[Elecraft] Switching P/S Horror Stories

Sat Apr 6 03:25:02 2002

Further to the discussion on switch mode supplies:

I bought a 4A 12V supply from one of our UK distributors as appearing ideal for
driving my K2.  There was nothing in the description to indicate it was a switcher,
and when it arrived there was not even a manufacturer's name or type number on it, so
of totally unknown origin.

The first problem was that the output (unadjustable) was only 11.6V, clearly
insufficient.  I investigated inside and found in fact it was a switcher, but not the
usual type.  This supply has a conventional mains transformer (quite large in fact)
and rectifier block at the front end.  This is followed by a dc-dc converter (ie
switcher) to produce the final output.  A little searching on the internet found a
datasheet on its Richtek RT34063A chip, and with this information I was able to
adjust the voltage defining resistors to produce a 13V output.  Fine, I thought.

Until I tried to use it with the K2...

As it is not a line switcher there is no radiation back into the mains line, and
there is no detectable radiation when it is sitting near the K2 but not connected.
When used to power the K2 however there is wideband hash, worse on the lower bands.
It is S5 on 10MHz, so effectively unusable.

I have tried filtering the dc output first with a large electrolytic (reduces the 1V
switching spikes but does little to reduce the hash), then with a series torroidal
inductor in various combinations in the output leads with no luck either.  I have now
got some EMC type ferrite rings and will have another go with those, but it does seem
a difficult problem to sort out.

In the mean time I am using my Farnell 15V variable bench supply which does the job
admirably.  I was a bit worried the other night though - I must have caught the knobs
by accident and suddenly realised it was supplying 16V to the K2.  Fortunately all
seems OK!!

Dave G3YMC
K2 #2498
[email protected]
[email protected]
http://www.dsergeant.btinternet.co.uk