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Old 11th October 2012, 11:22 AM   #1
bego2 is offline bego2  Croatia
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Default lm338 regulated power supply

Hello guys... Need a little help, first i didnt know where to put the thread because is not the power for amp it just a testing power supply...

Im going to use lm338k, I have 2x12V 120VA transformer, is good for 10A+. Would love to have 2xlm338k parallel, but i need some easy circuit because I will do this p2p and I dont wont to have a lot components...

I found this, but it uses one lm338k, I asked a friend could i put a two in parallel, he said that this circuit would not work...

Power Supply

Thanks for your help.
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Old 11th October 2012, 12:03 PM   #2
AndrewT is offline AndrewT  Scotland
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120VA 12+12Vac is a 5Aac transformer.
When feeding a capacitor input filter the continuous output DC current (per winding) is ~ 2.5Adc, NOT 10Adc.

If your load requires a continuous steady current then I'd suggest you limit your continuous DC current to ~ 50% of the maximum rating. i.e. ~1.3Adc continuous.

If your load has a variable current demand and the average demand is <~10% of the maximum, then the transient current demand can be considerably greater than 2.5Apk. The rectifier and capacitors must be rated to meet this peak demand.
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Old 11th October 2012, 12:09 PM   #3
bego2 is offline bego2  Croatia
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12+12=24V

I have 5A on one winding (12v), so in serial 5A+5A=10A, already tested, holds 10 amps for 10min no problem, i dont need more or time or amps.

right?
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Old 11th October 2012, 12:20 PM   #4
AndrewT is offline AndrewT  Scotland
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you must derate the the transformer using the manufacturers derating information when feeding a capacitor input filter.

That derating results in approximately half the continuous AC current value being available as a continuous DC rating.

For two parallel windings, the derated DC current is 5Adc continuous.
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Old 11th October 2012, 12:21 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by bego2 View Post
12+12=24V

I have 5A on one winding (12v), so in serial 5A+5A=10A, already tested, holds 10 amps for 10min no problem, i dont need more or time or amps.

right?
Wrong.
Series connected adds voltage, the current rating remains at 5Aac.
If you parallel connect the windings then the voltage remains the same at 12Vac and the current available doubles to 10Aac.
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Old 11th October 2012, 12:56 PM   #6
bego2 is offline bego2  Croatia
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Why then holds 10a at 24v?
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Old 11th October 2012, 01:14 PM   #7
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Maybe it is a 2x12V, 2x120VA transformer?
Derating is still necessary. The reservoir capacitor will charge to the peak of the AC secondary, ~16V or ~33V. With this increase in voltage over the nominal RMS value, the current must decrease. Power out cannot exceed power in.
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Old 11th October 2012, 01:34 PM   #8
bego2 is offline bego2  Croatia
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just tested it, with 7a load, voltage drops to 20V from 25, just with bridge rectifier without capacitor, Im planing to put 2x10000uf..
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