Any cheap small log potmeters <50 ohms?

[Joerg]

Hello Folks,

Are there small and cheap potmeters 50 ohms and lower that are log or
audio-taper?

Reason I ask is that I want to make an adjustable current source out of
just two parts, an LM334 and that potmeter. Well, plus maybe a limit
resistor for the upper current range (or a mechanical peg on the pot).
The current shall be adjustable from about 1.5mA to 8mA. No big deal but
with a linear potmeter the upper part of that range is being squished
into the last 20% or so of that potmeter, making it kind of fickle to
adjust. The current setting of an LM334 is proportional to 1/R.

I am looking for one of those cheap transistor radio type potmeters. The
alternative would be a more elaborate voltage control circuit but it's
larger and also I am limited to very small voltage drops. Can't waste
many battery volts in overhead here.

Regards, Joerg

 

[Pooh Bear]

Hello Folks,

Are there small and cheap potmeters 50 ohms and lower that are log or
audio-taper?

The lowest value audio taper I've ever seen is 5k.

Graahm

 

[Phil Allison]

"Joerg"

Are there small and cheap potmeters 50 ohms and lower that are log or
audio-taper?

** No.

Reason I ask is that I want to make an adjustable current source out of
just two parts, an LM334 and that potmeter. Well, plus maybe a limit
resistor for the upper current range (or a mechanical peg on the pot). The
current shall be adjustable from about 1.5mA to 8mA. No big deal but with
a linear potmeter the upper part of that range is being squished into the
last 20% or so of that potmeter, making it kind of fickle to adjust. The
current setting of an LM334 is proportional to 1/R.

** To operate in the normal clockwise direction - you need a reverse log
pot.

For fine adjustment a 100 ohm, 10 turn WW pot will do.

Or just use a 12 way switch and some fixed resistors.




........... Phil

 

[Joerg]

Hello Graham,

The lowest value audio taper I've ever seen is 5k.

Well, that's where the market is, unfortunately. The lowest log potmeter
I have used was 1K and that was a rather large hi-rel version.

Regards, Joerg

 

[Joerg]

Hello Phil,

** No.

:-(


** To operate in the normal clockwise direction - you need a reverse log
pot.

That would not be an issue here. It'll be thumbwheel and just be mounted
the other way around.

For fine adjustment a 100 ohm, 10 turn WW pot will do.

Or just use a 12 way switch and some fixed resistors.

Both way too expensive and too large.

Regards, Joerg

 

[JeffM]

Are there small and cheap potmeters 50 ohms and lower

that are log or audio-taper?
...with a linear potmeter the upper part of that range
is being squished into the last 20% or so of that potmeter
Joerg

Fake it.
http://groups.google.com/group/sci....d78bf21f28d/bbf7da1cd0ef41ad?q=That-kicks-ass

 

[Joerg]

Hello Jeff,

Fake it.
http://groups.google.com/group/sci....d78bf21f28d/bbf7da1cd0ef41ad?q=That-kicks-ass

Thanks, but won't work in this case because it isn't a voltage divider.
It's an adjustable series resistor.

But that is a great link, it can save some spreadsheet time. In case
others are interested in the 'potmeter curve bending' plots here is the
link from the thread:
http://www.wavefront.mcmail.com/pot.htm

Regards, Joerg

 

[Sjouke Burry]

Hello Jeff,


Thanks, but won't work in this case because it isn't a voltage divider.
It's an adjustable series resistor.

But that is a great link, it can save some spreadsheet time. In case
others are interested in the 'potmeter curve bending' plots here is the
link from the thread:
http://www.wavefront.mcmail.com/pot.htm

Regards, Joerg

If you let the moving part of the pot carry that kind of
current,it will not live very long.(or work very well).

 

[Phil Hobbs]

Hello Jeff,


Thanks, but won't work in this case because it isn't a voltage divider.
It's an adjustable series resistor.

But that is a great link, it can save some spreadsheet time. In case
others are interested in the 'potmeter curve bending' plots here is the
link from the thread:
http://www.wavefront.mcmail.com/pot.htm

Regards, Joerg

Yeah, the only problem in this case is that you want the resistance vs
shaft angle curve to be concave upward, which means you have to use a
negative resistance in parallel.

OTOH, you can do this, e.g. you can linearize a platinum RTD
*beautifully* with a little positive feedback. Not much use to Joerg,
but oh well.

Cheers,

Phil Hobbs

 

[Roger Hamlett]

Yeah, the only problem in this case is that you want the resistance vs
shaft angle curve to be concave upward, which means you have to use a
negative resistance in parallel.

OTOH, you can do this, e.g. you can linearize a platinum RTD
*beautifully* with a little positive feedback. Not much use to Joerg,
but oh well.

Cheers,

Phil Hobbs

Idon't know about 'cheap', but the Bourn's 81/82 ranges are available in
log taper, down to 50R value, as are the 85/86 range units, but I'm fairly
sure these are more expensive.

Best Wishes

 

[Spehro Pefhany]

Idon't know about 'cheap', but the Bourn's 81/82 ranges are available in
log taper, down to 50R value, as are the 85/86 range units, but I'm fairly
sure these are more expensive.

