# Digital Control Voltage using LM317

Discussion in 'Microcontrollers' started by Suraj143, Aug 27, 2009.

1. ### Suraj143Active Member

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I want to make a digital controlling voltage source.

Can somebody help me to interface LM317 & a PIC micro to change the output voltage of LM317 digitally.

Thanks.

2. ### mdorianMember

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How many steps do you need and what is the minimum and maximum output voltage?

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Use digital potentiometer, or the cheapest way is use pwm

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5. ### mdorianMember

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The digital pot have a end to end resistance tolerance of 20% , output voltage must be less than 6.25v ,5v (maximum voltage on the pot ) +1.25 (LM317 regulated voltage).
See the tipical use of LM317 (fist page) http://www.electro-tech-online.com/custompdfs/2009/08/LM117-4.pdf and look at the formula at the bottom of the page:
The current on the ADJ pin is somewhere between 0 and 100uA , using a 10k digital pot this will giwe an error of R2*Iadj = 10000*0,0001 = 1V , so you have to use for R2 a smaller value than a digital pot can have. If you don't need a very good precision you better use a resistive net connected to the portb this will give you in the worst case an error of 0.6 V

6. ### Suraj143Active Member

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Hi mdorian I like 0.5V steps. I like the range 1.25V to 12V.What do I have to do with LM317?

7. ### atferrariWell-Known Member

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Digitally controlled.

This comes from EDN magazine, online edition. Could not find the note accompanying this.

I think you can implement and test it with no further help.

PLease note that depending of the number of steps required you could need to extend the R2R network using more bits.

Last note: keep in mind that you can play with Rf and Rin of the op amp too.

Could you post the outcome? (Usually, nobody does)

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9. ### Mr RBWell-Known Member

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I would use the PWM output from the PIC CCP module, then filter it with a 2 stage low pass filter (requires only 2 resistors and 2 caps) then just connect that DC voltage to the Adj pin of the LM317.

That will be fully linear and PIC output of 0v-5v will give LM317 output of 1.2v-6.2v.

10. ### Suraj143Active Member

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Hi thanks for the replies now I got what I have to do.

I have two options to choose.

1)Using R2R Ladder Feeding via the opamp to LM317
2)Using PWM frequency via Low pass filter to LM317.

Thats great.

First I'll look into the 2nd option.& after that I'll study the 1st option.

I can generate any frequency with PWM module.

Mr RB Can you tell whats the purpose of using low pass filters for this job?Why can't I direct feed the PIC output to the LM317 ADJ pin?

Last edited: Aug 30, 2009
11. ### Mr RBWell-Known Member

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It depends if you want the LM317 to generate a DC voltage or a nasty pulsating DC voltage...

12. ### mdorianMember

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1 - The Rds of 2N5461 is temperature dependent so the output voltage will be temperature dependent
2 - The PWM output is referenced to PIC supply voltage
If this is a school project I think the idea is to use the precision and the temperature stability of LM317 to build a digital controlled voltage source , both ideas have the same lack , if you multiply (first case) or add (the second case) an imprecise value to a precise value you will have an imprecise value at the end. Sorry for the criticism , I'm still searching for an working solution.

13. ### mdorianMember

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Improved version of digital pot solution, the Iadj will go through the op amp repeater output , not through R1 R2 divider

You will need a calibration procedure because the digital pot have a tolerace of 20%.

Last edited: Aug 31, 2009
14. ### Suraj143Active Member

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I need a smooth DC not pulsating DC.

mdorian

In your last diagram where to connect the PIC?

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16. ### mdorianMember

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Last edited: Aug 31, 2009