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Old 9th June 2004, 09:07 AM   #1
Pablo is offline Pablo  Spain
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Default input output diferential in LM317

I want to make a power supply with a LM338, the Lm338 can give 5A but only when the input-output differential is less than 10V aprox, and I have 50V at the input fron the rectifier.. if I want output ranges less than 10 V...
Can somebody explain how can I do to make that the regulator sees only 10V betwenn input and output?
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Old 9th June 2004, 09:16 AM   #2
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You can use a simple preregulator, like an emitter-follower, that lowers the input voltage to say 20V. But then that preregulator has to dissipate 30V at 5Amps = 150Watts!

This kind of application screams for a switching regulator, 48V in, 10V out.
Or maybe you can change the raw supply to something less than 50V?

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Old 9th June 2004, 09:37 AM   #3
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Pablo, take a look at National's "Simple switcher". I don't know if they have 5 A out but some of those regulators are very easy to get going... with a good pcb. I have used those at work but then it was only 500 mA or so.
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Old 11th June 2004, 01:20 AM   #4
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and I have 50V at the input fron the rectifier..
Or maybe you can change the raw supply to something less than 50V?
If those 50V are coming from a ~ 35AC xfmr winding which is FWB rectified into a capacitive load you could use a FW C input rectifier instead.
This would give you ~ 25VDC at full xformer secondary current capacity and a lot easier to step down but at 5A this would still mean a whopping 75W of heat.

I'm just mentioning this alternative as it often seems to me as if everyone automatically thinks FWB and C input which in cases like this can work against you.

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Old 15th June 2004, 04:26 AM   #5
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Default Transistorised Zener

Construct a zener (with transistors) of Vin max of LM338 and use it as a shunt preregulator, with a resistor in series, and use smaller filter C.

Gajanan Phadte
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