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Why are transparent trimpots so rare?

J

Joerg

Jan 1, 1970
0
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.
 
J

Jim Thompson

Jan 1, 1970
0
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
 
S

Spehro Pefhany

Jan 1, 1970
0
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.

Duuno, but I never figured out why US manufacturers could get away
with shipping trimpots apparently set at random from one end to the
other. The Japanese ones were always set very close to the middle
right from the factory. If your design was good the pot would hardly
have to move, since most of the time the variations will (mostly)
cancel out. Also helps identify outliers in case something else is
wrong.


Best regards,
Spehro Pefhany
 
B

Bob Eld

Jan 1, 1970
0
Joerg said:
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.

Most designs adjust the pots to some measured parameter, voltage, current,
time, pulse width, amplitude, etc. If the physical position is important,
why do you need a trim pot? Just use a fixed resistor of the corresponding
value.
 
L

Lostgallifreyan

Jan 1, 1970
0
Most designs adjust the pots to some measured parameter, voltage,
current, time, pulse width, amplitude, etc. If the physical position
is important, why do you need a trim pot? Just use a fixed resistor of
the corresponding value.

If you need a fixed gas pressure in a pipe and have a regulator, does this
mean you don't need a gauge?

The idea of seeing the position of a multiturn pot IS a good one, have you
ever tried tweaking a device when direct physical feedback is worth more
than the hassle of attaching some kind of metering to do it? If not, then
you haven't done much tweaking.
 
J

John Larkin

Jan 1, 1970
0
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.

Transparent, unfilled plastic melts too easily. We've had that same
problem with surface-mount LEDs; they tend to fail when reflowed, and
if you try to replace one, your tweezers just squish them flat when
you try to solder them.

The surfmount led's we use now are either on a ceramic substrate, or
have a clear plastic inner region and an opaque overmold that can take
the heat. Osram has some great side-firing surface-mount led's that
are rugged, have great optics, and come in all colors.

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.

Last few boards we've done had no pots at all; everything is in serial
eeprom or flash. Well, I did use some pots to calibrate gains in a
couple of optical/electrical converters. It's surprising what
bandwidth you can use a pot at.

John
 
J

John Fields

Jan 1, 1970
0
"So rare. So rare."

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

---
Yup.

Here's a link to a sample:

http://www.cleverocity.ca/sorare.htm

Andy Williams also did it, but I don't know when.

From another web site, but I lost the link:


"Words ands Music by John Rufus Sharpe and Jerry Herst

Charted in 1937 by Gus Arnheim (# 2) and Guy Lombardo (# 3) and a #
2 hit for Jimmy Dorsey in 1957. Andy williams also did it, but I
don't know when.


(So rare, so rare)
(So rare, so rare)

So rare, you're like the fragrance of blossoms fair
Sweet as a breath of air fresh with the morning dew
So rare, you're like the sparkle of old champagne
Orchids in cellophane couldn't compare to you

You are perfection, you're my idea
Of angels singin' the Ave Maria
Or you're an angel, I'd breathe and live you
With every beat of the heart that I give you

So rare, this is a heaven on earth we share
Caring the way we care, ours is a love so rare

(So rare, so rare)
(So rare, so rare)

(You are perfection, you're my idea)
(Of angels singin' the Ave Maria)
Or you're an angel, I'd breathe and live you
With every beat of the heart that I give you

So rare, (so rare) this is the heaven on earth we share (so rare)
Caring the way we care (so rare) ours is a love so rare

(So rare, so rare)
(So rare, so rare)"
 
L

Lostgallifreyan

Jan 1, 1970
0
Transparent, unfilled plastic melts too easily. We've had that same
problem with surface-mount LEDs; they tend to fail when reflowed, and
if you try to replace one, your tweezers just squish them flat when
you try to solder them.

There are several clear epoxies that could be used. The expense might be
higher, needing a two-part moulding that has to be cemented together, but I
think the current construction of multiturn pots isn't much different.
 
L

Lostgallifreyan

Jan 1, 1970
0
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.
 
B

Bob Eld

Jan 1, 1970
0
Lostgallifreyan said:
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.
 
J

Joerg

Jan 1, 1970
0
Jim said:
"So rare. So rare."

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

I still had to have my diapers changed back then :)

BTW, Jim, when I answered your 2nd email on Friday it came back with
some kind of spam refusal message. Just wanted to let you know since
that might also happen with client email.
 
J

Joerg

Jan 1, 1970
0
John said:
Transparent, unfilled plastic melts too easily. We've had that same
problem with surface-mount LEDs; they tend to fail when reflowed, and
if you try to replace one, your tweezers just squish them flat when
you try to solder them.

