# basic properties of a trimmer pot

Discussion in 'Electronic Basics' started by Jim Alexander, Aug 24, 2005.

1. ### Jim AlexanderGuest

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?

2. ### John LarkinGuest

Never seen such. Trimpots usually span their full spec'd range.

John

3. ### Andrew HolmeGuest

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.

4. ### John FieldsGuest

---
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%.

5. ### Jim AlexanderGuest

]> 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.

6. ### Tom MacIntyreGuest

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

Tom

7. ### Jasen BettsGuest

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

8. ### Bob MyersGuest

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

Bob M.

9. ### ehsjrGuest

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