# Adjustable power supply too sensitive

Discussion in 'The Projects Forum' started by Max12345, Oct 18, 2013.

1. ### Max12345 Thread Starter Member

Aug 27, 2013
63
0
Hi
I'm almost too embarrassed to post this problem here as this LM388 chip has 10 pages devoted to it in this forum, but I couldn't find anything on sensitivity problems.
I built this circuit using a 16V 7A transformer with a 3 300uF smoothing cap. The LM388 has a heatsink and a mica washer with paste on both sides of the washer.
My problem is that the pot to vary the output voltage is too sensitive and I would like to reduce this sensitivity.
I have detailed the issue on the attachment, with some questions at the end.
Thank you
Max

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2. ### wayneh Expert

Sep 9, 2010
14,862
5,346

Smaller pot? That's a good option if you don't need full range adjustment. Consider two pots for a coarse and fine arrangement if you do.
I imagine that 10:1 or 5:1 is a better choice than 100:1.

If you do the math to figure out the min and max resistances you need, it would become more obvious what arrangement of resistors you need. I usually try to get the target voltage near the center of the variable resistor's range. Use fixed resistors to ensure the user can't turn the pot down to zero ohms.

tailoring potentiometers.pdf

This and this, "The Secret Life of Pots", too.

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3. ### w2aew Active Member

Jan 3, 2012
219
64
What you can do is put a fixed resistor in series with the variable resistor. Pick the fixed resistor so that the output will be the minimum desired output (with pot dialed to min). Pick the pot so that the sum of the pot (at max value) plus the fixed resistor give you the max desired output.

4. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
510
Put two pots in series, the smaller (fine control) being about 10% of the larger one.

5. ### Max12345 Thread Starter Member

Aug 27, 2013
63
0
Greetings all
Thanks to all for your replies. They have been of great help.

Thanks to you and bountyhunter for the tip on fine pot size. It seems that 10% of the coarse pot is the way to go.

w2aew, your tutorials look amazing...I will definitely spend some time on your site. Thanks.

The attachment shows the 2 options I am going to try.
I am not too sure about what the last paragraph means, though...

Max

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6. ### wayneh Expert

Sep 9, 2010
14,862
5,346
The first comment is noting that the end-to-end resistance across the pot is changed when you tailor it. This changes the current flowing through it. If the circuit was designed for just enough current through, say, a 5k pot and you've changed that to 1k, there could be a problem. This concern is not relevant in your application. I'm pretty sure I don't follow their 2nd comment.

Max12345 likes this.
7. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,432
1,312
It's important to put the 10uF cap between the ADJ pin and ground, NOT across the pot.

That will ensure stability even when (during) the switch is flipped.

8. ### Max12345 Thread Starter Member

Aug 27, 2013
63
0
Great. Thanks for the tip, THE_RB.