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Old 2nd March 2006, 07:23 PM   #1
Nordic is offline Nordic  South Africa
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Default 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 0.367 = 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?
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Old 2nd March 2006, 07:34 PM   #2
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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.
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Old 2nd March 2006, 08:55 PM   #3
Nordic is offline Nordic  South Africa
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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?
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Old 2nd March 2006, 09:10 PM   #4
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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.
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Old 2nd March 2006, 09:25 PM   #5
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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.
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Old 2nd March 2006, 09:47 PM   #6
Nordic is offline Nordic  South Africa
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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?
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Old 2nd March 2006, 09:54 PM   #7
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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
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Old 5th March 2006, 07:18 PM   #8
cpemma is offline cpemma  United Kingdom
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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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