Bill, W6WRT wrote:
>ORIGINAL MESSAGE:
>
>On Sat, 18 Dec 2010 14:26:29 -0800, Vic K2VCO <vic@rakefet.com> wrote:
>
>>
>>Some of my happier mods were the addition of a glitch resistor and
>>back-to-back 1N5408
>>diodes across the meters,
>
>REPLY:
>
>Just a word of advice - back to back diodes across the meters adds zero
>protection. Diodes don't conduct until the voltage across them reaches
>about .6 volts and by then the meters will be toast. You need to add a
>resistor in series with the meter of such value that the voltage across
>the meter + resistor equals about .6 volts when the current is just
>above full scale. Then place the diodes across the combination of meter
>+ resistor. Use heavy diodes such as 6A10 and your meters will be nearly
>bulletproof.
>
Alternatively, use a low-value resistor in series with the current path,
and configure the meter as a voltmeter using a 'multiplier' resistor in
series.
The protection diodes are then connected in parallel with the low-value
resistor. As Bill says, use large diodes like the 6A10. The aim is that
NOTHING should burn out in an overload, except a power fuse or breaker.
Aim for 0.5V across the silicon diodes when the meter is at full scale.
This is -just- below the threshold of conduction, so the meter will read
correctly all the way up to full scale; but beyond this point, even a
slight overload will make the diodes conduct. Thus 0.5V gives maximum
protection to the meter without affecting normal operation.
There is no need for elaborate calculations or precision resistors. Use
a trimpot for the meter multiplying resistor and then calibrate the
whole thing by pushing a known value of current through it from a small
external power supply. Simply adjust the trimpot until the reading on
the meter agrees with an accurate DMM.
It's much easier to do this than to describe it. There are more details
in the user manual for the Triode Board (via the link below).
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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