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@bresnan.net>
To: <amps@contesting.com>; <rfamplifiers@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@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
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