6922 for a Noob - Page 3

SY · 29th June 2009, 08:53 AM

Quote:

Originally posted by Fuzzymuff
I don't have a resistor/s anywhere near that value. Guess I will have to wait till I can go shopping.
Is this where it should be added?

It is used instead of the reg, not in addition to.

tomchr · 29th June 2009, 06:41 PM

I doubt the current limiter in an LM317 is a foldback design. I think the device will output max current at close to no voltage until it goes into thermal overload. That's how the LM7805's behave.

One thing that's noteworthy from the Datasheet is that the max output current depends on the voltage difference Vin-Vout. The 1.5 A is only for Vin-Vout <= 15 V. See "current limit" half way down page 5.

Also note that you need 0.1 uF from LM317 input to ground if the LM317 is more than 6" away from the filter cap. That looks to be the case in your circuit. So I'd add that cap... It's needed for stability.

You can always try using the LM317 as a reference for a pass transistor. Take a beefy power NPN like a 2N3055, TIP42, or the like and bolt it to your heatsink. Connect the output of the LM317 to the base of the transistor. Connect the collector to the filter cap (unregulated supply). The emitter is then the output of your regulator. The output voltage should be about 0.7 V (one Vbe drop) lower than the voltage of the LM317 output. You'll lose all thermal overload protection, current limiting, etc. by doing this, though. The line and load regulation sucks too, but as an experiment it could be worthwhile.

~ Tom

tomchr · 29th June 2009, 06:43 PM

Oops, sorry. TIP42 is the PNP. You'd want the TIP41...

~ Tom

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29th June 2009, 06:41 PM	#22
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	I doubt the current limiter in an LM317 is a foldback design. I think the device will output max current at close to no voltage until it goes into thermal overload. That's how the LM7805's behave. One thing that's noteworthy from the Datasheet is that the max output current depends on the voltage difference Vin-Vout. The 1.5 A is only for Vin-Vout <= 15 V. See "current limit" half way down page 5. Also note that you need 0.1 uF from LM317 input to ground if the LM317 is more than 6" away from the filter cap. That looks to be the case in your circuit. So I'd add that cap... It's needed for stability. You can always try using the LM317 as a reference for a pass transistor. Take a beefy power NPN like a 2N3055, TIP42, or the like and bolt it to your heatsink. Connect the output of the LM317 to the base of the transistor. Connect the collector to the filter cap (unregulated supply). The emitter is then the output of your regulator. The output voltage should be about 0.7 V (one Vbe drop) lower than the voltage of the LM317 output. You'll lose all thermal overload protection, current limiting, etc. by doing this, though. The line and load regulation sucks too, but as an experiment it could be worthwhile. ~ Tom

29th June 2009, 06:43 PM	#23
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	Oops, sorry. TIP42 is the PNP. You'd want the TIP41... ~ Tom

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