Pages: [1]   Go Down

Author Topic: Schematics for SWR protection circuitry that read REF as a % of FWD??  (Read 5045 times)

2E0ILY

  • Member
  • Posts: 237

I am looking for schematics for a standalone SWR protection circuit. Most seem to just compare REV power to a fixed input voltage to a comparator. I want a circuit that works at all power levels, so it has to compare the relative levels of FWD to REV voltages. As by illustration say I am running 1 Watt QRP. A high reflected power may be a tiny voltage out of the bridge compared to a safe REV level when running 1kW. I am sure someone has addressed this before and built SWR protection that looks at the relativity between FWD and REF voltage levels, but I can't find much, and the one I have found is hand written and illegible, unfortunately. Please bear in mind I am a beginner so I need real schematics and not general ideas to create my own Wink It may be easy peasy to you, but I need to build to a known working schematic at my level, although I do try and understand what's occurring! I have the bridge built and working.

Thanks for reading.
Logged
Best regards, Chris Wilson.

G4AON

  • Member
  • Posts: 2178
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #1 on: March 11, 2017, 12:38:07 PM »

Chris, you normally protect a PA based on several factors, forward power, reflected power and temperature. The PA generally doesn't care what the SWR is, provided it's not at high forward power.

Not all amps need SWR protection, the 10W PA by QRPproject! works well, I use them in a couple of projects without any protection. http://www.qrpproject.de/UK/qrppa2008.html

There is a lot of useful info, including diagrams and a board for QRO amp protection at:
http://www.w6pql.com/amplifier_control_board.htm

73 Dave

Logged

W9IQ

  • Member
  • Posts: 8866
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #2 on: March 11, 2017, 01:03:08 PM »

Chris,

You mention a range of 1 watt to 1 kw. This is a design constraint that makes the circuit more complicated. Is this really what you need in a single circuit?

Some of the higher end SWR meters have an SWR alarm in them. Is this what you are effectively trying to accomplish? Perhaps a little bit more information about your intended application would help.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

2E0ILY

  • Member
  • Posts: 237
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #3 on: March 11, 2017, 01:11:49 PM »

I want a poor SWR to shut down the amp, which is a 1kW push pull Class D on 136kHz, It can do that by changing the state pf a pin on the FET driver IC easily enough, via a flip flop, so it stays shut down pending investigation. The mini SWR above looks like it could be ideal, I could even have a min rotary switch to choose how MUCH reflected power would shut it down. I tend to build things in separate boxes with a view to using them on different projects and would probably do the same with an SWR shut down circuit. The bridge is in a separate box and I use it several different amps. Being FET I probably won't bother with over current, by the time a drama with FETs has occurred that has caused a current rise they are usually dead. A fuse will do ;) Over temp is a good call. Thanks.
Logged
Best regards, Chris Wilson.

2E0ILY

  • Member
  • Posts: 237
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #4 on: March 11, 2017, 01:49:32 PM »

 Ooops, missed out the link, I think I can use the circuit of this device which someone has alerted me to, with a few mods:

http://www.n5dux.com/ham/files/pdf/Pocket-Size%20VHF%20SWR%20Meter.pdf

I could even use a rotary switch to select how much percentage of FWD power seen as reflected causes a trip I believe?

Thanks
Logged
Best regards, Chris Wilson.

W9IQ

  • Member
  • Posts: 8866
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #5 on: March 11, 2017, 03:09:50 PM »

Chris,

That schematic seems to have what you are looking for. You will need different sensors than the ones shown which will also necessitate changing some of the resistive values on the front end. A rotary switch connected to the LED array will do the job to trip your amp's protective lock-out.

Have fun!

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

2E0ILY

  • Member
  • Posts: 237
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #6 on: March 13, 2017, 03:38:44 AM »

Thank you Glenn, I will build it and have a fiddle about with it on a breadboard and see how I go.

Thanks to Dave, too, of course, all the best.
Logged
Best regards, Chris Wilson.

