The AM Forum

I know this has been hashed out before, but I'm still not sure on what to do here. Last year SH4 shunt resistor blew on my Johnson Ranger frying the meter so now after repairs and locating a new meter I'm a little gun shy the last thing I want is to have another event. Is it really possible to protect the Johnson panel meter with some type of diode combination or maybe even low current fuses or is this a futile endeavor?  Reading one person says 1n4001 diodes the next says schottky diodes and the list goes on.... So what's my best option here?

Dave

I recall that the meter requires 0.1V for full scale. For example, consider the plate current position:

The shunt is 0.51 ohms. This is in parallel with the meter's internal shunt resistance... which I believe is 15 ohms. The parallel combination is 0.49 ohms

When the plate current is 125mA, that current is passing through 0.49 ohms.

125mA x 0.49 ohms = .062V

So... when the meter is reading about 2/3 of full scale (125mA of plate current) the voltage across the meter is 0.062V

To protect the meter, I suggest the following:

1. Remove the 0.02uF capacitor-to-ground on each side of the meter. [I.e. why hold the voltage on either terminal of the meter, even briefly, at 700V, by including these capacitors between each terminal and ground? Why create sparks that damage the meter switch contacts when changing the meter switch position from "plate" (plate current) to any other position? These capacitors were probably part of the original TVI-proofing done on the Ranger]

2. Place one of the 0.02 capacitors directly across the meter (between the two terminals) to slow down the rate at which the voltage across the meter can change.

3. Place a 1N4007 diode across the meter is each direction (2 diodes, in opposite directions) to bypass current around the meter when the voltage across the meter rises above a few hundred millivolts.

The above should protect the meter from brief transients... like briefly shorting one side of the meter to ground when the meter switch is in the plate current position.

Stu

 

Dave,

Besides all the good suggestion that Stu has made, you can use multiple shunt resistors in parallel.  Its unlikely that all the resistors would fail at the same time.   You would know instantly when one of the shunt resistors fail since your meter would read higher.   The advantage of a schottky diode would be that the clamping voltage is much lower than the standard silicon diode.  As long as the the normal voltage across the meter does not cause the diodes to go into conduction, you could used that lower clamping voltage solution.  Look at the conduction curve for the particular diode you have in mind.  A Schottky diode "nominal voltage drop is approximately .15 to .45V.  Low current power diodes are nominally .6 to .7V. 

Joe, GMS   

Stu,

 Looks good to me I'll give your suggestions a try. I really didn't know which way to go there seemed to be so many different opinions on meter protection some were simple to very involved. You mentioned the internal meter resistance at 15 ohms I seem to remember 20 ohms, but close enough for now. Hopefully I will be checking back with a positive report on the Ranger working again.

73, Dave

Dave

Okay

On the meter's internal resistance:

It is probably 20 ohms, as you suggest.

Note that 20 ohms in parallel with 3 ohms is 2.6 ohms.

The currents flowing through 2.6 ohm meter shunts (including the meter in parallel) will produce approximately 5x as much meter rotation per milliampere as the currents flowing through 0.5 ohm meter shunts.

Note that on the concentric meter scales, the 20mA tick mark on the medium scale is in the same place as the 100mA tick mark on the highest scale.

Likewise, a 20 ohm shunt in parallel with the meter results in a 10 ohm shunt.

A current flowing through a 10 ohm shunt (including the meter in parallel) will produce approximately 4x the meter rotation per milliampere as a current flowing through a 2.6 ohm shunt (including the meter in parallel).

The lowest scale 5mA tick mark is in the same position as the 20mA medium scale tick mark.

Stu