[Elecraft] K3 power switching.

[Fred Townsend]

Alan by now you know the offending parts are located on page 3 of the RF Board. Perhaps taking their queue from QST Magazine they go against convention and flow power from right to left so look on the right side for the power entry. Also because these components are supplied as part of an assembly their part numbers are not individually shown.  

As for what type parts I do not have the assembly before me. Check with Elecraft for verification of what I list. The part information is my best guess and is provided to assist you in locating the defective parts. Use at your own risk. Digikey will also lead you to the manufacture's data sheets and give you a picture of what you are looking for.

Fuse F1 similar to Digi-Key Part Number	20LR600SU-ND  Thermal fuse
Zener D28 similar to Digi-Key Part Number 497-6686-3-ND  Manufacturer's # BZW50-15

One more note. The schematic shows a zener diode. However the part number is actually a Transient Voltage Suppressor (TVS). These are slightly different parts.

73,
Fred, AE6QL
See more comments below:

-----Original Message-----
>From: G4GNX <G4GNX at theatreorgans.co.uk>
>Sent: May 13, 2015 4:29 AM
>To: Elecraft Reflector <elecraft at mailman.qth.net>
>Subject: Re: [Elecraft] K3 power switching.
>
>Forgive me Fred, but so far I've not been able to locate those components on 
>the schematics. Do you know which page they're on?
>
>It's also possible that the zener had failed previously, so this time it 
>didn't cause the fuse to trip. On the other hand, what sort of fuse is it, 
>mechanical, thermal or electronic? 

Thermal, self resetting

As we all known, chips are the fastest 
>fuse on multiple legs and given a gross voltage overload, I doubt that a 
>mechanical or thermal fuse would be fast enough.

The normal failure mode for the TVS is shorted. Therefore I doubt it was already failed. The fuse is another matter. They have a soft failure mode which is hard to detect.
>
>I'd prefer to see a full-blown crowbar circuit at the start of the power 
>circuit and of course the same in the PSU although I do admit to having 
>defeated a crowbar once.

This is pretty much a full blown crowbar circuit except it is self resetting and it does not disconnect the source. Also the rest of the circuit is very robust and has voltage regulators which protect the critical circuits.

 I'd built a 12V 10A variable PSU which would go up 
>to 15V before the crowbar tripped in and the regulator then saw the overload 
>and shut the output down. For some reason I had connected the PSU across a 
>car battery and then absent mindedly increased the PSU voltage past 15V at 
>which point the crowbar shorted the output, but of course car batteries 
>don't care about that, so considerable current hit the crowbar and destroyed 
>it! No other damage, so I replaced the thyristor and learned from the 
>experience. :-)
>
>73,
>
>Alan. G4GNX
>
>-----Original Message----- 
>From: Fred Townsend
>Sent: Wednesday, May 13, 2015 7:36 AM
>To: G4GNX ; Elecraft Reflector
>Subject: Re: [Elecraft] K3 power switching.
>
>Alan the power circuit on the K3 is very robust and is protected from over 
>voltage. There is a 15 volt zener that should trip a self resetting 6A fuse. 
>It may be that zener over heated and shorted but even so there should not be 
>smoke.
>
>I agree your psu should be set so it can not exceed 14.8 volts.
>
>73
>Fred, AE6QL 
>
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