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### Topic: Coarse and fine adjustment knobs (Read 5598 times)previous topic - next topic

#### btfdev

##### May 03, 2012, 04:33 pm
For a typical lab power supply with adjustable coarse and fine knobs for voltage adjustment, can anyone tell which knob uses a 10 turn pot and which one uses 1 turn?

#### MarkT

#1
##### May 03, 2012, 05:14 pm
Neither, coarse+fine is an alternative to a 10-turn pot.  A good 10-turn pot gives better stability I suspect.  However they are tedious to adjust all the time - best used for a situation where fine tuning is a common operation.

For a lab power supply you often want to zoom between 5 and 12V, say, and occasionally want to carefully set the value with the fine knob - the split into coarse and fine works well here.

I suppose you could have a 10-turn as "coarse" and a standard pot as "very fine" adjustment...  Certainly the tolerance and stability of the coarse-adjust pot is the more important.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### btfdev

#2
##### May 03, 2012, 10:58 pm
Thanks for the response.

Lets start again by rephrasing the question a bit.

If we are to build a lab power supply equipped with both coarse and fine knobs, which combination of pots do we need? Here are the options:

(i) 10-turn 10K
(ii) 1-turn 1K
(iii) 10-turn 1K
(iv) 1-turn 10K

Assuming we pick two from above and join them in series. Of course the operations would be:
- Use coarse knob to turn from 0V to 12V quickly
- Use fine knob to fine tune (i.e. 12V to 12.20...etc)

Thanks, BTFDev

#### robtillaart

#3
##### May 04, 2012, 09:27 am

recently did this - http://arduino.cc/forum/index.php/topic,104236.0.html -

same construct could be used to get a 16 bit value that controls an DAC ?

placing 2 pots in series like this should work, the 3rd resistor (e.g. 10K) restricts the current be

+--------+        +--------+
|        |        |        |
v        |        v        |
+---------+   |    +---------+  |    +-----------+
GND ---|         +---+----|         +--+----|           +-----24V
+---------+        +---------+  |    +-----------+
|
|
|
+
Output

It is quite simple to build and test with a voltmeter...
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

#### jackrae

#4
##### May 04, 2012, 12:55 pm
10k for the coarse with 1 k for the fine.  The 1k is wired as a 2-terminal rheostat (wiper connected to one of the other two terminals) and in series with the supply to the upper terminal of the 10k. The lower terminal of the 10k goes to ground and the variable sense voltage comes from the 10k wiper terminal.  Of course you cannot use such an arrangement to supply "power"; only extremely low currents or as a sense voltage to your regulator system.

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