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Any cheap small log potmeters <50 ohms?

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

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

Graahm

Reply to
Pooh Bear

Hello Graham,

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

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Reply to
Joerg

Fake it.

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Reply to
JeffM

Hello Jeff,

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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:

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Regards, Joerg

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Reply to
Joerg

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

Reply to
Sjouke Burry

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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

Reply to
Phil Hobbs

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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

Reply to
Roger Hamlett

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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

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Hello Roger,

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

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Reply to
Joerg

Hello Spehro,

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

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Reply to
Joerg

Hello Sjouke,

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

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Reply to
Joerg 

--
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
Reply to
John Fields

Hello John,

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

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Reply to
Joerg

"Joerg" schreef in bericht news:7WxBf.14170$ snipped-for-privacy@newssvr13.news.prodigy.com...

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.

--
Thanks, Frank.
(remove 'q' and '.invalid' when replying by email)
Reply to
Frank Bemelman 

--
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?
Reply to
John Fields

Hello John,

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

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Reply to
Joerg

Hello Frank,

Thanks. The cost would certainly be ok here but the Howland requires a path to ground, or a Hi-Z load sense. Since there is no power switch here it would drain the battery.

Regards, Joerg

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Reply to
Joerg

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