Why are transparent trimpots so rare?

[Joerg]

While stuffing a BOM with Mouser part numbers for a prototype order I
found that pretty much all the multi-turn trim-pots were in stock.
Except for the transparent ones. None, zip. Arrgh.

How can that be? I was wondering why anyone would want to use a
non-transparent trim-pot where you cannot see the position it is in. So
the demand for transparent ones should be a lot higher.

--
Regards, Joerg

http://www.analogconsultants.com

 

[Jim Thompson]

On Sun, 10 Dec 2006 17:20:09 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

While stuffing a BOM with Mouser part numbers for a prototype order I
found that pretty much all the multi-turn trim-pots were in stock.
Except for the transparent ones. None, zip. Arrgh.

How can that be? I was wondering why anyone would want to use a
non-transparent trim-pot where you cannot see the position it is in. So
the demand for transparent ones should be a lot higher.

"So rare. So rare."

Wasn't that a top ten tune hit from the late '50's ?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Joerg]

John Fields wrote:

On Sun, 10 Dec 2006 21:11:34 GMT, Joerg
notthisjoergsch@removethispacbell.net> wrote:



This is for prototyping and we need some fine adjustment capability for
experiments. It's a one-off board, basically to see whether the concept
I came up with is feasible and recovers nicely and fast enough when
kicked in the shins. My designs for production never contain pots or
variable caps either (we used to call that "rubber engineering" in the
80's).


---
I worked for Racal-Milgo in the 70's, and one of the things we were
never allowed to do was to let pots leave the premises. I learned a
lot about slop and how to keep it from biting you in the ass back
then.


Good policy. I remember Racal as a great company.

If other mainstream companies do use lots of potmeters in their products
that, to me, is not an indication that it's a good thing. Not at all.

--
Regards, Joerg

http://www.analogconsultants.com

 

[Guy Macon]

Joerg wrote:

While stuffing a BOM with Mouser part numbers for a prototype order I
found that pretty much all the multi-turn trim-pots were in stock.
Except for the transparent ones. None, zip. Arrgh.

How can that be? I was wondering why anyone would want to use a
non-transparent trim-pot where you cannot see the position it is in. So
the demand for transparent ones should be a lot higher.

I have, on occasion, used two single-turn pots for course and
fine adjustment of a parameter.

As for temperature and plastics:

Typical Deflection Temperatures of
Polymers at 0.46 MPa (degrees C)

Polyethylene Terephthalate (PET)......70
Acrylic...............................95
Polyethylene, HDPE....................85
Polystyrene...........................95
ABS...................................98
Polypropylene........................100
Polycarbonate........................140
ABS + 30% Glass Fiber................150
Acetal Copolymer.....................160
Nylon 6..............................160
Polypropylene + 30% Glass Fiber......170
Acetal Copolymer + 30% Glass Fiber...200
Nylon 6 + 30% Glass Fiber............220
PET + 30% Glass Fiber................250

Guy Macon
<http://www.guymacon.com/>

 

[Charlie Edmondson]

Jim Thompson wrote:

On Sun, 10 Dec 2006 21:40:24 GMT, Joerg
notthisjoergsch@removethispacbell.net> wrote:


Lostgallifreyan wrote:


[snip]

The point could be debated beyond inanity but there's no denying that basic
visual feedback of position of a moving object is a sensible requirement.


Amen! It is somehow a good thing to come back from lunch and be able to
verify that nobody had turned the trim-pot.


In my more slender disciplined days I ate 6 almonds for lunch... then
walk a few miles.

So while my troops had headed out to McD's or whatever, I'd roam the
labs seeing what they were up to.

The most fun I ever had was checking out a junior engineer's
breadboard.

He had complained of strange behavior before he left for lunch.

I found the mis-wire and fixed it.

When he returned from lunch and fired up his breadboard he was very
befuddled.

I didn't tell him for two hours ;-)

...Jim Thompson
Of course, then there is software...

A former boss of mine told the story of a smart munition they were
testing, and artillery shell that was supposed to seek an armored
target. It worked great in the lab, and in their tests, and when it
came time to do the official demonstration tests for the military, he
had all the programming connectors cut off, and strictly announced "No
Changes!"

Came the day of the test, they fired the shell, and it zigged left when
it should have zagged right. They failed and lost the contract.

During the investigation afterwards, one of the software engineers said
"It shouldn't have done that! The update I did didn't touch that code..."

Update? What update?

Why, the update I did the night before the test. I had this great idea,
but you wouldn't believe how long it took me to solder on new connectors
so I could re-progam the thing...

Moral of the story: You can make something fool proof, but you can't
make it damn fool proof!

Charlie

 

[Lostgallifreyan]

Guy Macon <http://www.guymacon.com/> wrote in news:Ev2dnSbkQqpL5-
PY4p2dnA@giganews.com:

Typical Deflection Temperatures of
Polymers at 0.46 MPa (degrees C)

Polyethylene Terephthalate (PET)......70
Acrylic...............................95
Polyethylene, HDPE....................85
Polystyrene...........................95
ABS...................................98
Polypropylene........................100
Polycarbonate........................140
ABS + 30% Glass Fiber................150
Acetal Copolymer.....................160
Nylon 6..............................160
Polypropylene + 30% Glass Fiber......170
Acetal Copolymer + 30% Glass Fiber...200
Nylon 6 + 30% Glass Fiber............220
PET + 30% Glass Fiber................250

Is PET dramatically unusual in combination with glass fibre, or is that top
one meant to be lower, with value 170?

