Yes, crowbar circuits "disable" the power supply, but some crowbar circuits are self-resetting. They shut down the power supply only as long as the over-current condition remains and automatically return power when it's gone.
Some of the K3 owners here have run into that. Their K3 shut down unexpectedly, so they hit the POWER switch and it came back on. They transmit again and the K3 shuts down again (because the power supply did). Sometimes it's not apparent what is happening because they don't notice that the power supply is actually shutting down. I like the more conventional crowbar in the Astron because it requires cycling the Astron's power switch to reset it. That makes it very clear why things shut down. My Astron 20 is "stock"; I have not modified it in any way. It will deliver a full 20 amps, perhaps an couple of amps more, before the crowbar trips. That's enough to run the K3 at 100 watts on all bands if the SWR is low. A well designed supply like the Astron can deliver full rated power - in this case 20 amps ICS - 24/7 without any problems as long as the demand is ICS -- "Intermittent Commercial Service" -- meaning there are periods when the full current is not being drawn such as during receive. In the K3 running CW or SSB, that demand is intermittent even during transmit since the full current is drawn only on momentary voice peaks or while the CW key is actually closed. Ron AC7AC -----Original Message----- From: Jim W7RY [mailto:w...@arrl.net] Sent: Saturday, December 20, 2008 9:56 PM To: Ron D'Eau Claire; elecraft@mailman.qth.net Subject: Re: [Elecraft] K3 Power out 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 > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
