I built the following voltage regulator and do not have as fine as control as I would like. First I used a regular 1 turn pot, then a 10 turn pot but when I turn the pot, the numbers jump 2 or 3 hundredths of a volt at a time. The 10 turn pot is a 10K pot instead of the 5k pot that the circuit called for but I tried it a couple of ways;first with a 10K ohm in parallel which made it a 5k pot like the circuit called for, then at the 10k rating. Still no change. It still bounces around. Am I asking too much to have the numbers change with more control?

Thanks

CJ
​

 

Hello,

Do you mean that the ten turn potentiometer is making "steps" of 0.03 - 0.04 volts?
If you want to adjust "fine" use a second potentiometer of 100 Ohms in series with the original potentiometer.
The original potentiometer will be "course" adjust and the 100 Ohms "fine" adjust.

Greetings,
Bertus

 

Does the final setting also jump by .01 volt at a time? Or is the control noise only evident as the setting is changed? Is it a new or a junk box 10 turn pot? What happens if you use a 22 turn trimmer?

At any rate, you can try a .1 uF capacitor to ground from the pot's wiper.

 

Your value for R1 is 240 ohms which is for the more expensive LM117. The cheaper LM317 needs to have a value that is 120 ohms max and then the value of the pot also must be half.
Or the output voltage will rise when there is no load.

 

I built the following voltage regulator and do not have as fine as control as I would like. First I used a regular 1 turn pot, then a 10 turn pot but when I turn the pot, the numbers jump 2 or 3 hundredths of a volt at a time. The 10 turn pot is a 10K pot instead of the 5k pot that the circuit called for but I tried it a couple of ways;first with a 10K ohm in parallel which made it a 5k pot like the circuit called for, then at the 10k rating. Still no change. It still bounces around. Am I asking too much to have the numbers change with more control?​

Thanks​

CJ​

I think you need to remove the 10uFd capacitor off of the ADJ terminal. That can cause the LM317 to oscillate.

hgmjr

 

I think you need to remove the 10uFd capacitor off of the ADJ terminal. That can cause the LM317 to oscillate.

hgmjr

No.
The datasheet recommends the capacitor to improve ripple rejection by 15dB and to improve the transient response.

 

Your value for R1 is 240 ohms which is for the more expensive LM117. The cheaper LM317 needs to have a value that is 120 ohms max and then the value of the pot also must be half.
Or the output voltage will rise when there is no load.

Actually, both the LM117 and LM317 load regulation specifications are given with load on the output that is 10mA <= load <= Imax.

I believe you're thinking of the LM317L (100mA version), which requires a minimum load of 5mA for guaranteed regulation.

For the benefit of our OP, since the regulator requires a minimum 10mA load for guaranteed regulation, and Vref (measured between ADJ and OUT) may be anywhere from 1.2v to 1.3v, a 120 Ohm resistor between ADJ and OUT causes the necessary minimum current to flow from the OUT terminal.

 

I think you need to remove the 10uFd capacitor off of the ADJ terminal. That can cause the LM317 to oscillate.

hgmjr.


no i think it is for filtering the output voltage

and D-2 should be in between output of 317 & C-3 to the J-2(output)
and D-1 may not be required i think so


Do you mean that the ten turn potentiometer is making "steps" of 0.03 - 0.04 volts?
If you want to adjust "fine" use a second potentiometer of 100 Ohms in series with the original potentiometer.
The original potentiometer will be "course" adjust and the 100 Ohms "fine" adjust.

Greetings,
Bertus



and yes it is possible to have the fine tuning in the ref. ckt.

 

no i think it is for filtering the output voltage

Location is everything in electronics. The adjust point is a control point, you don't need the capactor there. The diodes are only needed if you are worried about current back flowing, such as what a large capacitor on the output of the regulator might create.

 

Location is everything in electronics. The adjust point is a control point, you don't need the capacitor there. The diodes are only needed if you are worried about current back flowing, such as what a large capacitor on the output of the regulator might create.

yes you are right
the location is everything in the electronics ckt's.
the capacitor should be in between ground (0)and supply(+) no need of D2 in between
The adjust point is a control point, you don't need the capacitor there.
yes the adj. point is a control point you don't required any capacitor there the C-2 should be in the place of C-3
sorry i can't draw the ckt. as i don't have the s/w for the same
pl.ref. any free s/w for the ckt. drowning
thanks

 

No.
The datasheet recommends the capacitor to improve ripple rejection by 15dB and to improve the transient response.

I found the circuit that the OP is using in the LM317 in the National Semiconductor datasheet. I see that the use of a capacitor is recommended. I missed that circuit in previous datasheet reads. Very interesting.

hgmjr

 

I built the following voltage regulator and do not have as fine as control as I would like. First I used a regular 1 turn pot, then a 10 turn pot but when I turn the pot, the numbers jump 2 or 3 hundredths of a volt at a time. The 10 turn pot is a 10K pot instead of the 5k pot that the circuit called for but I tried it a couple of ways;first with a 10K ohm in parallel which made it a 5k pot like the circuit called for, then at the 10k rating. Still no change. It still bounces around. Am I asking too much to have the numbers change with more control?

Thanks

CJ
​

A 10k pot with 5mA passing through it will give you 5V per turn for a 10 turn pot. That's about 14mV per degree of rotation. That's why you are having problems.

 

You absolutely need the cap! Especially when using a something like a pot or a switch to control the voltage.

My favorite bench supply was modified, I changed the main voltage adjust pot to a 10-turn pot (full sized, not a trimpot). It's nice, I can tweak the output voltage to within a couple mV.