Re: QMX diode or Zener PA protection?
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QMX is still young, but I believe so far the only two instances of BS170 failure I have seen reported here can be connected with a failed clock generator leaving the gates of the BS170s stuck high. One case is suspected, one is confirmed. That latter QMX belongs to Mooneer.
Regarding a damaging inductive spike on power outage while transmitting, the capacitance of the power supply plus QMX's (small) internal 12V capacitance will provide some cushion.
JZ KJ4A?
Can the firmware protect against a
power failure during transmit?
I need to press the button on a Digikey
order for a number of things, including the Zeners
(#1N4756AFS-ND).? Ordering 10, could send you some, assuming the
right thing arrives.
Just a thought, are you able to suggest
capacitor values for a 17/15 LPF?? Could add them to the order.
Chris, G5CTH
On 15/08/2023 09:01, Hans Summers
wrote:
Hi Chris
QMX doesn't do a sudden switch on or switch
off so there's no spike on QMX.?
I'm keen to know any detrimental effects of
zeners on transmit harmonics, and receiver noise floor and
dynamic range. Hopefully none all round.?
I'd measure it myself but still no 47V zeners
here. The supplier keeps sending me BZY47... Which is
9.1V... hi hi
Hi Hans,
I hear what you are saying but I am seeing a
significant number of reports of blown BS170s and
sometimes their driver IC, I don't think they are all
down to people thinking 12V means 13.8V.
The inductor (L502) will generate high voltages if
the current is changed suddenly; theory says this and
people have seen the spikes.? A high SWR could have
differing effects depending on the electrical length
to what's generating it, and this will depend on the
frequency and the characteristics of the output
transformer and LPFs which AIUI will be part of the
transmission line from the PA.
I'd like to mitigate the possibility that the BS170s
are damaged. I'm not perfect, It's inevitable that my
QMX will be inflicted with a high SWR at some point,
especially if it's running with a temporary antenna.
I'm surprised that no one with the equipment has
checked and posted. I don't have suitable equipment
myself but I'll ask round my radio club, it's possible
that someone does have a spectrum analyser. Or maybe
this is an excuse for my getting one of those tiny
SAs.
I'll get some Zeners but will try to defer fitting
them until after I've checked the spectrum before so
can come up with before and after plots.
Already have some spare BS170s.
Chris, G5CTH
On 14/08/2023 14:57, Hans Summers wrote:
Hi Chris
It isn't a "fix" because nothing
is broken... Also not a "solution" because there
is no problem... If these are not good enough
reasons not to do it, as far as I know there is
no known harm that comes from it but on the
other hand nobody has tested it thoroughly
either. If you do it and have capability to make
accurate spectrum analyzer measurements of
harmonics on transmit, and measure the noise
floor on receive, I think it would be
interesting information to post here.?
73
Hans G0UPL
I'm planning to fit
some form of over voltage protection to the PA
stage of my QMX.
I understand that the power management in the
firmware should avoid this but I'm a believer
in belt and braces fixes.
The question is whether to go for a diode
across L502 or Zeners in the D503 and D504
places. I like the Zener option because it
protects against some bad SWR problems as well
as switch off surges and there are places for
them but most recent talk has been of the
diode solution.? Is there a good reason not to
go for the Zener fix?
Chris, G5CTH
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