On Sat, Jan 11, 2025 at 04:08 AM, WK4DS wrote:
I have done some informal tests into a 500 watt dummy load to see how well the power output will hold up on the IRF520 and I can get 55 watts continuous?on FT8 (on 80 mtrs) as long as I am willing to transmit without failure, but after about 5 minutes the power starts to fall off, I assume from the heat, and ends up at about 50 watts. In practice this is probably a poor signal so?I dont operate here. It is too high for the other parts too as I found out the hard way when it killed one of the filter switching diodes on me. These mosfets are almost bomb proof when biased properly and operated under 40 watts. I am glad Ashar did the hardware change to use them, I have operated FT8 in POTA?parks at SWRs of 2.3:1 and had no damage, the IRF24N would never tolerate that...
You've got a lot more experience than I have!? ?Ashhar has probably poured hundreds of hours into the reliability issue and he knows a lot lot more than I ever will.? ?All I want is a bit of "headroom" and a modest amount of power for digital operations or CW.? ?I've made WINLINK connections with uBitx's with only 4 watts in good conditions, so amazing things can often be accomplished.
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What DID surprise me was the difference between different manufacturers' versions of IRF520.? ? Perhaps stupidly, I ordered some generics from Amazon.? ?Then I noticed that one datasheet I was examining for IRF520 cited a breakdown drain-source voltage of only SIXTY volts.? ? I think it has to do with the reverse-biased "body diode"? ? I looked at it multiple times, and sure enough, that is what it cited as "minimum."? ?The same manufacturer cited an input (gate) capacitance of nearly 500 pf -- much more than the Vishay device, which is stated to have 100V drain-source breakdown and 360pf input capacitance.? ? (I havent thought hard enough to consider the miller capacitance, sorry Allison!!)? ?
? ? So I think I'm going to give that device a try next.? ? I ordered some from Mouser.? ?Have to wait for them to arrive.? ? ?In the process I did some reading and better understood the use of the "linear" portion of the device characterization for a transmitter.? ? It is very understandable why Ashhar went back to the tried-and-true IRF510 for the commercially available kit.? ?
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With all the trials and experiments and learning, this transceiver has been a huge benefit to me.? ? It might not be the best choice for an "appliance operator" -- but it has been excellent for me, and this group has taught me a TON.? ? What fun learning!
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Gordon KX4Z
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