0-30volts Lab power supply

[pier]

I have made the 0-30 volts lab power supply which is there on this site . I want to know if it was possible to add a FINE TUNE provision for accurate selection of the O/P voltage . Please help me .

[shafton]

I dont think it is possible, or else the author would have done that .

[audioguru]

Hi Pier,
Of course you can add a fine tuning pot. Just add it in series with the existing pot. Make it have a value less than the existing pot. You might need to reduce the value of the existing pot a little.

[pier]

Thanks a lot AG , I was about to wind up after shaftons reply . Will this changes be sufficient ?

[esp1]

hi pier,
Assuming that the existing 10K pot gives you a linear range of 0 to 30Vout, then the 100R [centred] will give you a +/- 0.15V adjustment, that should be OK.

[pier]

hello esp1
  Actually i just guessed with no practical calculations . But since u say its going to be fine i just have to try it out . But since AG said you should reduce the value of the existing pot as well, I am confused .

[audioguru]

The 10k resistor provides an output voltage range of from 0V to about 34 (but the project can't go that high). Since 100 ohms is 1/100th of 10k then its fine tuning range is from 0V to only 0.34V. The 10k pot doesn't need to be changed.

This is the voltage setting pot which is a voltage divider. I was thinking about adding another fine tuning pot in series with the current setting pot which would change the amount of current and require a lower resistance for the original pot so the total resistance is correct.

Many people found the voltage and current of the project reachs max before the pots are at max. They fix it by adding calibration trimpots in series with the existing pots.

[pier]

Hi AG
        So is the 100 Ohms pot fine for the fine tune . What should be the value if i need to get a fine tune of one volt ?

[ante]

I use this type (10-turns) of potentiometers for PSU

[audioguru]

So is the 100 Ohms pot fine for the fine tune . What should be the value if i need to get a fine tune of one volt ?

If 10k ohms gives 34V then 10k/34= 294 ohms gives 1V.
It won't be accurate since pots have a tolerance of 20%.

[pier]

Thanks for the value AG but is it a standard value that we get ? . I feel ante's selection is a good one but do you call them a multiturn pot ? or does it have a specific name ??? ?

[shafton]

sorry for wrong information

shafton

[audioguru]

Pots are made only in certain resistance values and their resistance has a tolerance of about 20%. So you won't get one that is 294 ohms. Maybe 200 ohms and 500 ohms are the closest.
Use a multi-turn pot instead like Ante uses.

[ante]

For these multi-turns I try to find heavy machined aluminum knobs with a small crank or one with a finger hole; this gives you a smooth and accurate feeling.

[pier]

Hi Ante
          Do You mean the outer diameter of the knob should be big for lesser torque we apply, so that it goes smooth ? Can u give me a picture of the knob you use ?

[ante]

Yes, bigger and heavier gives a nice feeling when incrementing the voltage. Here are a few examples of crank-type knobs: http://www.surplussales.com/ShaftHardware/Knobs-12.html I am sure you could find many more of a similar type at other suppliers. The finger hole knobs are often used for tuning DX-receivers and other ham stuff.

[pier]

Thanks Ante I've got the Idea now .