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QRT or Silent Key!?? ha.. ha

Another reason for fast blowing fuses, and not Poly Fuses for reverse protection.

And you still may have to change the Mosfet.

For over current situation (not blowing the fuse with a diode for reverse polarity protection) a fast fuse still may not work?

A fast comparitor (over current detection) and high side P Fet as suggested by Bill is probably the best answer?

Thermal protection may be given over to the Raduino to do, if there are enough ports left??

Or, besides thermistors, there are a lot of temperature sensors out there, Microchip makes some cheap ones.? Thirty cents or so.

73's

Mike, WA6ISP

Nice Bill,

?I fear that Thermal shut down would be rather slow and doesn't hel\p. By that time the mosfet etc might gone QRT.

?Current sense and fasrt action only could save from damages like bad SWR etc.

Thanks

and rtegards

?sarma

?vu3zmv

Regards
MVS Sarma
?

On Mon, Jan 29, 2018 at 8:19 PM, K9HZ <bill@...> wrote:

After I sent this note, I figured someone would say “is there a board available” so even though I’m on vacation at our villa in the Caribbean I sat down with a piece of paper and started working on something.? So far I have designed a power FET switch to control voltage to the PA and a toggle trip mechanism.? I thought about the what conditions were desirable to trip the switch off and came up with 1. Over current, and 2. Over temperature.? I need to finish the designs to interface those parts and then bread board it.? I’ll design it so that you can implement either or both by just deleting or adding the interface parts, plus leave the trip line available so that you can design you own interface circuits (like high SWR trip too).

?

Sheesh I better retire soon… this is getting to be a full time job.

?

?

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

?

Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois

?

Owner – Operator

Villa Grand Piton – J68HZ

Soufriere, St. Lucia W.I.

Rent it:

Like us on Facebook!

?

?

email:? bill@...

?

From: [email protected] [mailto:[email protected]] On Behalf Of Leland Lannoye
Sent: Monday, January 29, 2018 10:12 AM
To: [email protected]
Subject: [SPAM] Re: [SPAM] Re: [SPAM] Re: [BITX20] IRF510 amplifier failures

?

Is there a circuit or, better yet, a board available? It sounds like this could be current limiting as well.

?

?

?

Sent from my Verizon, Samsung Galaxy smartphone

?

-------- Original message --------

From: K9HZ <bill@...>

Date: 1/29/18 08:58 (GMT-05:00)

To: [email protected]

Subject: Re: [SPAM] Re: [SPAM] Re: [BITX20] IRF510 amplifier failures

?

Now with this said… you could measure the current draw to the FET using a small Hall-effect device and use that to make a simple resettable trip circuit.? Very easy to do.

?

?

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

?

Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois

?

Owner – Operator

Villa Grand Piton – J68HZ

Soufriere, St. Lucia W.I.

Rent it:

Like us on Facebook!

?

?

email:? bill@...

?

From: [email protected] [mailto:[email protected]] On Behalf Of K9HZ
Sent: Monday, January 29, 2018 9:42 AM
To: [email protected]
Subject: [SPAM] Re: [SPAM] Re: [BITX20] IRF510 amplifier failures

?

I think you are missing the point.? Tim is somewhat correct in that limiting the current to the FETs at any point along the characteristic operational curve in any form causes non-linearity and distortion because of it.? It’s more akin to flat-topping although maybe not as sharp.? In any case, it’s not what you want to do.

?

?

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

?

Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois

?

Owner – Operator

Villa Grand Piton – J68HZ

Soufriere, St. Lucia W.I.

Rent it:

Like us on Facebook!

?

?

email:? bill@...

?

From: [email protected] [mailto:[email protected]] On Behalf Of Arv Evans
Sent: Monday, January 29, 2018 1:44 AM
To: [email protected]
Subject: [SPAM] Re: [BITX20] IRF510 amplifier failures

?

Tim AB0WR

No modulation involved.? The operation is as a switch between ON and almost
OFF.? Gain of the two BJTs makes it operate quickly and drop the output voltage

to the point where the current sensing resistor has less than the 0.65 threshold
voltage across its base-emitter junction.

