Michael Tope wrote:
>
>Is a back-to-back pair of single 1N4003's sufficient to protect the plate
>current meter and
>grid current meter in the Drake L7, or do I need to series several of these
>diodes to
>avoid shunting significant meter current thru the forward biased diode? IOW,
>will a single
>diode affect meter accuracy?
>
You're asking for two different things: protection against faults, and
low errors due to current flowing through the diode in normal operation.
Both depend on the voltage drops across the individual meters.
A silicon diode will not affect meter accuracy if the voltage drop
across the meter is 0.5V or less. It will have started to conduct by
0.6V or more.
A good-quality meter may have a voltage drop of only 0.1V at full scale,
so a shunt diode won't even *start* to provide protection before the
meter is 6x overloaded. With a big current surge, the meter may be 8-10x
overloaded or more, which is not what you'd really call "protection".
Ironically, the diode does a better job of protecting a cheap, poor-
quality meters because many of those have a full-scale voltage drop of
0.4V. That means the protection starts to kick in at only 1.5x overload.
The other point is the current involved in an arc or other kind of
"near-crowbar" short from B+. If you have a surge limiting "glitch"
resistor in the B+ supply, the current may peak at 50 amps... and if you
don't have that resistor, the peak current will be considerably more. If
you want to be sure of protecting the meter, you need diodes with a
better surge capability than a 1N400x. Use a 1N540x as a minimum - it's
only pennies more.
There are ways to configure the meter resistors to increase the full-
scale voltage drop to 0.5V and thus get optimum protection - see
http://www.ifwtech.com/g3sek/boards/triode/triode-3.htm
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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