Subject: Bad LM338K?
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Date: Wed, 11 Dec 2002 19:19:54 GMT
NNTP-Posting-Date: Wed, 11 Dec 2002 20:19:54 MET
Organization: Telenet Internet
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 load)
... the transformer delivers 35 volts DC to the input of the LM (no ripple).
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 ... and
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 "safe
state" and does not want to provide 24 volts ... it almost does not produce
any heat ... could it be damaged? Or is there a simpler explanation?