Never seen such. Trimpots usually span their full spec'd range.
John
basic properties of a trimmer pot
Aug 25, 2005
9 Replies
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
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