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LM317 with pass transistor

Hi, I'm looking at a circuit in the TI datasheet for the LM317 voltage regulator. Fig 14 on page 12 shows a circuit with a 2n2905 PNP transistor and a TIP73 pass transistor. I am wondering if I can put in multiples of the TIP73 parallel to get higher current.

I would like to balance the system so that the overcurrent limit of the LM317 works to shut down the whole thing if the complete system is overcurrent.

Can anyone give me some idea about how they arrived at the value of 22 ohms for the input side of the LM317 and 5k ohms for the base of the

2n2905?

Also, I have a pile of 2n3055 transistors, I would probably use those instead of the TIP73 pass transistor. I would also put in some .1 ohm emitter resistors on each of the pass transistors, like in other pass transistor setups I've seen.

My goal is to get to about 25 or 30 amps of capacity.

Reply to
Chris
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Over what voltage range are you looking to run this regulator circuit? If you're looking to get a variable voltage over the full range of the LM317, then you'd better have a *BIG* heat sink available, or a tracking preregulator ahead of the LM317 regulator circuit. What you're asking the LM317 to do with minimal circuitry is a real stretch.

More directly to your question, If you look at the LM317 datasheet from National Semi at

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you'll see a circuit example on page 18 that you could possibly modify to meet your current requirements. Again, you'll have to take serious precautions to handle all the heat. Here are a few links to suggest some circuits that you might investigate.
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I'd like to offer another approach. Since a switching regulator for this current level is not trivial, I suggest that you look into the Astron linear power supplies. Most models of their linear supplies are based on the LM723 regulator, which has user-defined current limiting feature built-in. Find lots of info and discussions as well as some of the manuals and schematics for the Astron supplies at

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Cheers!

--
Dave M
masondg44 at comcast dot net

One good thing about Alzheimer's; you get to meet new people every day.
Reply to
Dave M

Sorry, I left out the voltage information. In the end I just want a 13.8 volt output with 20-25 amps. So I don't have to preserve the wide range of output voltages available with the LM317.

All of the three-legged regulators have these neat features of overcurrent protection, etc. I was just wondering how to preserve those while handling much more current.

Thanks to both Charles and Dave M. for lots of good links. I will dive into that info.

Chris

Reply to
Chris

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