[Amps] Need Zener for 2KD-3

[Manfred Mornhinweg]

Bill,

> I need a zener for my 2KD-3.  It's a 1N4561.  It appears that they are 
> really hard to find and exorbitantly expensive when you do.

Power zeners are very much out of fashion. They have a tendency to burning out 
after some time, even when used perfectly within rated specs, and it seems that 
this made them impopular in new equipment.

 >  Has anyone found a source for a replacement at a reasonable cost?

My way to replace power zeners is to use a common low power zener together with 
a power transistor and a small resistor. The zener goes collector to base, the 
resistor base to emitter. The transistor is selected to comfortably handle the 
voltage, current and power, the zener is a low power unit (typically either 1 
watt or 0.4 watt) with a voltage 0.6V lower than the original power zener, and 
the resistor's value is chosen to bias the small zener to about 10% of its 
maximum current, with 0.6V across the resistor.

It's your choice whether to use an NPN or PNP transistor. If one side of the 
power zener goes to ground, it's often very convenient to select the transistor 
polarity so that its collector will be at ground. That way you can bolt it to 
the chassis for heatsinking, without needing any insulation. Of course you must 
connect the zener in the proper way, according to transistor polarity.

This solution has a more stable voltage than a series string of diodes, uses 
fewer components, and doesn't need any extra support, because the two small 
parts are simply soldered to the transistor, which is bolted to the chassis or a 
heat sink. It's also suited to replace higher voltage zeners, which is 
impractical using diode strings.

Manfred

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