Hi, All,

can anybody tell if conductive plastic pots are non-inductive and can be
used for HF/50MHz ?
Same question re cermet pots.

Thks

Ivan Makarov wrote:
Hi, All,

can anybody tell if conductive plastic pots are non-inductive and can be
used for HF/50MHz ?
Same question re cermet pots.

The short answer is yes, but the long answer, as always, is more involved.

Nothing is truly non-inductive -- even a short straight piece of wire
has inductance. But the inductance of a conductive plastic or cermet pot
is only due to the physical length of the element. That is, it's as
small as it can possibly be for the physical size of the pot. That's in
contrast to a wire-wound pot which has many turns and a correspondingly
much higher inductance.

Whether you can use them at a given frequency depends on the particular
application. If you can tolerate a wire at that point in the circuit
which is the length of the pot's element (plus the distance to the
external connections), then you can tolerate the pot. Otherwise, you
might or might not be able to use the pot, depending on the relationship
between the inductance and resistance.

Because the element length is shorter, physically smaller pots have less
inductance than larger ones.

Roy Lewallen, W7EL

Thanks, Roy,

and multiturn conductive plastic/cermet pots still have the same flat
element, say , 340 degree, but incude a vernier dial that changes turns
ratio?

Thks

"Roy Lewallen" wrote in message
...
Ivan Makarov wrote:
Hi, All,

can anybody tell if conductive plastic pots are non-inductive and can be
used for HF/50MHz ?
Same question re cermet pots.

The short answer is yes, but the long answer, as always, is more involved.

Nothing is truly non-inductive -- even a short straight piece of wire
has inductance. But the inductance of a conductive plastic or cermet pot
is only due to the physical length of the element. That is, it's as
small as it can possibly be for the physical size of the pot. That's in
contrast to a wire-wound pot which has many turns and a correspondingly
much higher inductance.

Whether you can use them at a given frequency depends on the particular
application. If you can tolerate a wire at that point in the circuit
which is the length of the pot's element (plus the distance to the
external connections), then you can tolerate the pot. Otherwise, you
might or might not be able to use the pot, depending on the relationship
between the inductance and resistance.

Because the element length is shorter, physically smaller pots have less
inductance than larger ones.

Roy Lewallen, W7EL

Ivan Makarov wrote:
Thanks, Roy,

and multiturn conductive plastic/cermet pots still have the same flat
element, say , 340 degree, but incude a vernier dial that changes turns
ratio?

I've seen pots of both types, but I think the only ones I've seen with a
multi-turn helical element were wirewound. I'd say look at the shape of
the pot -- if it's a flattened cylinder like a conventional pot, it's
probably got a single flat element. On ones I have like that, the
planetary vernier drive is obvious. But if it's shaped more like a tin
can, I'd suspect a multi-turn element.

Roy Lewallen, W7EL

Small cermet pots should be no trouble at 50MHz, assuming you aren't
going to extreme resistance values. You will always have parasitic
inductance and capacitance, but if a carbon pot would work in your
application, a cermet of the same size and general design should behave
similarly.

Beware of the conductive plastic ones...some of those are "hybrid" pots
that have a wirewound element under the plastic, to get very good
linearity, and the conductive plastic provides interpolation between
the turns of wire. I once used one of those to read out an angle, and
it worked great, but I wouldn't consider trying to use it at RF. If
it's not a hybrid (no wirewound element under the plastic), it should
be OK.

You may also want to consider the amount of (RF?) current in the wiper.
It's generally a good idea to keep the wiper current low, unless the
pot is specifically designed for high wiper current.

Cheers,
Tom

Tom,

how does one know if it is of a 'hybrid' type? Is it in the datasheet?

Thks

"K7ITM" wrote in message
oups.com...
Small cermet pots should be no trouble at 50MHz, assuming you aren't
going to extreme resistance values. You will always have parasitic
inductance and capacitance, but if a carbon pot would work in your
application, a cermet of the same size and general design should behave
similarly.

Beware of the conductive plastic ones...some of those are "hybrid" pots
that have a wirewound element under the plastic, to get very good
linearity, and the conductive plastic provides interpolation between
the turns of wire. I once used one of those to read out an angle, and
it worked great, but I wouldn't consider trying to use it at RF. If
it's not a hybrid (no wirewound element under the plastic), it should
be OK.

You may also want to consider the amount of (RF?) current in the wiper.
It's generally a good idea to keep the wiper current low, unless the
pot is specifically designed for high wiper current.

Cheers,
Tom

Hi Ivan,

Yes, I'd expect it to say its a hybrid pot in the data sheet. They
tend to be quite expensive.

What are you doing with this particular pot? It always helps in giving
a useful answer if we have all the relevant information, and "50MHz"
isn't a whole lot of information.

Cheers,
Tom

Tom,

it is the old good ARRL noise bridge. It has a pot in its output bridge
circuit.
I changed the layout of the elements a little from what is in the Handbook,
and found it works reasonably well up to 50MHz. The signal is still audible
on 2m, but I did not concentrated on evaluation of the circuit on that band.
The original question came up because I had unacceptable results with the
carbon pots (they are not good at their minimum level). My hope was cermets
or plastic ones may have better stability.
Unless the pot is of a wirewound type, parasitic inductance of the element
could be compensated as described in the article.
And because the null can be very sharp, a multiturn pot would be of a big
help.

My best regards,
Ivan


"K7ITM" wrote in message
oups.com...
Hi Ivan,

Yes, I'd expect it to say its a hybrid pot in the data sheet. They
tend to be quite expensive.

What are you doing with this particular pot? It always helps in giving
a useful answer if we have all the relevant information, and "50MHz"
isn't a whole lot of information.

Cheers,
Tom

the single turn variety and the cemets are non inductive
Saandy 4Z5KS