End whistling into your mic with this suggestion from David GW4AKZ:
When using an antenna tuner on SSB, we could easily add a tone generator and switch to do that.
I have fitted a switchable -6db pad between the output of the pcb and the antenna socket. With the circuit switched in the worst case SWR is about 1.65 to 1 with an open or short circuit. This prevents the finals from ever seeing a poor SWR.
Tune is easily achived with the key down and reduced radiated power when tuning up.
I use 2 watt carbon resistors 33 ohms in a t attenuator circuit. 2 in parallel to give 16.5 ohms which is close enough to the calculated value. A relay is s used to switch this attenuator in and out.
Chris suggests replacing the 51 ohm resistors with 2x 100 ohm 3watt resistors.
The Virtual VRS2H relays in the µBITx are not easy to find, but there are substitutes available from Panasonic.
The VRS2H datasheet can be found here.
Arv K7HKL notes that the problem with IRF510 RF amplifiers failing seems to be a recurring one for those who are not quite careful with antenna matching, bias level, and drive level.
As a way to start looking into this situation Arv performed some on-line searches to see how others were working around this problem.
There are a number of potentially useful ideas contained in those articles and discussions, but nothing that obviously applies directly to the problem of blowing IRF510 devices at only a few watts of power if the antenna is mis-matched.
Marco – KG5PRT notes from one of the stories reviewed …
… In the middle of the page:
“I destroyed many IRF510 FETs during testing. In fact I blew a small hole in one and another into several pieces. It was quite a shock when the first one was destroyed because it made a loud noise like a rifle being fired.
Once I got tired of replacing the FETs, I built a current sense circuit, which shuts off the bias once the amplifier draws more than about 3 amps from the PSU. I think this circuit is essential. You can build it into the Power Supply or into the Amplifier. I built it into the Amplifier because the power supply, which is also homemade, does not limit until 7 amps. With the current limit circuit the amplifier now survives transmitting into any SWR from an open circuit to a short.”
He notes that the current limiting circuit has only 7 parts.
And Arv K7HKL suggests implementing this bias control circuit used by AC2CZ in his 50 watt IRF510 linear. He suggests you could probably do this using 2N3906 transistors. Arv likes the fact that it includes a warning
Or this one:
Conclusions about protecting the IRF510 devices
Arv’s conclusions to date are:
- Power supply current limiting (2.5 to 3A) seems to work.
- Bias voltage shutdown on high current (3A) seems to work.
- Driver impedance to IRF510 gate should be quite “stiff”. Use a 50 ohm pad.
- Use a low resistance (4.7 ohms) in series with each IRF510 gate lead.
- Keep IRF510 leads short.
- Impedance presented to the IRF510 drain should be 12.5 ohms.
- RF output transformer should be 1:2 windings for 1:4 impedance transition. A Binocular core may work better…?
- Avoid the problem by using some other MOSFET.
- Use plastic fuses at 3A for self-healing action.
- Fuse rig power at 2 to 3A to avoid IRF510 failures.