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LM338 Help

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linguist

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I have a LM338 voltage regulator, I need to regulate it to a 5v output, on the Datasheet I found they have a circuit that has an ouput of 5v with an electronic shutdown using a transistor as well.
The circuit is attached.

I have no need for the electronic shutdown so can I just leave the transistor out & use this circuit as is without the transistor.

Also will 1/2 Watt resistors be ok for this type of circuit, I need about 5v 2Amps output from the regulator.
My input voltage will be around 15v.

Cheers
 

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Hi, Your resistor values will be fine for 5v output R1 120r and R2 360r 1/2watt resistors will be fine.

Now here is the problem, as you said you are inputing 15v and wish to draw a maximum of 2A. Because the output voltage is 5v you are wasting 10v this has to go somewhere.

10v x 2a = 20watts of power, this power needs to be dissipated through a large heat sink!

Looking at the data sheet of the device it has a thermal resistance junction to ambient of 35c/w. Which means for every 1 watt of power the LM338 will be 35c + the room temperature. This is with out a heat sink so in your case 35x20=700c+room temp. The LM338 would cut out long before it gets anywhere near this temp.

So you will need a large heat sink or a computer cpu cooler.

Other options would be to reduce the current draw or reduce the input voltage.

To understand more about thermal resistance and heat sinks watch this video.



Hope this helps

Don

A quick calculation shows that the following heat sink should work**broken link removed** resulting in an heat sink temp of approx +80c for 20watts dissipation.
 

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Thanks for the reply Don,

Yes, I get what your saying with the power that needs to be dissapated, I was thinking about it wrong in that I want 5v @ 2A =10W output which I can't change.
So I thought the input would only equal the 10W also & not have to dissipate the difference. I was not thinking in terms of a voltage regulator.
That's basically the reason I asked the question as I was unsure about it.

So maybe I can drop the input voltage as mentioned.

Cheers, yes, it has helped!

Just noticed the link to the Heatsink but I can't seem to open it.
 
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Thanks again Don,

After viewing the heatsink I have a couple I can use that are very simillar & just a bit larger.
I'll check out the Video as well.

Very helpful info

Cheers
 
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