Voltage regulator using lm338

Discussion in 'The Projects Forum' started by rock94, Jun 19, 2013.

1. rock94 Thread Starter New Member

Apr 12, 2013
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I need help with the LM338 variable voltage regulator circuit.http://www.google.co.in/url?sa=t&rc...YI1HakR9ZkIWVlw&bvm=bv.48175248,d.bmk&cad=rjaI'm building the one with the improved ripple rejection
I'm using a 24v 3A transformer with a bridge rectifier and a 4700µF filter capacitor at the input and I'm having trouble deciding the values of R1 and R2.I would like an output of around 3-20v(+/- 2v at the input and output).The circuit in the datasheet uses a 5k pot.However the potentiometer I have is marked 4.7k,but when measured is close to 7k.So if I use a 10k pot what should be the value of R1?What would be the output current of this setup?Also can the tantalum capacitor at the output be replaced by an electrolytic capacitor?If so then what should be the value of the electrolytic capacitor?
Thanks.

2. Dodgydave AAC Fanatic!

Jun 22, 2012
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So if you have a 10k pot for R2, then R1 would be 625 ohms, that should give you a supply output from 1,25V to 21.25 V. As for the output caps i would use 1000uF to 2200uf on both the input and output sides also a 0.1uF caps on both sides too.

The current will be limited to the Regulator limit prob 10 to 12 Amps bigger heatsink would improve it.

see page 8 of datasheet for formula

Last edited: Jun 20, 2013
rock94 likes this.
3. rock94 Thread Starter New Member

Apr 12, 2013
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How can the output current be 10-12 Amps, as the input would be 25v @ 3 Amps.Just a little confused there.What would be the current output at max output voltage?
Thanks.

4. Dodgydave AAC Fanatic!

Jun 22, 2012
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As long as the supply transformer will give upto 10Amps your regulator can maintain it, you only get out what you put in less efficiency.

5. GopherT AAC Fanatic!

Nov 23, 2012
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I think your LM338 regulator is limited to 3 amps no matter how big your heat sink is (check the datasheet to confirm max amperage) . Small surges above may be ok but the case has a limit of how much heat can escape so you will eventually overheat it and it will shutdown if you have sustained high current draws.