Re: OT Question for Iggy

On 2009-09-03, Ignoramus22805 <ignoramus22805@xxxxxxxxxxxxxxxxxxxx> wrote:
On 2009-09-03, Carl Ijames <no@xxxxxx> wrote:
Oh well. I assumed it was long gone but just in case I figured buy from
someone I know :-). There are several on ebay at a range of prices, I'll
keep shopping and pick one up in the next few weeks. I told my old adviser
I'd charge a 3 tesla supercon magnet for him when he gets an experiment
ready to go, after I heard that he got a quote of $28k from the

I do not see any on ebay in the range of prices. The one for $133.12
is bad.

Note that sometimes "bad" means that the vendor does not know
how it is supposed to work.

I got a pair of "bad" Power Designs 2005 power supplies (20 V,
5 A) which were set by multiple concentric switches in steps of 1,
0.100, 0.010, and 0.001 V, plus a toggle switch which adds 10 V to the
total. There is also a pot for tweaking the output from 0.000 to 0.001
V.

Anyway -- what was "wrong" with the supplies, and what I
suspected was the problem was that the "sense" terminals on the back
were not connected to the V+ and V- terminals. Normally, they are
connected, and the front panel terminals are used -- but for use in
rack-mounted test setups (and these two were in a dual rack mount frame)
it is common to run the power and the sense wires from the back of the
supply, to leave the front panel uncluttered by cables. Running the
sense wires to where the load is assures that the regulation produces
the set voltage *where the sense and power leads join*, instead of at
the panel, allowing the loss of several milivolts or more depending on
how long the cables are. The sense leads should be run back in a
shielded pair cable to keep noise pickup from affecting the output
voltage.

Anyway -- once I got them home and replaced the missing sense
jumpers, both worked just as designed. And I got them for what I
considered to be a particularly good hamfest price for the period. (It
helped that I used to use these at work, and knew what they would do so
I wanted them. :-)

Not any good on eBay -- unless the vendor includes photos of the
back panel and you can see that there are no jumpers between the power
terminals and the sense terminals there. On something capable of 100 A,
the sense terminals will probably be normal sized Jones barrier strips,
and the power terminals on the back will be on an extra heavy duty
barrier strip -- or even on stud terminals on the back.

Of course -- if you can see the sense wires are connected in the
photo -- don't bother bidding. :-)

Good Luck,
DoN.

--
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