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From: "Christopher R. Carlen"
Subject: Re: Bad LM338K? Extra question ...
Date: Thu, 12 Dec 2002 08:55:25 -0800
Organization: Sandia National Laboratories, Albuquerque, NM USA
NNTP-Posting-Date: Thu, 12 Dec 2002 15:53:44 +0000 (UTC)
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> When I first got the PCB made up, I noticed that the heavy 10.000 uF
> capacitor blew my fuse at the primary stage of my transformer ... I had to
> install something bigger than a 2 Amps fuse to survive the inbound current
> rush from the capacitor ... I did not learn about NTC resistors used to
> prevent this till after this design :-(
> Can this damage the LM338K? As the capacitor is located before the LM, it
> does not harm the LM? Or is this a faulty assumption? I've got the LM
> protected by 2 diodes, one from the output to the input line and one from
> the adjust leg to the output leg.
That won't damage the regulator, which only sees the filtered voltage
from the capacitor, which slowly rises to the final peak value with no
There are basically these possibilities:
1. Raw DC voltage is drooping at the input to the regulator. It isn't
enough to just know that you have a 10A transformer. You must monitor
the raw DC with a voltmeter while changing the load on the reg. output
to be sure it stays at least about 3V higher than the desired load
voltage at all times.
2. The regulator is overheating. You have indicated that it doesn't
appear to be very hot so...
3. The regulator is oscillating. You can only determine this with an
oscilloscope. Do you have an oscilloscope and what is it telling you?
4. The regulator is broken. Try another one.
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
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