Recently, someone explained that diodes alone don't protect meters. The 0.6v
across the
diode is enough to seriously damage many meters.
I found out the hard way the other day when I was trying to fix something that
wasn't
broke and accidentally shorted the screen terminal of a tube to ground. The
screen meter
was destroyed, despite the back-to-back diodes.
Here is the original discussion by W6WRT and GM3SEK -- needless to say I'm
going to follow
their advice:
>> 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).
>
On 5/27/2011 6:48 PM, Mitch Cox wrote:
> BTW, those diodes need to be much larger than 1N4004's.
> ----- Original Message -----
> From: Paul Decker
> To: amps@contesting.com
> Sent: Friday, May 27, 2011 9:42 PM
> Subject: [Amps] Meter issue
>
>
>
>
>
>
> I've been trouble shooting a problem in my amp with the Ip and Ig
> metering. I've got parallel 1n4004 diodes with opposite polarity accross the
> meter terminals of each Ip and the Ig meter, nothing new, pretty standard.
>
>
>
> It's easy enough to replace the diodes, but what would cause one of the
> diodes in both meters to fail in the shorted case? There are no other
> apparent problems and the amp still works save seeing the meters deflect.
>
>
>
> Thanks
>
> Paul
>
>
>
>
>
>
>
>
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--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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