bad precision pot?

[jorgen.bergfors at idg.se]

:::Well, the settability isn't always as good
:::as you might expect. You often have to turn quite a bit befor anything
:::happens. And if you turn too far and have to turn back, there is even more
:::"dead" travel befor the setting changes. I'm not sure if the setting
:::changes if you knock on them, but it could. Read Bob Pease's book and
:::see what he thinks about multiturn trimmers...

>I always use these trimmers, almost never carbon types.  I have never
>experienced exessive "dead gear", but I have never checked if they
>change with vibration. This could be. But the usual carbon types have
>wipers that also act like a spring, there is mechanical strain in order
>to get contact, there is certainly a "dead travel" and maybe also change
>with vibration.

I too always use the cermet trimmers. No carbon trimmers at all. Mostly I use single turn cermets and sometimes 15 turn.
My experience is that you get maybe 3 or 4 times the setting precision with a 15 turn. And not 15 times as you might expect.
It does not work to put in a multiturn if the trimming range is too large to start with. Like fine tuning the VCO frequency with a trimmer connected directly to 15 volts and ground. I have tried this and it was sensitive to knocks as well as some sort of random quantisation due to limited resolution. I don't know what caused the latter. It is always better to lower the range by putting fixed resistors in series with the trimmer. Then you often don't need a multiturn at all.

/Jorgen