Best Wishes

In the old days, CTS would make whatever we wanted. Just pop in and
have a word with their engineers, place a reasonable minimum order and
the stuff would be ready in a few weeks. You can probably still do
that- but not in North America.


Best regards,
Spehro Pefhany

 

[Don Lancaster]

Yeah, the only problem in this case is that you want the resistance vs
shaft angle curve to be concave upward, which means you have to use a
negative resistance in parallel.

OTOH, you can do this, e.g. you can linearize a platinum RTD
*beautifully* with a little positive feedback. Not much use to Joerg,
but oh well.

Cheers,

Phil Hobbs

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Joerg]

Hello Roger,

Idon't know about 'cheap', but the Bourn's 81/82 ranges are available in
log taper, down to 50R value, as are the 85/86 range units, but I'm fairly
sure these are more expensive.

I am still waiting for a call back from Bourns Engineering. Hopes aren't
high though. I remember 50ohm log taper pots for RF attenuators and
still have one here. But these were rather huge. Also, with the advent
of chips like the AD603 their market might have shrunk too much by now.

Regards, Joerg

 

[Joerg]

Hello Spehro,

In the old days, CTS would make whatever we wanted. Just pop in and
have a word with their engineers, place a reasonable minimum order and
the stuff would be ready in a few weeks. You can probably still do
that- but not in North America.

Haven't found anything under 100ohms at CTS. Long term if this goes in
series production it will be a custom pot. Probably one of those dual
paste screening deals like we use with hybrids.

Regards, Joerg

 

[Joerg]

Hello Sjouke,

If you let the moving part of the pot carry that kind of
current,it will not live very long.(or work very well).

Yes. It depends on the type of wiper. In this app it's not going to need
much precision and would be somewhat of a disposable.

Regards, Joerg

 

[John Fields]

Hello Folks,

Are there small and cheap potmeters 50 ohms and lower that are log or
audio-taper?

Reason I ask is that I want to make an adjustable current source out of
just two parts, an LM334 and that potmeter. Well, plus maybe a limit
resistor for the upper current range (or a mechanical peg on the pot).
The current shall be adjustable from about 1.5mA to 8mA. No big deal but
with a linear potmeter the upper part of that range is being squished
into the last 20% or so of that potmeter, making it kind of fickle to
adjust. The current setting of an LM334 is proportional to 1/R.

I am looking for one of those cheap transistor radio type potmeters. The
alternative would be a more elaborate voltage control circuit but it's
larger and also I am limited to very small voltage drops. Can't waste
many battery volts in overhead here.

---
Can you use a pot with a resistor in parallel with the wiper and one
of the ends of the pot?:


+-----+
| |
[R] +----->ROUT
| |
+-->[POT]
|
+----->ROUT

 

[Joerg]

Hello John,

---
Can you use a pot with a resistor in parallel with the wiper and one
of the ends of the pot?:


+-----+
| |
[R] +----->ROUT
| |
+-->[POT]
|
+----->ROUT

Tried it but with our range it doesn't work. The problem with such
adjustable current sense resistors is that the resolution for higher
currents is poor. It all piles into one corner of the pot.

It looks like a custom potmeter or long term a little ASIC.

Regards, Joerg

 

[Frank Bemelman]

Hello Folks,

Are there small and cheap potmeters 50 ohms and lower that are log or
audio-taper?

Reason I ask is that I want to make an adjustable current source out of
just two parts, an LM334 and that potmeter. Well, plus maybe a limit
resistor for the upper current range (or a mechanical peg on the pot).
The current shall be adjustable from about 1.5mA to 8mA. No big deal but
with a linear potmeter the upper part of that range is being squished
into the last 20% or so of that potmeter, making it kind of fickle to
adjust. The current setting of an LM334 is proportional to 1/R.

I am looking for one of those cheap transistor radio type potmeters. The
alternative would be a more elaborate voltage control circuit but it's
larger and also I am limited to very small voltage drops. Can't waste
many battery volts in overhead here.

Perhaps a cheap low voltage sot-23 opamp will do, howland current source,
allows you to use any pot you like. LMV321, 21 cents a piece.

 

[John Fields]

Hello John,

---
Can you use a pot with a resistor in parallel with the wiper and one
of the ends of the pot?:


+-----+
| |
[R] +----->ROUT
| |
+-->[POT]
|
+----->ROUT

Tried it but with our range it doesn't work. The problem with such
adjustable current sense resistors is that the resolution for higher
currents is poor. It all piles into one corner of the pot.

---
I understand.

What would you like the the "open" resistance of the pot to be and,
say, its resistance when the shaft was at its halfway point around
the arc?

 

[Joerg]

Hello John,

What would you like the the "open" resistance of the pot to be and,
say, its resistance when the shaft was at its halfway point around
the arc?

Hmm, one of those cases where I couldn't see my own reply. Guess the
news server swallows some posts again.

Ideally full open would be 45ohms and minimum 9ohms. But that's not
critical, I could use a mechanical peg. A halfway point around 15ohms
would be nice. Or less, since the high current values will still
continue to cram themselves into the corner from there.

When using a series resistor of 9ohms to cap the max current the halfway
point would need to be well below 10ohms.

Regards, Joerg