Bill wa4lav wrote:
> I'd like to point out that you should not use "FAST" diodes.
>Use power diodes. If you use fast diodes you risk rectifying RF signal
and
>getting odd readings on your meters. I have actually seen this in a
friend's
>homebrew
>amplifier. Power diodes have slower reverse recovery time and will not
>rectify the RF
>voltages induced by the strong fields inside the amplifier.
Agreed, I have seen the same. Swapping the "fast" diode for a plain
vanilla rectifier cured the strange readings on Top Band.
>Also, they are self sacrificial
>devices like fuses except internally they fuse to a short when if a
large
>impulse of current passes thru them and voltage drop goes to zero.
There is absolutely no need to sacrifice the meter diode. The glitch
resistor instantaneously limits the peak current that the diode has to
handle, and a few milliseconds later the fuse or electronic trip will
limit the total energy content of the surge. You can then select the
diode that will live through the experience - and something like a 6A10
will do that.
> One other thing. If you try to arrange it so that the voltage across
the diode
>is around
>.7 volts just beyond the meter peg, the meter will not be accurate.
Don't
>forget that
>the diode draws current at lower voltages. The .7 volts is just an
>approximation used
>mostly for power supply design and testing purposes.
>
Agreed: 0.7V is a typical value when a large silicon diode is
conducting, but what we need here is a voltage BELOW the conduction
threshold where the diode is NOT conducting... almost conducting, but
not quite. If resistors are selected to give a voltage drop of 0.5V when
the meter is reading full-scale, that will be just about the optimum
balance between accuracy and protection.
The User Manual for the Triode Board contains a more detailed
explanation (link below).
73 from Ian GM3SEK
<www.ifwtech.co.uk/g3sek>
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