I see where it does look like some of the CB-era AM modulators because it

causes a drop in voltage when current exceeds the base-emitter threshold.? ?

?

I'm still looking at the current limiter with an eye toward possibly adding another

MOSFET to get both polarity protection and current limiting.? Problem with fuses

for protection is that most do not operate fast enough.? There are specialty
Fast-Acting fuses? and Slow-Blow fuses but most hams do not have a ready stock

of these.

?

Arv
_._

?

On Sun, Jan 28, 2018 at 9:59 PM, Tim Gorman <tgorman2@...> wrote:

It would seem to me that this type of operation is actually AM
modulation of final. When you limit the current flow in an FET that is
very similar to plate modulation in a vacuum tube. It certainly can't
be good for the linearity of the amplifier since the operational curve
would no longer be linear but, instead, flat-topped at some value.

Why not just fuse the power lead? It would tell you immediately that
you have a problem.

tim ab0wr

On Sat, 27 Jan 2018 12:13:49 -0800
"Arv Evans" <arvid.evans@...> wrote:

> Hello
>
> Still thinking about IRF510 failures and ways to avoid making
> smoke.....
>
> It seems that there may be two ways to limit current in BITX RF PA
> stages that use the IRF510 device.? First would be to incorporate a
> current limiter in the positive voltage supply to the RF PA stage.
> Second might be to use current sensing in the RF PA supply line to
> cause a voltage drop in the MOSFET bias line(s).?
>
> In the first instance it should be possible to use another IRF510 or
> preferably a higher power, MOSFET as a control that limits voltage to
> the RF PA section if the current exceeds a preset value (usually this
> would be 2.0 to 3.0 amperes).?
>
> This circuit has not been built or tested by me (too cold in Idaho
> hamshack in winter). The MOSFET could be an IRF840, or similar.
> Option-B? potentially solves another problem, that of low level noise
> from powered RF PA stage during receive mode.?
>
> In the second instance we could get by with smaller control devices
> if we used current sensing in the RF PA power line to turn off bias
> to the IRF510 device(s).?
>
> This bias control circuit used by AC2CZ in his 50 watt IRF510 linear
> ( ).? You could probably
> do this using 2N3906 transistors.? I like the fact that it includes a
> warning light/LED.
>
> Arv? K7HKL
> _._

?

-- 
Mike Hagen, WA6ISP
10917 Bryant Street
Yucaipa, Ca. 92399
(909) 918-0058
PayPal ID  "MotDog@..."
Mike@...

I think Sarma is right. The IRFxxx MOSFETS are
really on/off switches. They have a small linear region which
can be used for RF, but they are really designed to be "all on" or "all off".

That means that once they start conducting, they are very fast.
Whatever happens takes microseconds to accomplish. What
device is fast enough to stop that? That is the real question.
And even several amperes over a micro-sized circuit for even a few
microseconds represents a lot of joules of energy...which is why it
is relatively easy to blow up a IRF510. It goes along with the design...

john
AD5YE

Yes, an FET built for RF will be somewhat better behaved.
More linear, a wider Vgs active region, less gate capacitance, a tab at ground potential.

But I think Farhan made the right call to use the IRF510 for a $109 ($59 if the Bitx40) radio.
It's popular on amateur transmitters for a reason.? It's very cheap and it can work quite well.
Reread KB1GMX's post:? ? /g/BITX20/message/22597
and check the web for complaints about the WA2EBY amp:
? ??

You are absolutely right, Jerry.

It was not my intent to diss the IRF MOSFETS.
What I am saying is that they are different beasts
and one has to understand that to make them work well.

And it can be done easily enough. Start with the NA5N critique
and the WA2EBY amplifier design article. Follow through
with the notes and comments of KB1GMX. Then one should
have a fair appreciation for what these devices can and cannot
do...and what to avoid.

There is an excellent design of a 300-600W amplifier using the IRF510
in multiples that was built some years ago by the German Radio Club
(among others). These are fantastic designs and worth much study.
Search for them on the web.

john
AD5YE