Potentiometer with switch

[Blue_Shift]

Hi all,
I am looking for a potentiometer with a built in switch maybe 5ohms or less? I want it to be able to dim a 5w LS that is run on 9volts DD. I think that linear taper would be better. Anyone know where to get one?
Thanks,
Jason

 

[MR Bulk]

Try Radio Shack, it is where I found one to make THIS. However I have long since discarded the package and do not have the specs, although I do recall it was something like 25 ohms max.

 

[Blue_Shift]

I am looking for one that has a built in switch...

 

[Doug Owen]

You got problems.

First off, you need more power than typically available. I wouldn't worry about taper, you're only going to find linear tapers in 5 ohms (almost sure to be WW).

CTS is the only supplier I know of that fits the bill. They make a 24mm part (series 026), available with either a rotary or push pull switch:

CTS 5 Watt pots

Doug Owen

 

[RGBrainbow]

Hi,
if You use a standard 100k/lin-pot (with scwitch) to open and close the gate of a BUZ11 MOSFET you can use a cheap (and small) standard-pot and a cheap (and small) BUZ11 instead of an expensive (and big) power-pot.
Just an idea..
Joachim

 

[PaulW]

Blue_Shift,

I have a similar need. I have some Luxeon IIIs and want to mount one in a 6 volt lantern. The idea is to use variable resistance to allow currents as high as about 700 mA and as low as about 10 mA. I have given thought to a pot but the range that would be needed would make it awkward to use.

I think what I'll try is a 10 position switch that simulates a non-linear taper pot. The first position will be open, making it the off position. Each position would double the current -- 10, 20, 40, 80, 160, 320, 640 mA. Well, that's a total of 8 positions. I'll have to play around with the math, but you get the idea.

Each twist of the knob will double the output. At least that's the plan for now. It'll be a simplified version of this.

Paul

 

[Doug Owen]

[ QUOTE ]
PaulW said:

I think what I'll try is a 10 position switch that simulates a non-linear taper pot. The first position will be open, making it the off position. Each position would double the current -- 10, 20, 40, 80, 160, 320, 640 mA. Well, that's a total of 8 positions. I'll have to play around with the math, but you get the idea.

Each twist of the knob will double the output. At least that's the plan for now.

[/ QUOTE ]

Small, funny world, this. I've been fiddling with just this idea for some time. I found in testing that doubling (i.e. 'one f stop' change) wasn't really all that much, perception wise. I find that steps a factor of four apart (not two) make more sense. I also found I had no use for 'full blast' inside, so the current version (pun intended) lives on the kitchen table and has a rotary switch that goes off, 2 mA, 8 mA, 30 mA, 120 mA, off. That is I took out the high, moved all the steps up a bit and added one at the bottom. It works very well for my needs, runs 24/7 at one level or another (2 mA is actually very useful for nightlight use in a small kitchen full of stuff to trip over in the dark). Every few weeks I trade out the four NiMH AAs (originally 3, but I couldn't find a 3 cell holder that fit the box and I haven't got around to making a dummy or jumpering out a position in the last six months.....).

Doug Owen

 

[Blue_Shift]

Doug,
Where can I get these CTS pots?
Thanks for your help,
Jason

 

[PaulW]

Blue_Shift,

One of the steps in designing your circuit is to determine the current capability of the rheostat. Since Rated Power = (Max Allowed Current^2) times Max Resistance,

Max Current = Square root of (Rated Power / Max Resistance)

So a 25 ohm, 5 watt pot would be linited to

Max Current = SQRT ( 5 / 25 ) = SQRT (0.2) = 0.447 Amps

If you want to increase the resistance of the pot, the allowable current will go down. A 100 ohm pot, for example, would be limited to .224 Amps. A 5 ohm pot could run at 1.0 Amp. I would advise getting the largest value resistance that can handle the current. That way you'll maximize the range over which you can dim the light.

Paul

 

[Doug Owen]

[ QUOTE ]
Blue_Shift said:
Doug,
Where can I get these CTS pots?


[/ QUOTE ]

Oh, I don't know....

The link comes from Digikey, they rep them.....

At the bottom of the link there's the maker's URL, where you can check the stock on whatever part you want (as Digikey or whoever you *buy* one from would) and the link where you could request a sample.........

Beats me what you might do.

Doug Owen

 

[Doug Owen]

[ QUOTE ]
PaulW said:

If you want to increase the resistance of the pot, the allowable current will go down. A 100 ohm pot, for example, would be limited to .224 Amps. A 5 ohm pot could run at 1.0 Amp. I would advise getting the largest value resistance that can handle the current. That way you'll maximize the range over which you can dim the light.


[/ QUOTE ]

Gentle Friends,

I think it's about time to get some terms sorted out.

What Paul says is (in general) quite true for 'pots', that is potentiometers. Three lead devices where the *potential* (voltage) is applied across the 'two outside' connections and some fraction of that voltage taken between the 'wiper' (center contact) and one end. A "continuously variable voltage divider".

What we're talking about here is not a pot, it's a rheostat. A variable resistor, a two lead device. A " continuously variable resistance".

A pot can be (and often is) wired as a rheostat, but it then is no longer a pot, but a rheostat (and has to follow rheostat rules). One of these rules is maximum current is determined by the rest of the circuit and has nothing to do with the resistor itself. The 100 ohm pot described can easily run into the amps (in a headlong race to burned out part heaven with the LED) given enough voltage and low enough external resistance and Vf.

And not only do we have to worry about 'worst case current', but maximum power typically comes at part scale. Since only part of the resistor in the rheostat is being driven (and therefore getting hot), the total power dissipation available is not 'nameplate rating', and must be derated.

Often going to higher resistances will cause a failure where otherwise none would happen. Consider the 100 ohm version of the 5 ohm. Since this is a wire wound part, we're talking about lots of turns of very fine wire (as opposed to the few much thicker ones in the five). When we're at a few ohms (nearly full blast) we're going to be seriously overloading those few turns, they're going to fail. The 'watts per turn' (not the total watts) will be too high. Now there's an open at the front end, we go from full blast to dead.......we have a switch. In the case at hand (switch built in), by the time the shaft rotates enough for the switch to close, the wiper is off the good part, it never does light.

Such parts are hard to spec, the typical solution is overkill. The CTS part in question is such a solution.

Doug Owen

 

[UK Owl]

To do what you want with a Pot / Rheostat will require quite a chunky wire wound one, as when you are in the 'fully on' position, you may burn out your POT. Wht not make a PWM Circuit that switches a FET on and off. Not only will this be far more efficient, but it will cut down on component size, and with carefull choice of components you will be able to scale the POT as you wish, eg 25% to 90% or 0% to 100% etc.