Re: IRF510 amplifier failures


I agree, I blew a 510 by messing with the bias adjustment. My meter was
not reading the 100 ma, so I cranked up all the way full CCW. Out of the box
it worked! My uBitX works with over 5 watts out, I'm leaving it alone. If
it works, leave the power alone and skip to fix the other problems (pop,
agc etc.) Mic that came with unit works well without amplification.
Stay QRP and look for us Sunday 7178 7278.

On 1/30/2018 11:05 AM, Jerry Gaffke via Groups.Io wrote:
We have thousands of uBitx's and Bitx40's out there now, an occasional post to the forum about 
a blown IRF510 is inevitable

My opinion is that SWR is not what blows the IRF510's, especially when operating from 12v.
Biggest issue is that the heatsink is quite small, especially if operating continuous duty in some digital mode
and/or feeding the IRF510's more than 12v to get higher power.
Also, be aware that the gate bias adjustments are backwards (clockwise is minimum), and get very touchy
as you approach the transition region, so once the drain current starts going up it is not much of a turn 
to go from the optimal 100ma to a very hot IRF510 with 3A of drain current.
And be aware that the heatsinks as shipped are not insulated from the IRF510 tab (connected to the drain).

I'd guess most failures are due to overheating and messing up when adjusting for 100ma of drain current.
A temp sensor and foldback current limiting would avoid these troubles if you want a safety net.
Other failures due to screwdrivers in the works or flying leads touching the wrong spot and other such snafus.

Reverse DC is not a problem for the IRF510.  It has an intrinsic reverse diode from source to drain,
will blow the traces or coils between it and the power supply.  Or, with some foresight, the fuse
you put into that power supply lead.  Where you need reverse protection is going into the remainder
of the board, be it Bitx40 or uBitx.  A 1n4148 diode would suffice, a 1n400x would not get quite so hot,
but there is no problem with running all that stuff at 0.7 volts less.  

Having a full wave rectifier in line with the power supply to the IRF510's would reduce the voltage
there by 1.4 volts.  And have a significant hit on output power available.

In post 22597, KB1GMX reports:

My experience with IRF510s is extensive and much different.  I rarely kill one even during experimenting.  I use them for RF power though 6M.  How much 8 in a parallel 4x4 push-pull at 6M running 225W!  My favorite is two in a push pull amp (K500r/wa2eby design) with 28V running about 55W on 40m and 37W at 10M and after 6 years of wrong antenna and all sorts of usual havoc its still running with the same pair. 

So the IRF510 is capable of being worked very hard, and still surviving when driving a high SWR.

Testing an antenna system with no more than 12v into the IRF510 should be fairly bulletproof.
If you want to reduce power further, reduce the drive level to the final using R136 on the Bitx40, RV1 on the uBitx.
Again, note that these pots work backwards, fully clockwise is minimum drive.

Jerry, KE7ER

On Tue, Jan 30, 2018 at 04:54 am, John Brock wrote:

I have not received my uBitx yet. I’ve been  reading this forum for about two weeks. Looks like PA failures is a huge problem.

Are the two main causes reverse DC power and high SWR?

For the DC polarity problem how about a full wave rectifier at the DC input? Two diode voltage drop a problem with this approach?

What would be the best way to limit PA  power out to a very small value to allow a manual tuner to be used for the SWR issue?


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