LM338 (x3) power supply cuts out on high loads

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The power supply still didn't work. It was then I realized (again) that

example circuitry schematics found in data books are often just used as
illustrations, never left the desk, are not tested and may not work.

I ended up designing my own power supply. It worked. I think the whole
control room is still in operation 2 decades later because once in a while
they broadcast scenes from the room and you can see the LEDS working..

Have you considered the fact that all those hundreds of microfarads in
filtering are supposed to connect to the rectifier output, not the regulator
output?

The 700,000uF worth of capacitance is connected to the output of the
regulator not because I'm try to filter the output, but because I am
trying to charge the bank of capacitors for a completely different
purpose.

That the voltage on your secondary will be too high? (1.414*26)?

I am now finding that. With this discussion especially, I know I need
to lower the voltage into the regulator. Is there an easy way to do
this without having to buy a new transformer? They are somewhat
difficult to locate for a reasonable price in my area, at least at
surplus prices which is about the only option at this point.

Would half-wave rectification be an option? I know I would probably
need even larger filter caps, but if it lowers the voltage to a
reasonable amount (but not too low...) I would save myself the trouble
of trying to find a different transformer.

Regards,
Bob

Thanks
Kevin

 

[Michael A. Terrell]

Please tell me why I would lie about whether or not I have a filter
cap. I don't lying would help solve the problem I am having.

Please ignore the troll. Better yet, just kill file him.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[colin]

limit.

Would you happen to have / know of a sample schematic of what you're
talking about? I've seen a small (LM317) regulator drive a bank of
3055's before, but not quite what you mentioned. Would that accomplish
what you mentioned?

Thanks so much for your helpful response!

the lm317 ciruit you mentioned will probably do quit well, it needs one pnp
transistor wich drives the 2n3055s.
carefull to ensure its got adequate current limiting.

or roughly something like this ... used to apear in many op amp application
notes ... however linear supplies are probably considered old hat now.

30v___________________
| | | |
____|/ | | |
| |e | | |
| |__|/____|/___|/
| |e |e |e
| | | |
| Re Re Re
|__Z/|____|_____|____|________ op
| Z\| |
R3 R1
| /|______|
|_____/-| |
\+|->pot |
\| R2
|
0v______________|


4 x 2n3055 on suitable heatsink. (first transistor could be something
smaller)
Re=3ohm and zener = 3v for aprox 3 amp current limit, change to suit, max
current =3x5 amp due to SOA
Re needs to be suitable power.
op amp could be lm358,
pot needs to be a potentiometer conected to a reference voltage
chose r1/r2 for the right op range.
R3=1k ish.

also a second op amp could monitor the current ie average voltage accros all
3 Re and take the place of the zener.

If you not capable of debuging its best to find a design with more detail,
im sure there must be some out there,
or simulate it first with ltspice etc.

ooh a quick search showed up this ..
http://ludens.cl/Electron/Ps20/Ps20.html

Colin =^.^=

 

[ehsjr]

I'm not so certain I need a steady 15 amps. The cap bank will draw a
quick surge well over 15 amps. What I'm hoping for is a circuit that
can handle this surge (even if it takes longer to charge than it
normally would with a steady 15 amp output) without burning up.

If charging speed is not an issue, just use a big R - say
50 ohms (2 25 ohm resistors at 25 watts each, in case you
short the output somehow) - between the bridge and the
regulator. That will be kinder on the bridge & the xformer,
and you won't need 3 parallel 338's. It'll take a few minutes
to charge. (You could use a much smaller xformer and save
the 15 amp beast for a better purpose.)


Ed