[Sneap] Current sense resistor, Varian 6004 power supply

[James Kaiser]

Allen,

We have repaired these water cooled resistors in the past similarly to 
what David Weisser mentions. At least once, we abandoned the voltage 
sensing connections, drilled and tapped holes directly into the main 
copper bars and used them for the voltage sensing. This was, of course, 
not quite as stable but satisfactory for years. In one case we actually 
rebuilt a new resistor from scratch, buying strips of manganin, making 
new copper bars and silver soldering the whole thing together, finishing 
off with a coat of marine epoxy.

A few years ago I replaced the whole resistor with a standard 100mv 
meter shunt and a precision amplifier to boost the shunt voltage to 
around 5v so the rest of the system didn't need to be changed. This is 
in our most critical supply, the analyzing magnet supply, and it works 
satisfactorily. There's a warm-up period with some drift but after an 
hour or so of operation the stability is good enough for our purposes. 
One end of the meter shunt is actually mounted to one of the copper rods 
on the still-present old shunt. This provides a heat sink for the shunt 
since the water is still flowing through the old shunt.

I've had it on my list for a long time to try to incorporate one of the 
non-contact magnetic precision current transducers from Danfysik into 
one of these venerable old supplies to get rid of one of the most 
failure prone (and difficult to replace) parts. It hasn't happened, 
partly because the supplies are still working and while they're working 
there always seem to be better things to spend the money (and time) on. 
I don't see any reason why this couldn't be done, however. After all, 
that's how Danfysik does the current sensing in their own supplies! If 
we had a failure now, I'd want to go in that direction.

So it seems to me you have four options: repair the old one if possible, 
build a new one like the old one, try the meter shunt trick or switch to 
a new type of sensor. Or maybe someone will come through with an old one 
you can scavenge. Good luck.

Jim Kaiser

James W. Kaiser
University of Notre Dame
Physics Department
225 Nieuwland Science Center
Notre Dame, IN 46556
(574)631-6808



kerna wrote:
>
> Dear Sneap community,
>
>  
>
> This resistor failed today.  It is .0316 ohms.  760watts of power at 
> 155 amps.
>
>  
>
> Does anyone have an old Supply not in use that I could scavenge this 
> part?  This resistor has opened up.  I have not yet tried to fix it.  
> Has any one had experience fixing it?  I haven't had a chance to 
> locate a replacement, as I have just discovered what was wrong with 
> the supply a short time ago.
>
>  
>
> I have left a mess with someone at Varian, but I don't hold much hope 
> they will be much help with a supply that was built back in the 1660's
>
>  
>
> Thank you.
>
>  
>
> Allan Kern
>
>  
>
> Western Michigan University
>
> ------------------------------------------------------------------------
>
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