Potentiometer and range of power / resistance

[dcbrown73]

Hey all,

When I insert a potentiometer into a circuit.  It works fine, but not like in devices I own.    For instance, in a simple circuit to say control an LED.  I can control the LED's brightness, but how do you make the pot (or select a pot) that will use the full range of the pot which controls the full range of say light in the LED.

I use a pot now and from 0 to say 25% of the pot controls the full range of light.  The rest (26%-100%) does nothing.  Or the opposite, 0%-100% works in dimmimg the LED, but even at max resistances, the LED will stay on.

While I get the math behind ohms law, what I'm unsure of is how to match a pot to whatever load I'm trying to control so that the full range of the pot is used. (like a volume control and turning it up to 11!)

Do I have to build some type of scaffolding around the pot (or load) to employ the expected range of the pot's control over the load? 

[mariush]

I use a pot now and from 0 to say 25% of the pot controls the full range of light.  The rest (26%-100%) does nothing. 

In this case, you'd measure the wiper resistance value at something like 30% of the pot and then use another potentiometer that's around 30% of the original value.

For example, let's say you start with a 4700 ohm potentiometer and 30% is 1410 ohm ... so you can use a 1500 ohm potentiometer or a 2000 ohm potentiometer.

As for controlling a LED, you don't control a led by adjusting the voltage. LEDs are current driven devices, you must limit the current going into the LEDs otherwise they burn out.

A LED with light up when the voltage goes above a threshold, which is called forward voltage. This threshold varies from led to led, but it's typically around some value, depending on chemistry used.. ex 1.7v for red, 2.2v for yellow or green .. 3v for blue
This threshold also changes slightly with temperature, a warm led will have a lower forward voltage than a cold LED.

Then, the LEDs have 3 regions ...
1. below the forward voltage where the LED basically doesn't produce light, doesn't turn on.
2. a few tens of a volt around the forward voltage threshold, the LED will start to produce some light, but it behaves like there's a resistor inside the led, it resists current flow
3. above the previous region, the led is fully open and will light up as bright as possible... and if you don't limit the current it will burn itself out.

So if you use the potentiometer as a voltage divider, to turn on the LED, you're doing it wrong. Depending on temperature and actual led, the actual potentiometer value will be different, and the led will be fully on only in the 3rd region.
If you use potentiometer to limit current ... well, a led will be bright even with 0.5 mA ... it's kinda hard to limit a device at such low thresholds.

So what can I say ... I guess it depends on the circuit you actually use... maybe show the circuit so we can tell you where you go wrong.

[cur8xgo]

Determine the current range you want to operate your LED across. You can do this by inserting a small value resistor in series with your LED and measuring the voltage across it as you adjust the pot to whatever brightness min and max you want. Do some ohms law and you will have the min and max current you want to operate the LED at.

Do some arithmetic and you will arrive at a min and max series resistance you need to operate your LED across that current range with your given voltage source.

Now your circuit design goal is to make a variable resistance which goes from that min to that max. If your pot coincidentally had a min and max that matched, you'd be done, but it doesn't.

Lets say your input voltage is 5V and your LED has a forward voltage of 1.5V at the current range we are interested in.

Lets also say your min LED current is 1ma and your max LED current is 20ma.

So your max resistance needed is (5V - 1.5V) / .001A = 3.5k and your min resistance needed is (5 - 1.5V) / .02A = 175 ohm

A variable solution could be a 175 ohm resistor in series with a (3.5k - 175) = 3325 ohm variable pot

If you have a 10k pot, you need to "convert" it to a 3325 ohm pot. You could do this by putting a resistor in parallel with it so that when the pot is at max (10k) the combo resistance is 3325. In this case that parallel resistor would be 4981 ohms.


EDIT: Note that things will not be perfectly accurate here. The LED forward voltage may be different at 1ma versus 20ma, and your resistances will not be exactly the right value (including the pot at min and max). But for purposes of an LED dimmer you should be able to get the effective result you want no problem.

[dcbrown73]

Ahh, that makes sense.  I will play around and see what I can craft to do just that.

Thanks!

[Audioguru]

We use Pulse width Modulation (PWM) to control LED brightness or motor speed control. Wide pulses produce maximum power and narrow pulses produce much less power.
The pulses occur at a frequency that is high enough that it is not noticed.

[dcbrown73]

We use Pulse width Modulation (PWM) to control LED brightness or motor speed control. Wide pulses produce maximum power and narrow pulses produce much less power.
The pulses occur at a frequency that is high enough that it is not noticed.

How exactly are you controlling the PWM with the pot where it uses the full range of the pot in its control an acceptable PWM range?

[Jwillis]

Look at this video