The surfmount led's we use now are either on a ceramic substrate, or
have a clear plastic inner region and an opaque overmold that can take
the heat. Osram has some great side-firing surface-mount led's that
are rugged, have great optics, and come in all colors.

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.

Last few boards we've done had no pots at all; everything is in serial
eeprom or flash. Well, I did use some pots to calibrate gains in a
couple of optical/electrical converters. It's surprising what
bandwidth you can use a pot at.

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).
 
L

Lostgallifreyan

Jan 1, 1970
0
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.
 
J

Joerg

Jan 1, 1970
0
Lostgallifreyan said:
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.


T'is excactly the situation here. A laser diode where the delta between
smooth sailing and a bzzzt-poof to the tune of a few hundred Dollars is
rather small.
 
J

Jim Thompson

Jan 1, 1970
0
I still had to have my diapers changed back then :)

BTW, Jim, when I answered your 2nd email on Friday it came back with
some kind of spam refusal message. Just wanted to let you know since
that might also happen with client email.

I caught it right away. I run a self-checking loop because of all the
problems I've had with Cox.

<Begin Personal Opinion>

Cox is such a bunch of mean-spirited "Cox-suckers" they hung a spam
block on my SMTP relay (operated by GoDaddy).

<End Personal Opinion>

But GoDaddy is owned by Bob Parsons. He'll kick Cox' ass if they do
that again.

...Jim Thompson
 
J

Jim Thompson

Jan 1, 1970
0
I still had to have my diapers changed back then :)

I was already "testing" the back seats of cars ;-)

...Jim Thompson
 
J

Jan Panteltje

Jan 1, 1970
0
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 think that is rather strange.
I remember Tek scopes with many many many (one turn) trimpots, form
after the eighties (lets say nineties), also the digital storage ones.
I also remember prof video equipment with even more.
Because analog _has_ tolerances, trimpots serve a good purpose.
To make it a religion to avoid these at all cost is bad practice in
my view.
 
J

Joerg

Jan 1, 1970
0
Jim said:
I caught it right away. I run a self-checking loop because of all the
problems I've had with Cox.

<Begin Personal Opinion>

Cox is such a bunch of mean-spirited "Cox-suckers" they hung a spam
block on my SMTP relay (operated by GoDaddy).

<End Personal Opinion>

But GoDaddy is owned by Bob Parsons. He'll kick Cox' ass if they do
that again.

Can you dump them? I don't know who your telco provider is but good old
Ma Bell has been pretty good to me.

It can be worse: About a month or so ago I had all my emails to
t-online.de recipients bounce. Then I began to receive some intervention
requests from biz friends over in Germany. They didn't get reponses from
US vendors. T-Online is a major ISP in Germany. Turned out they had
verbatim blocked a whole SBC server and that cut the guys over there
off. To add insult to injury a customer rep of this ISP "service" was
quite cocky about it IMHO. If I had to run a biz over there I certainly
would know which service not to use.

And I am glad I still have a fax :)
 
J

Joerg

Jan 1, 1970
0
Lostgallifreyan said:
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.


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

Jim Thompson

Jan 1, 1970
0
Jim said:
On Sun, 10 Dec 2006 21:06:19 GMT, Joerg
[snip]
BTW, Jim, when I answered your 2nd email on Friday it came back with
some kind of spam refusal message. Just wanted to let you know since
that might also happen with client email.


I caught it right away. I run a self-checking loop because of all the
problems I've had with Cox.

<Begin Personal Opinion>

Cox is such a bunch of mean-spirited "Cox-suckers" they hung a spam
block on my SMTP relay (operated by GoDaddy).

<End Personal Opinion>

But GoDaddy is owned by Bob Parsons. He'll kick Cox' ass if they do
that again.

Can you dump them? I don't know who your telco provider is but good old
Ma Bell has been pretty good to me.

Cox is the only cable provider here. Qwest DSL is slo-o-o-ow by
comparison.
It can be worse: About a month or so ago I had all my emails to
t-online.de recipients bounce. Then I began to receive some intervention
requests from biz friends over in Germany. They didn't get reponses from
US vendors. T-Online is a major ISP in Germany. Turned out they had
verbatim blocked a whole SBC server and that cut the guys over there
off. To add insult to injury a customer rep of this ISP "service" was
quite cocky about it IMHO. If I had to run a biz over there I certainly
would know which service not to use.

And I am glad I still have a fax :)

This wave of anti-spam hysteria is almost as bad as the global-warming
hysteria.

Anyone complains the ISP shuts you off... so malicious crap is
happening... and I have enemies galore... leftist weenies can't cope
with feeling inadequate ;-)

But after a few law suits it'll stop ;-)

...Jim Thompson
 
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