hermans wrote:
>It might be a good praktis to use both diodes in serie , zener and normal Si
>diodes, temperature drift is opposit.
>
>Jos on4kj
>
>In a message dated 11/6/2006 1:11:42 P.M. Eastern Standard Time,
>W2RU@frontiernet.net writes:
>
>
>I need to replace a zener bias diode. I have a ton of 1N5408's. Any
>problems with using a string of 9 X .07v [sic] =6.3v diodes connecting
>the cathodes to the center tap replacing a 6.3v 50W zener ?
>
>
Jos --
Somebody "cut and pasted" wrong; I did not ask the question below. I
was the one who pointed out the diodes would have a negative temperature
coefficient.
In the "old days", we used to use one or more diodes to temperature
compensate a Zener, as you suggest below. However, the temperature
coefficient of a Zener depended on the Zener voltage, so it doesn't work
equally well for all Zener voltages unless you check on the spec for the
Zener voltage you want. Around 6 volts, Zeners have no net predictable
temperature coefficient, so no forward-biased diodes are necessary.
-2 mV / degree C for 9 diodes in a string = -18 mV / degree C. In
general, if there is enough loop gain, the potential for thermal runaway
exists. (Idling current through diode string heats junctions, and net
biasing voltage across the string decreases. This allows idling current
to increase, which raises junction temperature further. Etc.)
Bud, W2RU
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