Ron
A crowbar circuit is used to "disable" the power supply in the event of over
voltage. The current limit is what would limit the current output of a
power supply.
A typical crowbar circuit uses a SCR triggered by a voltage sensor. The SCR
conducts and shorts the output of the supply. Which in turn puts it into
current limit mode and decreases the output voltage to near zero.
And yes, a 20 amp supply is marginal for a 100 watt HF rig because of the
examples you have given. The 20 amp Astron can be fudged a little bit by
increasing the current limit to around 22 amps. The modification involves
adding a resistor to the current limit circuit of the 723 regulator.
73 and Merry Christmas to all!
Jim W7RY
----- Original Message -----
From: "Ron D'Eau Claire" <r...@cobi.biz>
To: <g...@doctorgary.net>; <elecraft@mailman.qth.net>
Sent: Saturday, December 20, 2008 9:40 PM
Subject: RE: [Elecraft] K3 Power out
Hello Gary, and congratulations! Opening up the K3 again is not nearly as
traumatic as you might think.
The POWER control will *ask* for more than the rig can deliver. It's
spec'd
at 100 watts, and that's what is guaranteed. The K3 can make a bit more
than
100 watts on most bands, but that's all it's spec'd to do. When you set
the
POWER control for a specific output, it tries to adjust the transmitter
circuits for that output, but can't do it if it's out of their range.
That's
what is happening to you at 120 watts.
The absolute maximum power will vary by band and to some extent from rig
to
rig. But since 120 watts is a whole 0.8 dB more than 100 watts, it's not
significant on the air.
You're probably crowding that 20 amp supply at 100 watts on some bands.
The
efficiency (and so the current required for a given output power) varies
from band to band. I'm running a K3 with a 20 amp supply, but I'm near the
limit on some bands at right on 20 amps for 100 watts out, if the SWR is
quite low. (Higher SWR's reduce the efficiency so the current goes up!)
I don't know whether your supply has a crowbar circuit in it, but my
Astron
does. It lets me know if I push too hard by simply shutting down. I have
to
cycle the power supply mains switch to reset it, then turn the K3 on
again.
Ron AC7AC
-----Original Message-----
From: elecraft-boun...@mailman.qth.net
[mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Gary Smith
Sent: Saturday, December 20, 2008 9:04 PM
To: elecraft@mailman.qth.net
Subject: [Elecraft] K3 Power out
Firstly
Got the K3 assembled. The transverter board was back ordered and so
is the subreceiver so I will be re entering the K3 and adding to it
at least twice over the next few months. I'm not looking forward to
the disassembly & re-assembly but doing the kit sure gave me a great
appreciation for the detail and function this marvelous radio has.
Absolutely amazing.
Kudos Wane & Eric, you surely have an amazing way of looking at
things. Most effective and clear.
With that there are a few questions but in the spirit of one at a
time;
The power supply I have been using in the setup is a Heathkit Power
supply which was made for one of their last Transceivers. It's a HP-
1144 and offers 20 Amps out.
After calibrating the K3 I tried getting 120 watts out to the dummy
load but after the 50 watt calibration, when I held tune down to see
how it read. I only got 102-105W out, not 120, though I set he power
out at 120W. I'm wondering if the Heath power supply is lacking and
that's the reason for the lower maximum output? It seemed to work
well as a backup for the Omni V & corsair.
Idears?
Thanks
Gary
(One year older as of yesterday...)
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