Re: #uBITX polyfuse? #ubitx

K9HZ <bill@...>

Depends upon the mode of failure i suppose.  My protection circuit posted on acts far quicker than a polyfuse could and is more certain to protect in any case.  

Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ


Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois


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Villa Grand Piton - J68HZ

Soufriere, St. Lucia W.I.

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On Apr 10, 2018, at 10:11 PM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

As a means of protecting the IRF510's, I think a polyswitch in the PA-PWR line should work great.
The IRF510's mostly fail due to overheating, a small polyswitch will have far less thermal mass
then the IRF510's plus heatsinks.

Often an LED and resistor in series are placed across the polyswitch,
such that when the polyswitch opens up the LED turns on.
That tells the operator to shut down power and let the polyswitch reset.

Would be interesting to experiment a bit, the IRF510's are cheap.
Turn up the gate bias pot till something gives.
My bets are on the polyswitch (rated for say 2 or 3A, will blow somewhat north of that).

I doubt you need a limiting resistor in this particular case. 


On Tue, Apr 10, 2018 at 07:38 pm, lostfrogsrecords wrote:
They work slowly, need a current limiting resistor to allow the polyfuse to reach cutoff temperature before the protected device fails.  Again, slow...

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