[ARC5] "Quiet" ARC-5 Keying

[Dennis Monticelli]

As long as one keeps in mind that the vast majority of HV transistors
(MOSFET or the new Bipolar one in the link) make great switches and
not-so-great linear control devices you will be fine.  The SOA (Safe
Operating Area) of these HV devices is no where near the amperage times
voltage rating.  I'm not talking about excess overall thermal dissipation
here; I'm talking about local "hot spotting" of the silicon chip in which a
very rapid destructive runaway condition manifests (we're talking
milliseconds) well before the overall package temperature rises.  Heat sinks
will not save you.

If one just follows a simple rule when applying HV transistors one can avoid
most SOA limitations. Always make sure that they are always either full OFF
or full ON and not ever in between, even for a few milliseconds!  The rub
usually comes in when you consider a possible short (or partial short)
condition of the circuit.  Should the transistor transition out of its fully
switched ON mode into something in between ON and OFF, thus allowing
substantial voltage to develop across it, a conventional fuse will not blow
fast enough to save the device from SOA failure.

As for replacing a relay: keep in mind that the HV transistor is unipolar vs
bipolar like relays. Yeah, I know they're called bipolar transistors, but
they aren't.  Also most HV transistors have parasitic diodes reverse
connected from input to output leads (source to drain or emitter to
collector).  In other words when you turn it OFF it only blocks current in
one direction and will pass it in the other direction.  Obviously, relays
don't exhibit this behavior.

BTW.  I like and use HV transistors, including in ARC-5 keying service.  I'm
just passing along some caveats.

Dennis AE6C


On Fri, Aug 26, 2011 at 2:06 PM, David Stinson <arc5 at ix.netcom.com> wrote:

> If you really want to keep the keying "quiet," key all the B+
> (meaning just the 550 volt line into the "correct" voltage divider
>  per the manual and a past post)   as Mike suggests,
> but do it with a Hi-Voltage MOSFET.
> Here's a new one that can handle 900 volts at 20 amps!
>
> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-8772-5-ND
> FETs for switching high voltages and currents are common now.
> I invite The Smart People here, who have helped me many times,
> to tell you all about it.
>
> 73 Dave S.
>
> P.S. Ian; was I supposed to send you some information?
> Please forgive as I tend to forget things lately... very
> troublesome...
>
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