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Old 10th November 2012, 10:30 AM   #1
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Default Simple 12v adjustable help

This will be simple for anyone that knows simple stuff really

Have a pc power supply that's got a 12v regulated output that I want to put into an lm217t to get an adjustable output. I have a B1k variable resistor to hand so would prefer to use that to get it to work and want as close as possible to 0v-12v variable output

Who can help...

Please post a simple schematic
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Old 10th November 2012, 11:33 AM   #2
DUG is offline DUG  Canada
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You will not get the full 12V output using the LM317 family of adjustable regulators when the supply is only 12V.
There will be some drop across the regulator that will vary with current drawn.
Review the data sheet for the actual (expected) drop vs current.

see jpg for typical application.

(capacitors for stability not shown)
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File Type: jpg LM317.JPG (12.0 KB, 186 views)
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Old 10th November 2012, 11:36 AM   #3
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Quote:
Originally Posted by DUG View Post
You will not get the full 12V output using the LM317 family of adjustable regulators when the supply is only 12V.
There will be some drop across the regulator that will vary with current drawn.
Review the data sheet for the actual (expected) drop vs current.

see jpg for typical application.

(capacitors for stability not shown)
Yeh I know I won't get the full 12v and that's fine just wanted the best optimization for this little circuit as possible

How many amps does the lm217t supply?

What caps would you recommend? Or do I not need them as it is a fully regulated power supply...
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Old 10th November 2012, 12:09 PM   #4
DUG is offline DUG  Canada
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Your question begs for a long answer to be complete.

Current depends on drop across and temperature of the regulator. No simple answer.

Check the data sheet: http://www.onsemi.com/pub_link/Collateral/LM317-D.PDF

Learn well.
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Old 10th November 2012, 01:27 PM   #5
AndrewT is online now AndrewT  Scotland
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Yes you can use your 1kVR to control the output voltage of your supply.
The range of adjustment for a 12Vdc input will be ~ 1.25Vdc to 10Vdc.

The dissipation when set to low voltage will severely limit the output current.

There are other Threads showing better ways to do this.

I mention what I have done.
2.75A (secondhand Ebay) Variac feeding a 100VA 25+25Vac transformer. Output is variable from 0+0Vac to 25+25Vac. This is fitted into the box holding my bulb tester and my mains output from the same Variac.
Two pair of 4mm banana sockets feed that variable AC into a pair of PSUs to give me +-0Vdc to +-35Vdc @ ~1Adc continuous and many Apk to power most of what I build.
I also have an Ebay second hand dual output Lab supply that does 0 to 30Vdc @ <=3.1Adc continuous, twice over, with variable current limiters and ability to parallel or series connect the two independent outputs.
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Last edited by AndrewT; 10th November 2012 at 01:30 PM.
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Old 11th November 2012, 05:35 PM   #6
DUG is offline DUG  Canada
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Oops.

Yes

1.25V min
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Old 14th November 2012, 12:54 PM   #7
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Ok so I got it all together and it works but I only get voltage control on the first quarter turn of the knob, so it goes from around 1.4 to 11v just in the first quarter turn

How can I get full adjustment across the whole knob? I really need it this way to get more accurate control...

Thanks in advance
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Old 15th November 2012, 07:21 AM   #8
MarianB is offline MarianB  Romania
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Is the pot logarithmic or linear?

LM317 needs around 3V minimum Vin-Vout difference to work well and it can provide a max current of 2A between 5 to 11V dropout.
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Old 16th November 2012, 03:15 AM   #9
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If the 12 volt supply is regulated, u need not go for an LM317. Just construct a variable voltage circuit using single transistor and couple of parts. Added advantage is that u will get a bigger span than the LM317.

Gajanan Phadte
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Old 16th November 2012, 03:28 AM   #10
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Hi,
You need to increase the variable resistor about 2K. Or add a 200 ohms resistor in series with the Ik. Also your input voltage need to be higher that 12 volts. Possible about 15 volts.
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