Confused on Diode Readings

Thread Starter

dltocci113

Joined Jan 9, 2024
10
Electronic novice here. As a project I am restoring a subwoofer amplifier board. I found one diode clearly reading "short". It's part number is 1N4742A.

Using a Klein multimeter a 2nd diode (also 1N4742A) located in the same part of the circuitry reads 0.54 with positive lead on anode and negative lead on cathode. Reversing the meter leads (positive on cathode/negative on anode) the reading is 1.8. The diode is part of a circuit board dated April 1997.

The diode label shows an "M" ( resembling Motorola's symbol) followed by a W, then 1N4742A then under the part number is 512.
I believe may be this one:
https://www.westfloridacomponents.c...+Voltage+Regulator+Zener+Diode+Motorola®.html

Question #1: Is it possible for a zener diode removed from the circuit board to have a reading of in both directions? Or is the 2nd diode with a reading of 1.8 when leads are reversed also faulty? (Attaching pic of the circuitry). I bought new 1N4742A diodes from a mgt Chazon, the readings are 0.53 with positive lead on anode and negative lead on cathode and Open (OL) when reverse. So, this has me wondering if the 2nd original diode is bad or of a different type that permits readings in both directions.

Question #2: I bought 1N4742A diodes by mfg Chazon via Amazon. They are glass coated. The original diodes are glass with metallurgy coating. Both have a max 200 centigrade temp and other specs are very similar. Would the metallurgy diode have some properties the glass coated do not?

I could not find any difference in specs. As fyi, I see 3 other glass coated diodes on the circuit board. Only the 2 diodes with metallurgy coating are of my concern. (Attaching sample old and new picture of diodes)

Thank you for your time.
 

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dl324

Joined Mar 30, 2015
16,943
Question #1: Is it possible for a zener diode removed from the circuit board to have a reading of in both directions?
Yes. If the voltage used by the meter was high enough for the zener to conduct in the reverse biased direction.

I just tested a 1N4742 and I got around 0.7V in the forward biased direction and open circuit when reverse biased.

I tested a 2.4V zener diode and I got around 1.5V when the diode was reverse biased.
Question #2: I bought 1N4742A diodes by mfg Chazon via Amazon. They are glass coated. The original diodes are glass with metallurgy coating. Both have a max 200 centigrade temp and other specs are very similar. Would the metallurgy diode have some properties the glass coated do not?
At room temperature, package should have no appreciable effect on zener voltage.
 

Thread Starter

dltocci113

Joined Jan 9, 2024
10
Thanks. So if the diode is removed from the PCB it should show a positive reading with + on anode and - on cathode, the OL when reversing the leads?
 

MisterBill2

Joined Jan 23, 2018
18,584
Consider that a zener diode IS a reverse breakdown device, and ALSO a forward conducting diode of some quality in the opposite (forward) conduction direction. Like most diodes, the transition from "just barely conducting" to full conduction is rather non-linear. Likewise the transition from blocking to reverse breakdown is not nearly instant.
What that means is that a zener diode is not so simple to evaluate. The specified voltage drop happens only over a rather narrow range of current, which has confused MANY folks over the years. So a check with a multi-meter will usually not be as useful as checking a regular power diode.
 

KeithWalker

Joined Jul 10, 2017
3,097
Thanks. So if the diode is removed from the PCB it should show a positive reading with + on anode and - on cathode, the OL when reversing the leads?
No, with the zener diode removed, there should be more than +12VDC on the circuit board cathode location, with respect to the anode.
1708657099766.png
 
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