N3QE

  • Member
  • Posts: 5664
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #7 on: March 13, 2017, 07:48:09 AM »

In decades past this could be done with analog computing. e.g. http://www.qsl.net/kl7jef/Directional%20Wattmeter.pdf

In more modern times the computation would be done with a microcontroller: http://www.arrl.org/files/file/Product%20Notes/2012%20Handbook/KAUNE.pdf

You don't need ratiometric computation of either analog or digital kind, to protect a final stage.

A more realistic way to protect the final is to add overcurrent and overvolt protection right at the active device. Shorts or low impedance loads will trigger the overcurrent protection, and open circuits or or high impedance loads will trigger the overvolt protection. And thermal sensing on top of that too, especially at the 100W or up level.
« Last Edit: March 13, 2017, 07:50:23 AM by N3QE »
Logged

2E0ILY

  • Member
  • Posts: 237
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #8 on: March 13, 2017, 08:11:11 AM »

Is over current protection actually viable on a Class D FET PA amp at high power? My experience so far is a trauma with drive or output kills a FET before over current can activate, they seem such precocious devices ;)  I had a fairly crude over current protection via a Hall effect sensor on an earlier iteration of this amp and it never seemed fast enough to save a FET, and then a simple fuse would stop the big amperage PSU from going into its own over current shut down. Temp control is something i have covered, that is always a good idea with semiconductors in PA's, for sure. Thanks for your reply.
Logged
Best regards, Chris Wilson.

N3QE

  • Member
  • Posts: 5664
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #9 on: March 13, 2017, 08:34:10 AM »

Is over current protection actually viable on a Class D FET PA amp at high power? My experience so far is a trauma with drive or output kills a FET before over current can activate, they seem such precocious devices ;)

I used to experiment with IRF510 and other (slightly uprated frequency, lower capacitance but still power MOSFET) TO-220 class Class D/Class E amps too.

I'm pretty sure what kills these is high voltage ringing on the drain. The device inevitably fails shorted causing high current, but the actual first step in the failure chain was high drain voltage spikes.

Of course the high gate-drain capacitance means that it is tough to drive the gate too, so I think to some extent it would've been possible that stiffer gate drive would've helped at least a little.

If I had kept on with that project, I think I could've gone through buckets of transistors and still not come up with a bulletproof amplifier that pushes the devices anywhere near the limits.
« Last Edit: March 13, 2017, 08:37:05 AM by N3QE »
Logged

2E0ILY

  • Member
  • Posts: 237
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #10 on: March 13, 2017, 09:04:03 AM »

This is exactly what killed my FET's before discovering this. It was a driver problem that sent bizarre gate signals as a WSPR session stopped, and every now and again both FET's either turned on at the same time, or random noise sent them "berserk"! Once sorted it runs 1kW or hours on end, stopped and started via WSPR sessions or OPERA sessions with total reliability. they are delicate little flowers, great until something like you say occurs. tubes are much tougher, but 4 kV floating about is worrying ;)
Logged
Best regards, Chris Wilson.

DL8OV

  • Member
  • Posts: 1061
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #11 on: March 18, 2017, 01:46:24 AM »

Sorry that I am late to the party but here is the idea that I implemented for my automatic SWR meter.

1) The unit is built as a 100W SWR meter and in the signal path for forward and reverse are two op amps operating as unity gain buffers.

2) If the input power decreases then the gain of BOTH op amps is increased till the forward power reads 100%, similar in function to the 'calibrate' control on a manual SWR bridge. You can also include a peak hold circuit at this point.

3) SWR can then be read as either a percentage of full power or as a ratio as the return power meter scale is calibrated for both. To read the forward power the fwd meter is connected to the input of the buffer, before the signal is amplified to 100% of full scale.

Peter DL8OV
Logged

W9IQ

  • Member
  • Posts: 8866
RE: Schematics for SWR protection circuitry that read REF as a % of FWD??
« Reply #12 on: March 18, 2017, 02:35:22 AM »

Peter,

Quote
If the input power decreases then the gain of BOTH op amps is increased

How is the gain of the op amps increased as the power decreases?

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.
Pages: [1]   Go Up