Arduino Forum

Using Arduino => General Electronics => Topic started by: calvingloster on Jun 16, 2014, 01:53 pm

Title: making a potentiometer that has a low range of resistance?i
Post by: calvingloster on Jun 16, 2014, 01:53 pm
I want to create a potentiometer that changes it's resistance from a minimum of 2ohms to a maximum of 15ohms. Is this possible? It's basically for an LM317 as a constant current source. I want to be able to change the value of the resistor by turning a knob. But potentiometers are so sensitive. They go from 5ohms then just to 1k if you barely even turn it
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: elac on Jun 16, 2014, 02:10 pm
How about a max of 22 ohm? or 12 ohm?
Use a 2 ohm resistor in series with a 20 ohm  potentiometer.
2 ohm min, 22 ohm max
Or 2 ohm resistor in series with a  10 ohm potentiometer.
2 ohm min, 12 ohm max
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: calvingloster on Jun 16, 2014, 02:36 pm
Oh ok thanx guys, me not having an electrical background I never knew u get pots with such low value's. Thank you
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: CrossRoads on Jun 16, 2014, 03:08 pm
You can get a multiturn potentiometer like this & dial it right in:
http://www.digikey.com/product-search/en?pv23=9&FV=fff40004%2Cfff80338%2C40445%2C1140050&k=potentiometer&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: retrolefty on Jun 16, 2014, 03:39 pm
At 2 ohms in that application the pot would dissipate around 0.7 watts of heat so small 0.25 watt trimmer pots are not going to cut it.
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: CrossRoads on Jun 16, 2014, 04:12 pm
Ok, than a 3/4W  multiturn pot
http://www.digikey.com/product-search/en?pv1=1093&pv23=13&pv24=2&FV=fff40004%2Cfff80338%2C800b5&k=trimpot&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: MarkT on Jun 16, 2014, 07:34 pm
Low resistance pots are normally called rheostats and are wire-wound (no use at RF).

If you explain exactly what you want to achieve I'm sure someone will know
how to do it without an exotic component like a rheostat...
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: CrossRoads on Jun 16, 2014, 07:38 pm
It's a variable resistor to set the output voltage of a LM317, as explained in the original post.
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: MarkT on Jun 16, 2014, 07:42 pm

It's a variable resistor to set the output voltage of a LM317, as explained in the original post.


I said "want to achieve" not "how I am imagining it should be done"
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: CrossRoads on Jun 16, 2014, 07:53 pm
Imagining should not come into plqy - the data for TI parts has several examples of how to wire up an LM317 different variable modes.
http://www.ti.com/lit/ds/symlink/lm317.pdf
Figure 4 would seem applicable here given the initial request; others might be more applicable.
A rheostat, which most would envision as a high power pot, might be the answer. A lower wattage part might also suffice.
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: MarkT on Jun 16, 2014, 08:16 pm
Exactly, that's not a particularly clever way to do a constant current supply.
an opamp + darlington with a fixed shunt current-sense resistor can be done
with voltage control - add a normal 10k pot to give a preset circuit.

An LM317 is already an opamp and pass-transistor packaged in
a particular configuration, but the same components are available
separately for the cases the LM317 doesn't handle (such as voltage
controlled current source).

Still don't know what the current source is for though (or how accurate or
compliant it needs to be or its bandwidth?)
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: KeithRB on Jun 16, 2014, 08:22 pm
Check out what radio shack used to call a "Speaker L-Pad" A pot designed to limit the voltage across an 8 ohm speaker. Low resistance and relatively high power.
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: jackrae on Jun 16, 2014, 08:23 pm

Low resistance pots are normally called rheostats and are wire-wound (no use at RF).


Not so.

A rheostat is a variable resistor and generally needs only 2 terminals.  It provides no potential divider capability.  Certainly rheostats are generally of low resistance but that does not  mean low resistance potentiometers should be called rheostats.
Rheostats are generally linear devices but can be obtained with a nonlinear function.

A potentiometer (whatever its resistance value) has at least 3 terminals (some have a fourth fixed value terminal at mid scale 50%) and provide a potentiometric divider function.  Track resistance can be logarithmic, anti-logarithmic or linear.

Potentiometers can act as rheostats by using one end terminal and the slider terminal.  It is generally good practice to also connect the "unused" end resistance terminal to the slider.  This ensures there is always a resistance in circuit even if the slider contact goes open circuit.
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: CrossRoads on Jun 16, 2014, 08:24 pm
Alright, let's hear what Calvin is doing with his constant current source ...
Title: Re: making a potentiometer that has a low range of resistance?i
Post by: fungus on Jun 17, 2014, 09:47 am

Alright, let's hear what Calvin is doing with his constant current source ...


Is he out of his resistor-burning phase yet?

You people have far more patience than me....