LM317T regulate current and voltage?

Nordic · 2nd March 2006, 07:23 PM

Hi I'm still quite a beginner at using regulators but I have been able to sling together a few low voltage PSU's using them...

I am just not sure if I can regulate current as well as voltage

Last night I was reading how it may be good to regulate the current in my valve's heater grid. So, I would like to give it a bash.

The heater is spec'ed as 6.3V - 365mA

I played around with a pencil and some paper now and this is what I get for using a LM317T regulator

The first resistor sees 1.25V, so if its set as 3R4 it should give 1.25/3.4=0.367mA over this resistor (which would see 3.4 x 0.367² = 0.46 Watts).

So then with an output resistor of 17R2, I should get 6.32V (which I think is close enough)....

Could someone please be so kind as to check if I have it correct?

analog_sa · 2nd March 2006, 07:34 PM

No, they can't regulate both current and voltage at the same time

What you can build is a CCS using the reference voltage of the 317 across a variable resistor.

The attached circuit (stolen from Triode Dick) illustrates the idea. The input voltage obviously has to be high enough but not necessarily regulated.

Nordic · 2nd March 2006, 08:55 PM

Is the second regulator for the current?
Is it 250mA?
What makes the voltage go from 7.5 to 4?
How would I aim for 6.3V (would proabably be self explanatory when I know answer to previous Q)

Also curious what would the circuit I proposed produce?

AudioFreak · 2nd March 2006, 09:10 PM

As already stated, you can't regulate current and voltage at the same time. You pick one and Ohm's law takes care of the other.

leadbelly · 2nd March 2006, 09:25 PM

The resistor value you need is 3.3 ohms, what you are calling the "output resistor". That's the only resistor. 1.25V/3.3ohms=379mA; close enough. Don't be worried about regulating the voltage, just supply it with something like 12V (you need to allow for voltage drop for the regulator to do its job), and all will be fine.

Nordic · 2nd March 2006, 09:47 PM

Mkkkkay will do that, have been running off 12v trannies anyway as everything else I used just sagged when actualy put to use... the 12V trannies works great, but needs a big heatsink on the regulator to keep it from switching off.

Could I not parallel 2 6R8 resistors to get 3R4 and a bit closer to the goal, or does this have hidden drawbacks?

leadbelly · 2nd March 2006, 09:54 PM

Quote:

Originally posted by Nordic
Could I not parallel 2 6R8 resistors to get 3R4 and a bit closer to the goal, or does this have hidden drawbacks?

Use 'em if you got 'em

cpemma · 5th March 2006, 07:18 PM

If you want a regulator that will regulate voltage and limit current the 2A 5-pin L200 is the one to use.

http://www.st.com/stonline/products/...re/ds/1318.pdf (datasheet)

http://www.st.com/stonline/products/...re/an/1678.pdf (app notes)

2nd March 2006, 07:23 PM	#1
Nordic diyAudio Member Join Date: Sep 2005	LM317T regulate current and voltage? Hi I'm still quite a beginner at using regulators but I have been able to sling together a few low voltage PSU's using them... I am just not sure if I can regulate current as well as voltage Last night I was reading how it may be good to regulate the current in my valve's heater grid. So, I would like to give it a bash. The heater is spec'ed as 6.3V - 365mA I played around with a pencil and some paper now and this is what I get for using a LM317T regulator The first resistor sees 1.25V, so if its set as 3R4 it should give 1.25/3.4=0.367mA over this resistor (which would see 3.4 x 0.367² = 0.46 Watts). So then with an output resistor of 17R2, I should get 6.32V (which I think is close enough).... Could someone please be so kind as to check if I have it correct?

2nd March 2006, 09:10 PM	#4
AudioFreak Warp Engineer On Holiday Join Date: Oct 2001 Location: Queensland, Australia	As already stated, you can't regulate current and voltage at the same time. You pick one and Ohm's law takes care of the other. __________________ - Dan

2nd March 2006, 09:25 PM	#5
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	The resistor value you need is 3.3 ohms, what you are calling the "output resistor". That's the only resistor. 1.25V/3.3ohms=379mA; close enough. Don't be worried about regulating the voltage, just supply it with something like 12V (you need to allow for voltage drop for the regulator to do its job), and all will be fine. __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

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2nd March 2006, 08:55 PM	#3
Nordic diyAudio Member Join Date: Sep 2005	Is the second regulator for the current? Is it 250mA? What makes the voltage go from 7.5 to 4? How would I aim for 6.3V (would proabably be self explanatory when I know answer to previous Q) Also curious what would the circuit I proposed produce?

2nd March 2006, 09:47 PM	#6
Nordic diyAudio Member Join Date: Sep 2005	Mkkkkay will do that, have been running off 12v trannies anyway as everything else I used just sagged when actualy put to use... the 12V trannies works great, but needs a big heatsink on the regulator to keep it from switching off. Could I not parallel 2 6R8 resistors to get 3R4 and a bit closer to the goal, or does this have hidden drawbacks?

5th March 2006, 07:18 PM	#8
cpemma diyAudio Member Join Date: Jul 2003 Location: S Yorkshire OK	If you want a regulator that will regulate voltage and *limit* current the 2A 5-pin L200 is the one to use. http://www.st.com/stonline/products/...re/ds/1318.pdf (datasheet) http://www.st.com/stonline/products/...re/an/1678.pdf (app notes)

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