[Amps] Don't do this

[Carl]

----- Original Message ----- 
From: "Ian White GM3SEK" <gm3sek at ifwtech.co.uk>
To: <amps at contesting.com>
Sent: Sunday, December 19, 2010 2:51 AM
Subject: Re: [Amps] Don't do this


> Bill, W6WRT wrote:
>>ORIGINAL MESSAGE:
>>
>>On Sat, 18 Dec 2010 14:26:29 -0800, Vic K2VCO <vic at 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


Ian, do you have any references to those diodes NOT protecting a SB-200/220 
meter?  I certainly dont and have seen many failed 1N400x diodes that saved 
the meter. Redesigining the Heath circuits is not an option to most users.

Carl
KM1H