[Amps] switchable zener cathode bias

[Karl-Arne Markström]

The easiest way would be to use a single-pole relay or switch contact to 
parallel the lower-voltage Zener with the higher-voltage one.

When in circuit, the lower-voltage Zener will go into conduction before the 
higher-voltage, which then will become "invisible" in the circuit.

I consider this being the safest method, as it positively prevents exceeding 
the collector-base breakdown voltage of the transistor when there is no 
Zener in the circuit, which may damage it.

Another way would be to provide a third Zener across the collector and 
emitter of the transistor, with a voltage rating sufficiently low to prevent 
collector-base breakdown.

73/

Karl-Arne
SM0AOM


----- Original Message ----- 
From: "Steve Flood" <kk7uv at bresnan.net>
To: <amps at contesting.com>; <rfamplifiers at yahoogroups.com>
Sent: Tuesday, December 30, 2008 5:53 PM
Subject: [Amps] switchable zener cathode bias


> Considering the common zener bias circuit (with the 2n3055 and resistor), 
> what precautions should be taken if I wanted two different zeners (and 
> associated bias voltages) selected by a switch or relay?   Is there a 
> danger in the moment when the switch/relay throw is transferrring from one 
> pole to the other and neither zener is in the circuit?   Likewise, if I 
> used a make-before-break setup, can the two zeners be in parallel for a 
> moment without causing trouble?
>
> Thanks,
> Steve KK7UV
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps


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