Re: 50 Watt Amp Final Transistors

Hans Summers

Hi Geoff

I don't believe there is any substantial benefit to the use of heatsink compound in either the 10W Linear or the 50W QCX Amp. 

The thermal resistance of the IRF510 silicon junction to the metal tab is already 3.5C/W. I believe the heat compound just doesn't make enough difference to be important. It isn't harmful, just not necessary. I never use it on any of mine. By all means add some, if is harmless... but I wouldn't bother to buy any heatsink compound if you don't have any. 

In a larger MOSFET device with a much lower internal thermal resistance, then yes it would be very important to improve the tab-to-heatsink thermal resistance by say, a half a C/W because that would be a huge proportion of the total. But in small systems like an IRF510 I don't think it's important.

I have no idea about tightening torque for the bolts, I have no tools for measuring such things nor mechanical engineering expertise to know what are appropriate recommendations. I don't think I'm strong enough to break the thread of the heatsink supplied. What I do sometimes, being right-handed, is tighten bolts using a screwdriver held in my left hand. That way I know what when it comes time to loosen them, I can use my right arm for the task... so I'll always have more power available for un-screwing than was used for the assembly. 

73 Hans G0UPL 

On Sun, Mar 1, 2020, 21:04 geoff M0ORE via Groups.Io <> wrote:


What are your thoughts on the use of a very thin smear of heatsink compound to assist in the heat transfer. I usually polish both surfaces prior to mounting and use a tiny amount of compound to help get better thermal contact. I confess I haven't read the assembly instructions for the 50 Watt amp but do you suggest a tightening torque for the bolts. The instructions for the 10 Watt Linear amp just says to tighten them but not too tight in case you strip the threads.

It would of course be difficult to supply a tiny amount of compound with the kit, I have a 10ml syringe that I purchased many, many years ago and still have 9ml left!


On 01/03/2020 17:14, Hans Summers wrote:
HI George 

Yeah a large number of IRF510s and other MOSFET types perished during my R&D work... some quietly, some very loudly like a gun.

They get hot fast because they are dissipating a LOT of power as heat! So say the thermal resistance from IRF510 to heatsink is 3.5C/Watt and you're dissipating 20W per device, then the temperature rise will be 70C (on top of the current heatsink temperature). Being a relatively small device, their heat capacity is relatively small and so the temperature rise is quite rapid. 

Do NOT sit on the key! Use the amp as for normal CW type of duty cycle and everything will be fine. Normal CW is something around 50% duty cycle on average. 

You want to tune up your antenna? So you want to sit on the key while you make your adjustments? No! You shouldn't be doing that at full power anyway! Tune up at much lower power. 

During all my development I operated sensibly. In an hour of back to back QSOing consisting of some average QSOs and a few rag chews, with a digital thermometer sensor squeezed into the heatsink fins, the heatsink temperature was around 40-45C. Say 20C over ambient. 

In my opinion understanding the equipment you use and its limitations, then treating it always with the respect and kindness it deserves, is key to its long life. 

My 50W amp prototype has been used for around 700 QSOs in the last 9 months. At a guess, that's around 100 hours of operation. In all that time I had one failure of a pair of IRF510s at switch on. Not due to over heat stress. Just a power glitch. I later modified the QCX firnware and added the resistor mod described here and flipped the power switch back and forth until I got bored, and couldn't make the glitch destroy anything again. 

So it's quite a robust amp as long as it is used respectfully. 

If you do burn some IRF510s then think of it as fantastic education and the cost of the new ones is the very low price of the education! 

73 Hans G0UPL 

On Sun, Mar 1, 2020, 19:45 George Korper <georgekorper@...> wrote:
The paragraph in the 50 PA on heat and is excellent. 
I was waiting for the right  Finals from Digikey, so I tried two different sets
IRF 510's  from Amazon. I can't believe how fast they heat up. What I don't understand
is why. What is wrong inside them? I am saying thank you because, it gave me a small taste of what
your experimenting is like. Buddy, you've got persistence. 

On Sun, Mar 1, 2020 at 11:37 AM George Korper <georgekorper@...> wrote:
THANK YOU ALLISON.  I tried your technique and you know what? 
You saved me from a great deal of grief! Again, I will just say, THANK YOU ALLISON!!!

On Sat, Feb 29, 2020 at 6:27 PM ajparent1/KB1GMX <kb1gmx@...> wrote:
No, none as adding lead length via sockets is a bad thing.

With popper care board damage should  not happen.

Hint if you destroy them,  disassemble, cut the leads at
the package,  (they are dead!) and extract the leads one
at a time with a sufficiently hot iron and tweezer.

Clear the holes by heating the solder and using inertia
(sudden stop while molten) should clear the hole.

Do not drill or use pins or try to poke the tip of the iron in.
That often does more damage.

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