bad precision pot?

[Martin Czech]

:::>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.
:::

Of course, limit the range to the necessary. For precision circuits it
is not recommended to use trimmers as resistor, better is to use them
as voltage divider if possible, this will cancel most of the TC. The
voltage can then be further attenuated via resistor network.

I should really have a look at my trimmers at home, are the garden variety
carbon types really better? I doubt.


m.c.