Crowbar

[Mike/W8BAC]

In another thread a crowbar circuit was mentioned to protect meter movements from over voltage conditions caused by breakdown of meter shunts. I'm wondering if something like this might save my meter from the same fate.

My transmitter is a Collins 20V-3. On the meter panel far right is a thermocouple ammeter. Like the great majority of these surplus transmitters my meter and thermocouple had been fried by a lightening strike or broken antenna or feed line. The thermocouple and ammeter are the last link in the RF chain except for a few caps. If the Ni chrome wire in the thermocouple is parted the transmitter output usually finds the meter next in the path. A moment later the meter is toast.

It's been a multiyear quest but I have found a replacement meter and thermocouple that matches the rig and I'm working to calibrate the mismatched pair. Thanks to Skip K7YOO for his help and generosity.

Not knowing much about a crowbar I did some web searches. I found a few hits but I'm hoping someone here might be able to explain what a crowbar circuit dose using a few diodes and if it might help to save my meter should the tiny thermocouple wire burn out or I forget to attach an antenna. Thanks in advance.

Mike

[KC9LKE]

Hi Mike:

It’s just a slang term, the conduction curve of a diode looks like a crowbar.
I’ve seen zeners used for overvoltage protection, not very resettable.
The diode ends up shorted or open after an overvoltage condition. They can save the circuit but sometimes destroy the power supply. It’s a real cheap way to go.
However I plan to protect my meters with diodes.

I know what You mean about the thermocouple current meters. I got lucky and found two for my AN/ART-13 on the cheap. Haven’t given much thought on protecting them though. Not installed one yet.

Interested on what others have to say.

Ted / KC9LKE

[w3jn]

I agree with Brian.  Putting diodes in a high power RF line is inviting disaster in terms of harmonics, TVI, etc.  Besides, if the thermocouple burns out the whole power of the 20V will go thru the diodes and most likely fry them anyway.

You can get very low current fuses, but not knowing what the FS rating of the meter movement might be, I don't know if there are any available with the proper rating.

[Mike/W8BAC]

You can get very low current fuses, but not knowing what the FS rating of the meter movement might be, I don't know if there are any available with the proper rating.

My thinking exactly John and Brian. My meter movement is a milivolt movement with no shunt or series resistance and requires only 118 milivolts full scale. I can't measure the actual meter resistance but it's very low. The fuse would have be in the microamp range I think. I have never seen one.

A few options I have been looking at would be to build an op amp (LM-301A) circuit that would help increase the DC output of the thermocouple that would do two things. Increase the output in a controllable way which I need and help protect the movement. The other option (which looks really good) would be to experiment with the circuit in this months Electric Radio.

The ER circuit/meter set uses a miliamp meter with a series resistor (I think) built in and I think those meters have higher resistance in the movement windings that would make them a bit less sensitive. I would have to make some changes in his design to work with my movement or find a different movement. The article was written by Tom/W3BYM. It uses a half wave rectifier using a 1N34 crystal diode and a pair of zeners to protect the meter.

I have another thermocouple to try which might have more output. My mismatched meter/thermocouple set works but is reading much to low. The need to protect the meter would still be with me.

[KC9LKE]

I agree with Brian.  Putting diodes in a high power RF line is inviting disaster in terms of harmonics, TVI, etc.  Besides, if the thermocouple burns out the whole power of the 20V will go thru the diodes and most likely fry them anyway.

You can get very low current fuses, but not knowing what the FS rating of the meter movement might be, I don't know if there are any available with the proper rating.

You could always put the fuse in series with the thermocouple instead.

BUSS, at one time, sold fuses with pigtails. Don’t use a fuse holder at RF. If it blows on occasion move it up a bit.


BTW My last post was referring to the previous thread about meter protection with diodes. That was DC. Certainly not for RF.

Ted