[Amps] LK-500 Metering

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Thu, 3 Sep 2009 17:51:05 +0300, "alexeban" <alexeban at gmail.com> wrote:

>...perhaps it might be worth considering the addition of a pair of diodes
>connected in antiparallel across meter.

REPLY:

If you do this, be sure to measure the meter's resistance first. Many current
meters have a very low resistance which means the current would have to rise to
an excessive amount before the diodes would begin to conduct at abut .5 volts.

If your meter is a low resistance type, you can place a low value resistor in
series with it and then connect the diodes across the meter + resistor. 

For example, a typical Simpson 1 amp meter will have a resistance of about .05
ohms. Without the series resistor, the current would have to rise to about 10
amps before the diodes conduct, almost certainly damaging the meter. By placing
a .47 ohm resistor in series, the diodes would begin to conduct at just over 1
amp, exactly what you want. The resistor need be only 1/2 watt or larger. 

Of course you must analyze the circuit first to be sure the added resistance
will not affect circuit operation, but in nearly all cases, it won't. Be sure to
bypass the meter + resistor for RF, typically with a .01 uF disc ceramic.

73, Bill W6WRT