for equal forward resistance".
>
> I have a Fluke 87 meter that has a diode test position.  I get similar
> forward voltage indications of about .510 volts.  I get similar resistance
> measurements of about 1.7meg.
>
> What confuses me is when I measure the forward voltage in the diode mode,
> then without disconnecting the leads switch to resistance mode, I get 
> either
> approx 40K resistance, or approx 380K resistance, randomly.
>
> When attempting to match for equal forward resistance, which procedure 
> seems
> correct?
>
> 1. Simply measure the resistance.
> 2. Measure the diode voltage, then switch to resistance mode and measure 
> the
> resistance, which gives me either approx 40K or 380K?
> 3. Measure the diode voltage, then switch to resistance mode, disconnect 
> and
> reconnect the leads, which gives me approx 1.7meg?
>
> Thanks for any help on this matter.
>
> Sincerely,
>
> Jim Temple
> Louisville, KY,
>
>
>
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