How do I identify this unmarked (zenner?)

Thread Starter

Tom Monaghan

Joined Dec 3, 2016
5
Does anyone know what this is I suspect a zenner diode but it is shorted so does not test as a diode of any kind, It is used in series with a grain of wheat lamp that has 8v on it at full brightness and the supply is 12v dc max
 

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MrChips

Joined Oct 2, 2009
20,335
If it is a zener diode then it is expected to drop about 4V so that the lamp sees 8V from a 12V supply.

You can use:
1N4730 - 3.9V 1W zener
1N4731 - 4.3V 1W zener

The 4.3V zener will make the lamp less bright.

Make sure that you connect the cathode lead (band on the diode body) to the +ve voltage supply otherwise you will likely blow the lamp.

A resistor of the appropriate value should also work if you know the current and voltage required by the lamp.

For example, if the lamp draws 20mA @ 8V then a 200Ω (180-220Ω) ½W resistor should be ok. (Mind you, 8V is uncommon for a lamp. 6V is more common.)

Did the component fail while the unit was in operation?
Did the lamp blow or did it just get brighter?
How do you know the lamp requires 8V?
What is this unit you are fixing?

Without knowing the full details about the unit I would choose to replace the zener diode with a resistor.
 
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Thread Starter

Tom Monaghan

Joined Dec 3, 2016
5
train circuit1.JPG train circuit2.jpg train circuit3.jpg
If it is a zener diode then it is expected to drop about 4V so that the lamp sees 8V from a 12V supply.

You can use:
1N4730 - 3.9V 1W zener
1N4731 - 4.3V 1W zener

The 4.3V zener will make the lamp less bright.

Make sure that you connect the cathode lead (band on the diode body) to the +ve voltage supply otherwise you will likely blow the lamp.

A resistor of the appropriate value should also work if you know the current and voltage required by the lamp.

For example, if the lamp draws 20mA @ 8V then a 200Ω (180-220Ω) ½W resistor should be ok. (Mind you, 8V is uncommon for a lamp. 6V is more common.)

Did the component fail while the unit was in operation?
Did the lamp blow or did it just get brighter?
How do you know the lamp requires 8V?
What is this unit you are fixing?

Without knowing the full details about the unit I would choose to replace the zener diode with a resistor.
Thanks for the quick reply MrChips the circuit is from a japanese train on which I can find no technical info. The circuit turns on one lamp, which are at their brightest around 8v (US trains use 1.5v lamps usualy). I have posted pictures of the working circuit, there are two on the train, one front one rear. These have my findings I'm still not sure what the blown device is after the testing.
 
Last edited:

Thread Starter

Tom Monaghan

Joined Dec 3, 2016
5
Ooops I posted last reply with a sentence half finished I meant to explain with train running in one direction one lamp comes on and both come on when voltage (hence direction) is reversed.
 
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