Nurse Quacky Pot Replacement

[jimmy54]

Hi fellas,

I'm going to build a nurse quacky and can source all the parts from a place in the UK (http://www.rshelectronics.co.uk/index.htm).  The only part they haven't got is the 25k linear pot for the range control (the closest they have is 10K or 47K).  Would use of a 47k be very detrimental to the project?  Are there any ways around this (apart from sourcing a 25k from somewhere else) or could you just use a 47k pot and only turn it up to the 27k point (if you get what I mean)?

Cheers

[B Tremblay]

You can use a 47k pot with a 47k resistor soldered between lugs 3 and 1 - that will get you close enough.  There's only a small section within the 25k rance that's actually useful anyway.

[jimmy54]

Cheers B Tremblay, that makes sense.  Are you effectively putting the resistor in parallel with the pot to give a resistance of  23.5K?

[B Tremblay]

Exactly!

[Mark Hammer]

I'm going to suggest, instead, getting a 10k pot and sticking a 6.8k fixed resistor on one side and 8.2k on the other.  That will give you 25k and will probably result in the full rotation of the pot actually being useful.

[B Tremblay]

I heartily second Mark's suggestion.  The window of useful settings isn't very wide with that control.