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Old 21st December 2010, 05:43 AM   #1
semi932 is offline semi932  United States
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Default Mathematically challenged help for lm 317-337

I know this is basic for the pros but I'm getting confused as to the calculations.

To keep it short I won't go into how I figured the resistor values. Tried to go by the data sheet but I guess I missed the point!!!!!!!!

Trying to drop from 33v dc to 29-30vdc.

Apologize for numbness but can anyone help me?

So far my calculations have me dropping way too much voltage in my tests and I'm spinning back and forth.

Thanks
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Old 21st December 2010, 05:52 AM   #2
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three volt drop is probably better done with a chain of diodes or a zener. Its right on the limit of the stable operation of an LM3x7 regulator.
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Old 21st December 2010, 06:09 AM   #3
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aardvarkash10 is correct, but in case you still want to do this:

Vout = 1.25*(1 + R2/R1) + Iadj*R2

neglecting Iadj*R2 for the moment, R2/R1 = (Vout/1.25) -1 = 23. So pick values to get the ratio close enough. Since Iadj = 50uA typical, a value of R2 in the range of 2K only changes the output by 100mV.
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Old 21st December 2010, 06:10 AM   #4
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But if you want to try, assuming your R1 is 240, I calculate 5k + 470 for R2.

BTW, check out Paul Falstad's circuit simulator applet (Java). It's great for roughing out small circuits like this.
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Old 21st December 2010, 02:53 PM   #5
semi932 is offline semi932  United States
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Quote:
Originally Posted by mightydub View Post
aardvarkash10 is correct, but in case you still want to do this:

Vout = 1.25*(1 + R2/R1) + Iadj*R2

neglecting Iadj*R2 for the moment, R2/R1 = (Vout/1.25) -1 = 23. So pick values to get the ratio close enough. Since Iadj = 50uA typical, a value of R2 in the range of 2K only changes the output by 100mV.

I seem to be getting confused between R1 R2.

I'm thinking to use 220R for R1 as I don't have 240 on hand.

I guess just adjust R2 until desired results.
I can drop 5v with no problem.

Just trying to regulate Gainclone 3886 supply to see if it makes improvement or unregulated.

Thanks
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Old 21st December 2010, 02:54 PM   #6
semi932 is offline semi932  United States
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Thanks for all the leads as I'll research and hopefully learn something.
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Old 21st December 2010, 04:25 PM   #7
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data sheet here: LM317 - 3-Terminal Adjustable Regulator

Also there was a thread a while back on optimizing the performance and frequency response of the LM317/337, do a search.
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Old 21st December 2010, 04:53 PM   #8
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LINK
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Old 22nd December 2010, 08:58 PM   #9
semi932 is offline semi932  United States
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Great info for a learner folks.

Thanks for all the good tips. I'll try to soak it up.

Got the job done with dropping 5v and running GC at 28v.
Does sound a little tighter but maybe not quite as quick.

Quick Q if anyone looks at this thread.
Can the 317 337 be input with more than 37v?
I couldn't figure it out from the data sheet. DOH

I've got a transformer that will put out about 45v after rectification and would like to be able to trim it to about 34-35v for another application.

Thanks again.
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Old 23rd December 2010, 12:27 AM   #10
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I think you're nuts to be using a regulated supply with the 3886, you have the 1.5A output current limit of the regulator in series with the current demands of the output stage. Any sustained high power draw will drain the reservoir capacitance and then your rails will collapse and the regulator will overheat.

It might make sense to do this if there were separate supply pins on the 3886 for the input stage and output stage, but the V+/V- pins are shared. See any of the books on amplifier design - Doug Self, Bob Cordell, etc. for a discussion on this.

On to your question: the limit on the 317/337 is the input to output voltage difference, not the voltage referenced to ground. See the link above.
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