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Old 11th October 2005, 08:36 PM   #1
Botch is offline Botch  Switzerland
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Default CLC design for LM317

I need some help with a LM317 supply I going to build. Actually 2 supplies but want to use the same basic design for both. Originally I was going to use a CRC circuit in the supply but have been looking into using a CLC instead.

The idea is as follow:

- AC in
- RC shunt
- DC rectifier
- CLC
- LM317
- C
- nice clean DC out

The load is going to be ~400ma/24V for one supply and ~50ma/9V for the other.

I'm just having some problem deciding what is the approriate value I should be using for the Inductor in the CLC filter (mH & dc R values). For the caps I'm looking at a 2200uF, the inductor and then 2x1000uF (2000uF in total) after it. These values are adjustable of course and can go up or down (within reason).

I'm also unsure about the resonant frequency and type of inductor (Axial, Asymmetrical, Symmetrical etc) I should be using. I want to use a pcb mount so size is important.

Can anyone give me some ideas here? Or recommend a site with some design info?

Another question is what max current rating for the inductor should I look at? The LM317 can provide upto 1.5A, I'm looking at a load of under 500ma during operation. I'm thinking 2A to be on the safe side but...

I have been making some plots in Duncan PSU designer II so have been guessing some values but would like to get some ideas from other who may have been here before.

Botch...
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Old 12th October 2005, 03:57 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
just registering an interest so that I can also benefit from the replies.
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Old 12th October 2005, 07:59 PM   #3
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Whaddya need the choke for? Low on headroom and need to conserve voltage, while keeping peak transformer current low?

Any three-terminal regulator is already better on PSRR than you could ever hope so it can't be noise you're concerned about.

Tim
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