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Old 24th February 2013, 08:37 PM   #1
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Default adding current limiting to a power supply

i picked up a homemade power supply from a swap meet recently and just opened it up to see how it works. it uses an LM338 as a voltage regulator at 22V and the transformer has a 24V/50VA secondary.

according to the datasheet, LM338 is rated for 5A and limits current at 12A. if my transformer is rated 50VA at 24V then doesn't that mean that i shouldn't draw more then 2.08A from it?

do i need to implement some additional current limiting circuit to ensure the LM338 does not draw more then 2A and damage the transformer? i was messing around with it today using it to power an amplifier and it was putting out 5A to 7A.
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Old 24th February 2013, 10:31 PM   #2
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Putting a 2A fuse between the secondary and rectifier is probably the easiest, cheapest, and safest mod.
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Old 24th February 2013, 11:27 PM   #3
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doesn't that mean the fuse will blow if i ever draw more then two amps? there must be some kind of current limiting circuit i can setup to limit the supply to 2A, right?
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Old 25th February 2013, 12:04 AM   #4
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Yes, that's the basic concept. It protects the transformer and other circuitry.
Yes, there's probably a crowbar implementation in the datasheet.
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Old 25th February 2013, 01:20 AM   #5
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i was under the impression that crowbar protection prevents an over-voltage situation, caused by a failure in the power supply, which can damage the load.

crowbar wouldn't help in a situation where i am powering a device which draws more current then is safe for my transformer, right?

i guess i'll install a 2A fuse for the time being, but maybe i should just find a bigger transformer with the same current rating as the LM338.
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Old 25th February 2013, 02:24 AM   #6
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"Crowbar" can apply to voltage or current. Think Ohm's law.
Perspective... If powering a device that draws too much current (let's use a loudspeaker as our example load), isn't it true that there is excessive voltage applied to the load?
There are active limiters, resettable circuit breakers, polyswitches, thermal switches, that you could seek out for your application. Then again...
You yourself can prevent a situation where you are powering a device which draws more current then is safe for your transformer, right?
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Old 25th February 2013, 02:43 AM   #7
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Maximum DC current output will be about 1.2 A because a capacitor input filter has a current form factor of about 1.8 with a typical 50 VA transformer.

With a 1.2 A (DC) output current the transformer secondary current will then be around 2 A (RMS). Using a 2 A fuse in series with the secondary should protect the transformer.
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Old 25th February 2013, 05:46 AM   #8
MarianB is offline MarianB  Romania
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Just like LM317, LM338 offers a simple and safe way for current limmiting on single supply, choose R3 by 0,65/Iout
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Old 25th February 2013, 07:15 AM   #9
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by MarianB View Post
Just like LM317, LM338 offers a simple and safe way for current limmiting on single supply, choose R3 by 0,65/Iout
This scheme has many problems: it will ruin the regulation, and in case of dead short (Vout<0.6V), it will not limit and it will fry the transistor .

Better to replace the LM338 with a LM317 or LM350.
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Old 25th February 2013, 07:25 AM   #10
MarianB is offline MarianB  Romania
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Quote:
Originally Posted by Elvee View Post
This scheme has many problems: it will ruin the regulation...
Only when reaching the threshold the output will be affected, but then again any current limmiting acts about the same, it reduces the voltage to keep the current at a needed level, so no problem here.

Quote:
Originally Posted by Elvee View Post
...and in case of dead short (Vout<0.6V), it will fry the transistor...
Easy fix:
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