 

[Rich Grise]

On Sun, 10 Dec 2006 20:19:22 +0000, Lostgallifreyan wrote:

John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in

But why use multiturn pots? You can see the position of single-turns,
and they have about the same working resolution and maybe even better
stability.

If you trim the current of a DPSS laser pump diode, the last thing you want
is some sticky one-turn pot to jerk through the max current point and fry
the diode. I've used both types of preset, and I find it's worth using
multiturn types even where single turn types might do, they work much
better when setting accurately. I take the point about stability, but I
don't think it's any better. Even single-turn pots have spring tension that
can upset it after you leave it. With a multiturn you can turn back a
fraction to release this, there is enough hysteresis there to allow it
safely. Try doing that with a signle-turn pot, and you'll be tweaking for
half an hour trying to repeat the same value, let alone make it stay there.

What's really frustrating is when your desired set point is between two
windings on a wirewound. At this one lab, the boss would ask, "Is that
in-fine-ite-ly variable?" and, sadly, I'd have to answer "no." It's
too bad that the R&D departmant at the company were such idiots - when
it's that sensitive, they should have used a much smaller value pot,
with a fixed resistor at either end, as needed.

Cheers!
Rich

 

[Rich Grise]

On Sun, 10 Dec 2006 21:13:12 +0000, Lostgallifreyan wrote:

"Bob Eld" <nsmontassoc@yahoo.com> wrote in
"Lostgallifreyan" <no-one@nowhere.net> wrote in message
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in

But why use multiturn pots? You can see the position of
single-turns, and they have about the same working resolution and
maybe even better stability.

If you trim the current of a DPSS laser pump diode, the last thing
you
want
is some sticky one-turn pot to jerk through the max current point and
fry the diode. I've used both types of preset, and I find it's worth
using multiturn types even where single turn types might do, they
work much better when setting accurately. I take the point about
stability, but I don't think it's any better. Even single-turn pots
have spring tension
that
can upset it after you leave it. With a multiturn you can turn back a
fraction to release this, there is enough hysteresis there to allow
it safely. Try doing that with a signle-turn pot, and you'll be
tweaking for half an hour trying to repeat the same value, let alone
make it stay
there.

So, do you set the current in the laser diode by the position of the
pot wiper or do you actually measure something? Something tells me you
don't use the wiper position as an indicator of the diode current.

No, because I can't see it. Seriously, the Chinese DPSS lasers often have
IC names ground off and reverse engineering is made to be difficult, so
instead of measuring an exact current when you don't know what it should
be, you just note the current position and the number and size and
direction of steps taken in turning. A lot of laser adjustments are made
this way, being optical so the habit forms.

The point could be debated beyond inanity but there's no denying that basic
visual feedback of position of a moving object is a sensible requirement.

I once worked at a place that made, essentially, a SEM. The display,
voltage, focus, magnification etc. was all digital, controlled with
pushbuttons. One day, the boss was trying to focus in on some sample,
using those stupid buttons, and he said, "I'd rather have a knob."

Cheers!
Rich

 

[Rich the Newsgroup Wacko]

On Mon, 11 Dec 2006 08:33:55 +0000, Jan Panteltje wrote:

You speak Dutch?

I can say, "zaadvragende ogen." ;-)

Cheers!
Rich

 

[Rich the Newsgroup Wacko]

On Mon, 11 Dec 2006 09:06:00 -0800, Joel Kolstad wrote:

"Ethan" <idethan@yahoo.com> wrote in message

So who is still using trim pots these days?

Not too many people?

Trimmer caps seem a little more difficult to get rid of than trim pots... even
in stuff like reasonably high volume commercial filters, you still see the
occasional trimcap. Most cavity and waveguide filters sold commercial also
still have visible tuning rods, except on the highest volume items just as
satellite TV receivers.

I'd like to see someone build a trimcap that changes the ratio of Ca/Cb while
simultaneously being "tapered" such that Ca || Cb is constant -- that'd be the
perfect tuning device for a tapped-capacitor matching network.

That shouldn't be hard - just use two stators and a semicircular rotor:
http://www.abiengr.com/~sysop/images/dual-stator-cap.png

Would that give you what you want?

Cheers!
Rich

 

[Joel Kolstad]

Hi Rich,

"Rich the Newsgroup Wacko" <wacko@example.net> wrote in message
newsan.2006.12.12.21.06.25.738547@example.net...

That shouldn't be hard - just use two stators and a semicircular rotor:
http://www.abiengr.com/~sysop/images/dual-stator-cap.png

This link isn't working, but from what you describe...

Would that give you what you want?

Yes it would... although I'd be wanting it in a tiny package too!