What size pot to act as an LED dimmer?

[ashbass]

I have an expression pedal that uses an optical sensor and LED to activate its effect. What I've already done is to insert a 5k ohm linear pot between the LED and power source to effectively dim the LED by controlling the power going to it.

Will a 1k ohm or even 500 ohm pot get me more control? Now, from about 3.5 to 5.5 on the pot I can dial in precise control. With a 500k and 1m pot, there was no responsiveness really.

So will dropping only another 4k or 4.5k ohms let me have a wider range of usable sweep?

thanks,
ashley

[Mark Hammer]

With a 9v supply, most "normal" red LEDs will generally provide little visible illumination once you stick more than 3.9k or so between the LED and power source.  I couldn't tell you about yellow or green ones because they are sort of hard to see anyways unless viewed against dark backgrounds in something other than broad daylight.  In my own experience, I can stick a 15k or even 18k resistor between ultrabright LED (i.e., >2000mcd) and 9V power source, and still see plenty.  So, the ideal pot value to vary max illumination will depend partly on LED type.

Perhaps more importantly, taper will matter in terms of producing usable differences without cramming everything usable into 5 degrees of pot rotation.

Since it is simply a variable resistance you are interested in rather than a voltage divider, maybe the smart thing is to use a small rotary switch with a half-dozen fixed settings identified through trial and error.

[David]

Ashley:

Try a 1K pot.  On some things I've built that power LEDs with a 9V supply, I used 1K dropping resistors and they worked just fine.  This could do the job for you.

[niftydog]

You'll first want to work out the maximum brightness you want from the LED.  ie; the maximum current the LED can handle without stress.  Work out the resistor that gives you this maximum current and install that in series with a pot.

for eg; say the max current specified for your LED is 20mA.  You have a 9V supply, so;

V = IR
9V = 20mA x R
R = 450ohms for maximum current allowed.

Now you can fiddle with pots all you like without accidentaly blowing the crud out of your LED!

From there, it's a matter of experimenting to find the minimum current/brightness level; which you can do by substitiuting different pots.

It will depend on what type of LED you are using as to the resultant pot value.  As Mark said, some LEDs work with tiny currents and others bomb out below 2mA or so.

[ashbass]

Thanks all.

Nifty, is it possible to blow the LED when all I've done is to cut the hot (red) wire going to it and connecting the resulting two wires to a pot? It seems to me that there is no way this can send too much current to the LED as the max that any pot could send is what the pot is receiving in the first place which isn't enough to blow the LED by design of the original circuit. Maybe you thought I was building from scratch. Rather, I'm modding an existing circuit.

When a pot is maxed on 10, is the total current that can pass through it limited by the value of the pot? OR does the ohm rating of a pot reffer more to the range of sweep that the pot will effect?

ashley

[petemoore]

When I check the wipers show a connection to whatever outside lug it's rotated to.
 Used as a V/R for LED trimming, I would make certain that it was never turned to zero resistance before connecting an LED, by shaft position and tagging a small resistor in series with the pot to set a minimum Resistance for safety in that circuit.

[niftydog]

is it possible to blow the LED when all I've done is to cut the hot (red) wire going to it and connecting the resulting two wires to a pot?

well no, if the existing current limiting resistor is still in place you shouldn't have a problem.  You're right, I did assume you were designing, not modding.  

I was coming from the angle that if you had simply connected 9V to a pot then to a LED you could easily turn that pot to 0 ohms and hence KABLOOIE!!!  (well, perhaps not as dramatic as that...)

So in light of these specifics, to get fine control via your pot, you need to make it a smaller value so that the maximum resistance (hence minimum current) still results in a dim glow from the LED.

When a pot is maxed on 10, is the total current that can pass through it limited by the value of the pot? OR does the ohm rating of a pot reffer more to the range of sweep that the pot will effect?

er... both!

Current is limited by the resistance of the circuit path, so the greater the resistance, the lower the current.   So the least amount of current that can flow through a pot is dictated by the maximum resistance value.

Be careful though; the MOST amount of current through a pot is only limited by the external circuitry!!  That is to say that, at it's minimum, a pot is a short circuit and it will try to pass as much current as can be supplied to it!

The ohm rating of a pot is the absolute maximum value you can acheive.  It actually tells you the total resistance of the "track" in the pot.  The "wiper" contacts the track at different points, hence resulting in the variable resistance.

[ashbass]

Thanks. I'm stilll fuzzy on the logic but no doubt it will be clearer as time passes.

At any rate, I got what I wanted by putting a 1k linear pot before the LED. Interestingly, there was bleed over in that even on 0 the pot still passed enough current to light the LED. So much in fact that the pot was pretty much useless for varying resistance. So, I put two 1.5k ohm resistors in parallel between the pot and the incoming current. This knocked enough current out to give my pot an acceptable range and kept enough current to let the receiver recognize when the LED was on "full".

Thanks again for all the details.

ashley