identify a zener diode

[countludwig]

Hello,
I've got a circut board with what appears
to be a bad zener diode that needs replacing.

is there a way to identify what the diode is or
what a suitable replacement would be?

here is a picture that will hopefully help.

http://www.ghetti.net/rov/diode01.jpg

thanks.

-Ron

 

[buzzp]

There should be some numbers printed on the side. Can you read them? Take it out if you have too. This is only way to determine what it is. Unless you have a good board that you could power up and take some measurements on.

 

[countludwig]

well, thats the problem, it's unreadable. ( I have pulled both of them off the board)
I do have access to a known good board though, can you
outline the measurements I need to take?

thanks.
-Ron

 

[nbucska]

With scope verify that it has DC across -- measure VTG.
Measure current ( or VTG accross series R )

These are enough to find replacement. <nbucska@pcperipherals.com>

 

[buzzp]

Measure the DC voltage across it while you are measuring the current through it. It looks like a 1 watt zener or smaller so the voltage measured should be enough to select a zener. I am assuming the zener is conducting all the time. If you read a small voltage, say less than a couple of volts, then it may not be conducting or is not a zener. Post your readings and I will tell you what I think. Good luck.

 

[countludwig]

Thanks for the info.
Sadly, this is part of an ignition system
and I get no voltage readings at all with the
unit powered up.
I imagine that the only time I would get a reading
is when the ignition fires.
my guess is that I would only have milliseconds
to take a measurement.

any suggestions ?

thanks.
-Ron

 

[electricshock]

what about recording the voltage profile on digital oscilloscope before and after iginition and analysing the stored profiles!!!

 

[buzzp]

Yep, a scope will be necessary unless you use the min/max function of your meter. That may or may not work depending on the meter and the duration of ignition. A scope will almost certainly work if you have a single shot wave capture and set it up to trigger on a rising edge. If the voltage rises up and flattens out at some level, then this would be near the zener voltage if it is indeed a zener. It may be a TVSS as well but I have not seen a glass TVSS before. Good luck

 

[countludwig]

Thanks for the suggestion Buzzp,
It is most definately a zener diode according to the silk screen on the board.
I currently don't have access to a scope at the moment,
but I do have a decent Fluke 88 dvom although a bit dated, with many functions including min/max, rpm and duty cycle.
where these ignition units seem a bit fragile to me, I hesitate to probe it with the engine running.

-Ron

 

[buzzp]

The meter will be adding high impedance to the circuit, so it should not affect anything at all. I would definately probe into it with the meter. It may not be quick enough to pick it up, especially an older unit. Just review the data sheet for the meter. If not, a scope may be the only way to go. You could look and see what is hooked across the zener and determine the max voltage of this part, then pick a zener slightly smaller than this. This may be the best way. I imagine there is a relay or something similar in parallel with the zener and this is what the zener is trying to protect from over voltages due to the large inductive load (coil for igniter). Good Luck.

 

[pebe]

You have access to a known good board, so if the owner has no objection why not remove the zener from that and test it.

If you have a constant current supply feed it with 5ma. If not feed it through a 4K7 resistor from 20volts.

Pass current one way and it should behave as an ordinary diode, ie. you'll get 0.6v across the zener. Reverse the current and measure the voltage again. That will be the zener voltage.

You can get a rough estimate of its wattage rating by comparing its size with others from a catalogue.

 

[repoman]

How about this: what part of the product is damaged? the power supply to ICs? the Power supply to a logic section?

Identify that and you'll know what zener you need. It looks like a 1Watt zener, so it's probably used for a cheap regulator (is there a series resister connected to it?)

 

From the above discussions, it is clear that you have access to a working board. carefully pull out the zener from the working unit and connect a 4k7 resistor in series. connect this to a 30V supply and a meter across the zener. measure voltage across zener. and buy any part number zener matching voltage and wattage. thats it!! the ckt is as shown below


+ ----|
/
/
/
|
30 V DC |----------|
supply -- |
/ -- meter
| |
|----------|
GND ----|

 

hi!
i have a problem! please send me some texts about zener diode!
thanks a lot!
rassul_babapour@hotmail.com

 

I have removed a defective zener from a TV (15&quot. It regulates the voltage for the tuner and sound portion, The only marking on the diode is a blue ring on the wire, a black ring and then a red ring on the glass. the glass is 1/4&quot; long and 1/8&quot; dia. There was a 50 ohm voltage dropping resistor in series with the diode. Can you recommend a replacement
Email BobSchmidt@t-online.de