UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Circuit tracing

Argus25 · 13th Sep 2018, 12:21 am

If you find a diode in a circuit and you are wondering if it is zener:

Power the circuit, check if the diode is forward or reverse biased. If its forward biased and dropping around 0.7V , likely it is not a zener or a diode used as a zener.

If it is reverse biased (positive on the cathode(line)) then it may or may not be a zener.

Typically , in most (but not all) zener circiuts, the zener anode is grounded (for a neg earth system) So if you trace out the circuit, and the diode anode is grounded, then it is likely a zener shunting a resistive load (Although other times its possible to see diodes with grounded anodes in flywheel and snubber circuits)

If in doubt, its easy to find out. Disconnect one leg of the diode in question and reverse bias it (positive applied to the cathode) with a power supply, say 15V to 20V with a series 4.7k or similar resistor to limit the current (don't just apply voltage without a current limiting resistor). Measure the voltage across the diode with a DVM. If it is a zener, and not a 1N4148 or signal diode of some sort, it will shunt the voltage down to some lower value close to the zener voltage.

13th Sep 2018, 12:21 am	#8
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Circuit tracing - zener, or not zener? If you find a diode in a circuit and you are wondering if it is zener: Power the circuit, check if the diode is forward or reverse biased. If its forward biased and dropping around 0.7V , likely it is not a zener or a diode used as a zener. If it is reverse biased (positive on the cathode(line)) then it may or may not be a zener. Typically , in most (but not all) zener circiuts, the zener anode is grounded (for a neg earth system) So if you trace out the circuit, and the diode anode is grounded, then it is likely a zener shunting a resistive load (Although other times its possible to see diodes with grounded anodes in flywheel and snubber circuits) If in doubt, its easy to find out. Disconnect one leg of the diode in question and reverse bias it (positive applied to the cathode) with a power supply, say 15V to 20V with a series 4.7k or similar resistor to limit the current (don't just apply voltage without a current limiting resistor). Measure the voltage across the diode with a DVM. If it is a zener, and not a 1N4148 or signal diode of some sort, it will shunt the voltage down to some lower value close to the zener voltage.