# Diode problem

#### ccnhumble

Joined Mar 26, 2020
8
Hi folks,

Just a stupid question ,how can I find out what spec of a diode is on a old pcb board? Thanks a lot

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#### Dodgydave

Joined Jun 22, 2012
8,866
The Red one is a Zener diode, the Blue ones could be Shotkley diodes or Ultrafast diodes.

#### ccnhumble

Joined Mar 26, 2020
8
The Red one is a Zener diode, the Blue ones could be Shotkley diodes or Ultrafast diodes.
Thanks, do you know how I can find out the spec of it? I’m trying to find a compatible one to replace it

#### AlbertHall

Joined Jun 4, 2014
9,331

#### ccnhumble

Joined Mar 26, 2020
8
There’s only 2 dots and a strip on it.

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#### Marley

Joined Apr 4, 2016
344
Do you know what their function is in the circuit?
You could take a guess and assume they are general purpose silicon diodes.
You could take one out and measure it. By measuring the forward voltage drop you can tell between standard diodes and Schottkey. Standard diodes will have a drop of about 0.7V, Schottkey about 0.2-0.4V at small currents (10 - 100mA).
The thing you can't measure (easily) is their reverse voltage breakdown rating. Unless they are zeners which are possible to measure. Best to investigate the circuit to get a rough idea on that. If standard diode you could replace with the 1N4000 range. The part number varies with breakdown voltage. Go high if not sure.

#### ccnhumble

Joined Mar 26, 2020
8
T
Do you know what their function is in the circuit?
You could take a guess and assume they are general purpose silicon diodes.
You could take one out and measure it. By measuring the forward voltage drop you can tell between standard diodes and Schottkey. Standard diodes will have a drop of about 0.7V, Schottkey about 0.2-0.4V at small currents (10 - 100mA).
The thing you can't measure (easily) is their reverse voltage breakdown rating. Unless they are zeners which are possible to measure. Best to investigate the circuit to get a rough idea on that. If standard diode you could replace with the 1N4000 range. The part number varies with breakdown voltage. Go high if not sure.
Thanks a lot, I’ll try to take it off and measure. Do you have any idea of the zener diode? It looks like burnt and it’s hard to see the number and letter on it. I can see a C and 6.

#### AlbertHall

Joined Jun 4, 2014
9,331
There’s only 2 dots and a strip on it.
Does that apply to the zener as well?

#### Dodgydave

Joined Jun 22, 2012
8,866
Chances are the Zener is a 6V one, you can take it out of circuit and look at the markings,,or test it for shorts on Diode test.

#### dl324

Joined Mar 30, 2015
10,072
Just a stupid question ,how can I find out what spec of a diode is on a old pcb board?
Get a schematic for the circuit. If one doesn't exist, trace the board and make one. Once you determine how they're connected, you can see what specs they need to have.

Diodes from the first post:

Zener diodes will have a part number printed on them. You're going to have to remove it if you can't make out the important markings. When I install them in a board, I bend the leads so the important information will be readable.

#### Marley

Joined Apr 4, 2016
344
Look at the ident on the PCB.
If ZD<number> then its a zener.
If D<number> then its a normal diode.

#### ccnhumble

Joined Mar 26, 2020
8
Look at the ident on the PCB.
If ZD<number> then its a zener.
If D<number> then its a normal diode.
Yea I found the zd but couldn’t find the value of it

#### ccnhumble

Joined Mar 26, 2020
8
Do you know what their function is in the circuit?
You could take a guess and assume they are general purpose silicon diodes.
You could take one out and measure it. By measuring the forward voltage drop you can tell between standard diodes and Schottkey. Standard diodes will have a drop of about 0.7V, Schottkey about 0.2-0.4V at small currents (10 - 100mA).
The thing you can't measure (easily) is their reverse voltage breakdown rating. Unless they are zeners which are possible to measure. Best to investigate the circuit to get a rough idea on that. If standard diode you could replace with the 1N4000 range. The part number varies with breakdown voltage. Go high if not sure.
Thanks I don’t really know the function, I’m new in electronics stuff. Just did some simple circuit back in uni. Trying to learn bit by bit

#### ccnhumble

Joined Mar 26, 2020
8
Hi
Do you know what their function is in the circuit?
You could take a guess and assume they are general purpose silicon diodes.
You could take one out and measure it. By measuring the forward voltage drop you can tell between standard diodes and Schottkey. Standard diodes will have a drop of about 0.7V, Schottkey about 0.2-0.4V at small currents (10 - 100mA).
The thing you can't measure (easily) is their reverse voltage breakdown rating. Unless they are zeners which are possible to measure. Best to investigate the circuit to get a rough idea on that. If standard diode you could replace with the 1N4000 range. The part number varies with breakdown voltage. Go high if not sure.
Thx Marley,
I am also going to replace this resistors, by reading the colour strips this is 62k 5%, am I right?
Thanks

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#### atferrari

Joined Jan 6, 2004
3,793
Make sure you have a concrete reason to replace them.

#### ccnhumble

Joined Mar 26, 2020
8
To be honest I don’t, i was just thinking if I need to take it off for testing then just replace them might be better.

#### dl324

Joined Mar 30, 2015
10,072
I am also going to replace this resistors, by reading the colour strips this is 62k 5%, am I right?
Looks like blue, red, brown, gold to me. 620 ohms, 5%.

#### MisterBill2

Joined Jan 23, 2018
5,242
Get a schematic for the circuit. If one doesn't exist, trace the board and make one. Once you determine how they're connected, you can see what specs they need to have.

Diodes from the first post:
View attachment 202448
Zener diodes will have a part number printed on them. You're going to have to remove it if you can't make out the important markings. When I install them in a board, I bend the leads so the important information will be readable.
You can read the voltage rating on the nearby large capacitors and get some clue about the reverse voltage specification of the black diodes. For the zene diode you can use a DC power supply and a medium value resistor to find it's zener voltage. BUT using salvaged diodes, especially those with very short leads, such as these, is not the best choice in most instances. The heat and mechanical stress of removing them can cause compromised performance.