basic properties of a trimmer pot

I've never actually designed a circuit that used a trimmer pot, and I'm a bit ignorant of their basic properties, and I do not have any samples on hand I could just hook up to my DMM. From the way their specs are specified, it sounds like they really live up to their name: the range of resistance they offer is very narrow. For instance, a 1M trimmer pot would offer resistances from about .9M to 1.1M over its range of adjustment. Contrast this with a regular 1M potentiometer which can offer resistances from near 0 all the way up to its 1M spec. Is this correct?

I want to use a potentiometer in the RC circuit input of a monostable multivibrator in order to adjust the output pulse length. The pulse length will need to be somewhere between 4 and 8 seconds, and I want fine control over that whole range. I was considering the use of a trimmer pot, mostly just because they are so small - the final device will have 7 of these, so space is an issue. This time constant will only be set once for each multivibrator, so the durability of trimmer pots isn't a problem. However, if the resistance range of a typical trimmer pot is only 10%, though, then I don't see how I could obtain a time constant over such a wide range. Are there variable resistance devices available with a wider resistance range, and big enough values to obtain time constants in the seconds range, but with the small footprint of a trimmer pot?

--

________ Jim Alexander __________________ jalex@cis.upenn.edu ________________
I have yet to see a problem, however complicated, which, when you looked at it
in the right way, did not become still more complicated.      -- Poul Anderson

No. Where did you get that idea from? Trimmer pots go from zero to max. resistance like any other pot. If you've seen one specified with a 10% tolerance, that relates to the max. resistance not the adjustment range. All resistors, fixed and variable, are manufactured to a specified tolerance.

--
No, a trimpot is the same as a regular pot, but smaller and
sometimes with a greater mechanical range of adjustment, say ten or
twenty turns instead of less than one for a regular pot.  What
you\'re thinking about is the total resistance a 1 megohm trimpot
with a tolerance of +/-10%.

In article , Andrew Holme wrote: ]Jim Alexander wrote: ]> I've never actually designed a circuit that used a trimmer pot, and ]> I'm ]> a bit ignorant of their basic properties, and I do not have any ]> samples ]> on hand I could just hook up to my DMM. From the way their specs are ]> specified, it sounds like they really live up to their name: the range ]> of resistance they offer is very narrow. For instance, a 1M trimmer ]> pot would offer resistances from about .9M to 1.1M over its range of ]> adjustment. Contrast this with a regular 1M potentiometer which can ]> offer resistances from near 0 all the way up to its 1M spec. Is this ]> correct? ] ]No. Where did you get that idea from? Trimmer pots go from zero to max. ]resistance like any other pot. If you've seen one specified with a 10% ]tolerance, that relates to the max. resistance not the adjustment range. ]All resistors, fixed and variable, are manufactured to a specified ]tolerance.

As I said, ignorance ;-) I was led down this incorrect path by the name "trim pot" combined with some ambigous wording in a data sheet I downloaded - it could have meant tolerance across samples or a small resistance range in each part. I am glad to be informed that it is the former.

--

________ Jim Alexander __________________ jalex@cis.upenn.edu ________________
I have yet to see a problem, however complicated, which, when you looked at it
in the right way, did not become still more complicated.      -- Poul Anderson

No, but a 100k trimmer on a 1M resistor will give you 1M to 1.1M.

Tom

Or a 200k trimmer on the 900k resistor would give you the originally- mentioned 0.9 - 1.1M range, of course.

Bob M.

trimmer pots are just like regular pots only without the spindle for mounting a knob. they are used to save space and to save money where adjustment is a once only or infrequent thing.

I think that 0.9 to 1.1 you see in the specs is the variation in total range of the pots that come from that manufacturer.

IE you order a 1.0M pot and you might really get a 900k or a 1.1M but the pots will be adjustable from somewhere near 1m right down to 0

7 RC ooscilators - are you tring to generate DTMF signals... there's cheap chips built just for that...

if it's not DTMF something needing 7 timers looks like a good task for a small mictorontroller and you get a crystal reference for your timebase which should make it more reliable in harsh environments etc...

Bye. Jasen

--
Right.  he needs something where the fixed resistor plus the pot
cranked to max R will result in a little bit longer that his longest
desired output time and the fixed resistor with the pot cranked to
min R will result in the shortest output time.

But he wants an RC of anywhere from 4 to 8 seconds: "I want to use a potentiometer in the RC circuit input of a monostable multivibrator in order to adjust the output pulse length. The pulse length will need to be somewhere between 4 and 8 seconds, and I want fine control over that whole range. "

A series 900K resistor and 200K trimmer won't do it.

Ed