I concur. At the minimum current setting, it won't take much current to
cause the supply to go into current-limit mode, at which time it will lower
the output voltage to cut back. Turning the current knob back up a bit will
raise the threshold and the supply will be allowed to put out more current,
so the voltage will rise in an attempt to reach the set voltage. Once the
set voltage is reached, it should stay at that point until you exceed the
current setting. This is how a fold-back current-limited power supply is
supposed to work.

Sounds like the supply is working just as it should. If you can find a load
that will run the current up to 35 amps at 14 volts, you should see it start
limiting there even with the current knob turned all the way up. That kind
of load will get hot, so take the appropriate precautions. The 35 amp
supplies are rated for about 25 amps continuous and 35 amps peak. If a
sudden current surge causes the 35 amp limit to be exceeded, the supply
could shut down completely, giving you one volt or less on the output
terminals. Cycling the power should bring it back to life.

Bob M.
======
"Franc Zabkar" wrote in message
...
On Thu, 20 Nov 2003 11:36:39 -0500, "Bruce W...1"
put finger to keyboard and composed:

My Astron VS-35M power supply (http://www.astroncorp.com/linear.shtml)
isn't working right. Unable to get support from Astron I'm hoping
someone here can help me fix it. I'd really hate to ship it to
California, this thing must weigh 30 lbs..

Here is a symptom of the problem:
When the power supply is set at 13V and the current knob at zero, ...

When you say "zero", do you mean "minimum"?

... on connecting it to a 3-ohm power resistor the current goes to
2 amps and the voltage drops to 6V.

Sounds like the PSU is working as designed, ie you have set the max
current limit to 2A and the PSU is obliging with foldback current
limiting. You will probably find that a 4 ohm load will produce a Vout
of 8V, a 5 ohm load a Vout of 10V, etc.

When the current knob is then set to full, the
voltage returns to 13V and the current goes to 4 amps (should be 35A).

No, it should be 13V / 3 ohms = 4.3A.

Here's voltages I measured on the LM723 voltage regulator:
[No load voltages]
Pin Voltage
2 13.69
3 13.26
4 6.38
5 6.38

Pins 4 and 5 are the inputs to the error amp. It looks like the PSU is
regulating properly.

6 7.18
7 0
10 14.25
11 29.33
12 29.31
13 15.51

Here's the schematic for the power supply:
http://www.didah.com/astron/circuit.gif
And here's the data sheet on the voltage regulator:
http://www.didah.com/astron/LM723.pdf

I'm no electronics expert (or I probably could have fixed it). But I am
good with a soldering iron and have lots of test equipment. Any
suggestions on what to look at?

Thanks for your help.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.