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T O P I C    R E V I E W
hillbillyboy_316 Posted - Dec 10 2009 : 12:31:47 AM
i build a copy of aarons power supply based on the lm338k regulator and i cant get any variable control out of the 5k pot. what could be the problem?
15   L A T E S T    R E P L I E S    (Newest First)
audioguru Posted - May 20 2013 : 8:11:25 PM
I guess you did not read the datasheet for the LM338.
The LM338 provides more than 5A (typically 8A) when it is cooled properly and the voltage from its input to its output is 10V or less.
The datasheet shows two circuits with three LM338 ICs with an obsolete opamp to provide 15A.
Also shown is a circuit with two LM338 ICs and an obsolete opamp to provide 10A.
Hardeen Posted - May 14 2013 : 11:37:50 AM
Hi Audio guru,My best regards.
One question---Is it possible to Place more than one LM338 or similar types reg. in Parallel to increase the current output capacity.Guess this has been done or tried before. Best regards.
Hardeen
Hardeen Posted - May 14 2013 : 11:30:26 AM
Hi audio guru,
My best regards---I am looking to get more than 5 amps from the LM 338K or similar types reg.What if I place three or four in Paralell,in theory it is supposed to work ok? have anyone tried it successfully?.just checking--- will try it later,Unless you tell me otherwise. Best regards.
Hardeen
audioguru Posted - May 27 2010 : 9:03:44 PM
Your 32VAC transformer has a peak voltage of 45.3V and the bridge rectifier charges the main filter capacitor to 43.3VDC when the load is 2.1ADC. The 3A transformer is overloaded when the DC current is more than 2.1A.

The LM338 has a minimum output current of 5A only when its Vin - Vout= 10V or less. So your power supply produces 5A only when its output voltage is from 33.3V to 43.3V.
When the output voltage is set to 13.3V or less then its typical max current is 1A but it could be less.
Rod Posted - May 27 2010 : 4:42:36 PM
Hi Aaron.
I have a problem with my power supply.
I designed it and it worked fine between 0-40 V, but it only gives 1.7A. The trafo is for 3A, the rectifier 6A and the regulator is a LM 338K (5A). It should give 3A max, but it doesn't.
Any suggestions ??
Help Please ...
1 x LM338K (National - 5A)
1 x Rectifier KBPC610 (6A - 1000V)
1 x Trafo 16x2 3A (China)
1 x C = 4700 uF (electrolitic)
1 x C = 220 uF (electrolitic)
1 x C = 10 uF (electrolitic)
2 x C = 100 nF
1 x R = 150 Ohm
1 x Pot = 4.7 kOhm

Rodrigo
audioguru Posted - Feb 18 2010 : 3:47:16 PM
quote:
Originally posted by madnoob

Hi all! I don' have an lm338 but i have 4 lm350k available. How could i adapt this schematic in order to get about 10amps of current from it at the maximum voltage posible. The transformer i have is 28 volts AC.Thanks in advance!


Your 28V transformer will make an unregulated supply that is too high for the LM350.

The datasheets for the LM350 and LM338 show a 9A and a 15A variable power supply but they use an obsolete opamp. You can probably find an available opamp that will work.
madnoob Posted - Feb 18 2010 : 2:49:11 PM
Hi all! I don' have an lm338 but i have 4 lm350k available. How could i adapt this schematic in order to get about 10amps of current from it at the maximum voltage posible. The transformer i have is 28 volts AC.Thanks in advance!
audioguru Posted - Dec 17 2009 : 5:31:50 PM
Yes, your LM338 is defective.
hillbillyboy_316 Posted - Dec 17 2009 : 2:16:42 PM
i replaced the lm338k with a lm317t and i get the 1.2 reference volts and i can get 1.2 volts all the way up to 32 volts, would that mean i had a bad lm338k regulator?
audioguru Posted - Dec 17 2009 : 10:59:31 AM
quote:
Originally posted by hillbillyboy_316

the 240 ohm resistor is 0.238

It should be 1000 times higher.

quote:
the 5k pot goes from 0 to 5.44

It should also be 1000 times higher. Maybe it goes to 5.44k ohms and you don't know how to read your meter.

quote:
and the IN voltage to the regulator is 34 volts

That is fine.

quote:
im not getting the 1.25 reference voltage off of my regulator im only getting .50 volts

That is very wrong. It should have 1.2V to 1.3V from the output to the ADJ pin.

I have never used the wrong resistance of 240 ohms with an LM317, LM338 or LM350 so maybe that is what happens when the resistance is too high.
Why don't you use 120 ohms or 100 ohms?

hillbillyboy_316 Posted - Dec 15 2009 : 7:11:24 PM
im not getting the 1.25 reference voltage off of my regulator im only getting .50 volts
hillbillyboy_316 Posted - Dec 15 2009 : 4:15:53 PM
and the IN voltage to the regulator is 34 volts
hillbillyboy_316 Posted - Dec 15 2009 : 4:08:29 PM
the 240 ohm resistor is 0.238 and the 5k pot goes from 0 to 5.44
audioguru Posted - Dec 14 2009 : 10:44:41 PM
Without a load then the output voltage will probably rise because the LM338 needs a 120 ohm resistor or less.
The more expensive LM138 can have a 240 ohm resistor. Then the pot should be changed to 2.5k.

Remove the 240 ohm resistor and 5k pot and measure their resistance then report what they are.
Measure the DC voltage across the main filter capacitor (the input to the LM338) and tell us what voltage is there.
hillbillyboy_316 Posted - Dec 14 2009 : 9:40:19 PM
it is a 5k pot with a 240 ohm resistor

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