basic properties of a trimmer pot

[Jim Alexander]

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

 

[John Larkin]

On Wed, 24 Aug 2005 21:07:58 +0000 (UTC), jalex@cis.upenn.edu (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.

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

John

 

[Andrew Holme]

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.

 

[John Fields]

On Wed, 24 Aug 2005 21:07:58 +0000 (UTC), jalex@cis.upenn.edu (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, 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%.

--
John Fields
Professional Circuit Designer

 

[Jim Alexander]

In article <deioli$ajl$1$8302bc10@news.demon.co.uk>,
Andrew Holme <andrew@nospam.com> 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

 

[Tom MacIntyre]

On Wed, 24 Aug 2005 21:07:58 +0000 (UTC), jalex@cis.upenn.edu (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, but a 100k trimmer on a 1M resistor will give you 1M to 1.1M.

Tom

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?

 

[Jasen Betts]

In article <deinje$adls$1@netnews.upenn.edu>, 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.

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.

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 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

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.

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

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.

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

 

[Bob Myers]

"Tom MacIntyre" <tom__macintyre@hotmail.com> wrote in message
news:l59sg1ls1hl0dtntvee25c5hpdnmt70dle@4ax.com...

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

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

Bob M.

 

[ehsjr]

Bob Myers wrote:

"Tom MacIntyre" <tom__macintyre@hotmail.com> wrote in message
news:l59sg1ls1hl0dtntvee25c5hpdnmt70dle@4ax.com...


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



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

Bob M.

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

 

[John Fields]

On Sat, 27 Aug 2005 21:58:02 GMT, ehsjr <ehsjr@bellatlantic.net>
wrote:

Bob Myers wrote:
"Tom MacIntyre" <tom__macintyre@hotmail.com> wrote in message
news:l59sg1ls1hl0dtntvee25c5hpdnmt70dle@4ax.com...


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



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

Bob M.



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.

---
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.

--
John Fields
Professional Circuit Designer