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From: "Roger Hamlett"
Subject: Re: Bad LM338K?
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Date: Thu, 12 Dec 2002 12:27:56 -0000
NNTP-Posting-Date: Thu, 12 Dec 2002 12:28:47 GMT
Organization: ntl Cablemodem News Service
"Werner" wrote in message
> Hi guys,
> I'm looking for a answer to a puzzling question ...
> I've build a power supply from a LM338K (TO3) with a 10.000 uF and a 100uF
> capacitor on the input line and a 1uF and a 0.1uF capacitor to output a
> fixed 24 volts DC supply.
> I 've got a 220 ohm resistor between LM output the adjust of the LM, and a
> 4k7 trimpot to the ground ... which I adjust to 24 VDC (with almost no
> ... the transformer delivers 35 volts DC to the input of the LM (no
> The transformer is rated min. 10 Amps.
> All correct so far? Fact is that it allows me to drive small loads ... eg.
> power resistor 51 Ohm ... 24 volts output is kept ... resistor warms up
> no problem ...
> Now I put a second 51 Ohm resistor in parallel to the first resistor ...
> the output voltage of the circuits drops 4 volts (to 19-20 volts), to the
> best of my knowledge this is way under the power specs of the LM338, as it
> should be able to drive 5 Amps with input-output difference of 11 volts?
> How can this be explained? At this state even adjusting the trimpot has no
> effect on the voltages anymore ... it's like the regulator has found a
> state" and does not want to provide 24 volts ... it almost does not
> any heat ... could it be damaged? Or is there a simpler explanation?
1) Check the actual input voltage to the regulator, Though you obviously
feel happy about the current rating of the transformer, how much voltage
drop is there on the diodes, and intermediate wiring?.
2) Make sure that you have a well laid out common ground directly to the
supply, the reference control pot, and the output. It is very easy to make
linear regulators oscillate, and give apparent output voltage changes if
this is not done.
The third possibility is that the regulator has a problem...
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