I had a similar problem. My board was unstable and I fried one
IRF.
After much fiddling I found that the mini coax cable that I soldered to
the board which ended in a BNC was the problem. After I changed the
cable all problems vanished, The ground on the crimp of BNC was
bad..
I suspect the same thing. Floating ground on the output line. The heat
sink is also RF live..
73, Raj
At 28-02-2017, you wrote:
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It is strange.
? Perhaps the op may have to check his wiring.
? The bias mechanism used by Mr.Farhan appears general standard. .
In the absence of Tx?? (PTT) command,
? the mosFET would see a termination of 47+party of the 10K pot.
? I suppose it would work out to say 2.7K?? Under these conditions ,
a charge can't remain on its gate.
? However, One?? should prevent touching mosFETs by finger.
Again .IMHO, the permanent power?? to IRF510 and controling it by
bias supply from Tx power (while PTT is ON") is the?? standard
of the HAM designs as most of us Know.
Sarma / vu3zmv
.
On Tue, Feb 28, 2017 at 12:28 AM, Andrew Kasurak
<va2qch@...>
wrote:
- Thank you both for the feedback. Yes, when I said into transmission,
that was probably inaccurate. There is a similar sound on the speaker as
when I switch to transmit, but when I look at the output, I don't see the
RF transmission I expect. I am measuring power use for the whole thing at
once. This is all when hooked up to the dummy load (it's 51.2 ohm, not
50, but this is close enough? I built it in a metal coffee can). I have
not tried transmitting, or touching the heatsink 'transmitting'
[pseudotransmit henceforth] on the magloop (so I'm not sure if it's
different. I found this high current mode when trying to get an accurate
frequency measurement..).
- The gate (pin1) seems to be at 7--10v when it goes to pseudotransmit,
from 0v while just receiving. I'm not super sure on that one though, as
getting leads in there makes it go even without my hands nearby. For
clarification, from before, when I touched it, I was not also touching
any other part of the circuit, or plugged in devices. Nor was I using the
PTT button. Touching was also via two layers of electrical tape. I
believe the 3A draw, as the heatsink got hot quickly. Pin 1 of relay k1
goes to 0.9v at pseudotransmit. Pseudotransmit happens more frequently
once it's been triggered once, even when the device has been shut off
(may be a heat thing, or some stray capacitance memory?).
- I did grab a couple more irf510's, I've heard this usage is a bit
hard on them.
- As I follow it, the suppressor resistor you are suggesting is from
gate (pin 1 of IRF510) to ground? Is this
( for the grenade RF section. Some
of the pictures on this site do not load.) the kind of fix (1k resistor)
the one you were referring to? Would you recommend the 10k drain-gate
resistor too? In message? ?
#22548? you say 10 ohms, this seems very low to me, is this correct?
From that webpage, the 'miniboots 5W PA' also has a setup from gate to
ground, but it is a parallel of 2k7 with 10r in series with a 1n4740.
There is also the section 'Protect your MOSFET final stage' which details
a zener + 10 ohm. Is this attacking the same problem? I get the
impression it is more to limit regular usage than accidental
triggering.
- I'm going to go ahead and give the 1k gate-gnd a go while I wait for
any replies :)
- Thanks,
- Andrew (VA2QCH)
--
Regards
Sarma
?
Raj vu2zap