Potem...Poter...Variable resistors (The saga continues)

Ok this question really pretains to my superlight but I thought I'd start a new topic on it so that we could go on and on about it and even get a little off topic if we want...

On the suggestion of a member for the dimmer on my Superlight I used a 10k Audio taper potemeter. Only problem is that I don't really get a smooth dimming effect, it's bright, then in a tiny fraction of a turn it goes dim, and then it gets dimmer and dimmer...

Is this because it's Audio taper and not a linear taper or is it a property inherent in the LEDs? If it's because it's audio and not linear I can fix that, but if it's a property of the LEDs, I'll live with it...it's not that big of a deal, just wondered if it could be easily fixed.

Potentiometer is the word you're trying for, I'd say

I'd say you're on the right track - a log pot will cause that kind of problem, I think. Hard to say without knowing how you've hooked it up though. Simplest thing to do would be to get a linear pot and try it out..

Graham

Yes you are using a logarithmic pot, when you use a linear pot, the dimming will be linear as well...

Probably you'll fry the pot if you connect it in that way (I think). Whats the wattage of your light? Most pots are max. 0.25W!
I think you should use a transistor in your circuit, so you get a voltage regulator (Darlington regulator or something?)
Any experts...?

The pots I am using are 0.5 W...

I would also try a 1k or 500 ohm linear. To get a really even dim effect, you might have to put together some kind of current regulator. LED's put out light proportional to current, not voltage, they're non-linear w/ voltage. (Yes, I know, over simplifying.)

Sounds like you've got *way too much* resistance in the potentiometer.

Make some calculations. Think about what I'm saying in case I'm wrong.

At full power I = .025 amps per led
V = maybe 3.6 volts
R pot = 0

At min power I = .002 amps per led
V = maybe 2.4 volts
R pot = full value

delta V = I R

max voltage drop = delta V = supply voltage - 2.4 = 3.6 - 2.4 = 1.2
minimum current when pot is on full = I = .002 amps per led

R = full range resistance of the potentiometer. Then for four leds:

R pot = dV / I = 1.2/(4*.002) = 150 ohms

When the pot is on max, the power it's burning up is:

P = dV I = 1.2 * (4*.002) = .0096 watts

In the middle:

P = dV I = .6 * (4*.012) = .0288 watts

At full blast, the power coming in is:

P = V I = 3.6 * (4*.025) = .360 watts

Did I get that right??

If I did, then you want a .5 watt, 150 ohm potentiometer.

If it's too hard to find one, get a dip switch and some resistors and make a four speed transmission; 0 ohms, 50 ohms, 100 ohms, 150 ohms.

Audio pots give less change in R per turn of the knob at the beginning of the range. That allows finer control when the volume is low.

You're close, this is all running off a wayne dat2zip Madmax board set at 3 Volts max.

See if you can use the mosfet circuit half way down this page.

http://www.geocities.com/SiliconValley/Bay/4397/

Dan

Nuts!! *!@# Java scripted pages....it's under "LED and LED Driver Circuits".

Dan

Looks nice, but answer me this--what's the advantage of the Mofset over a plain ole POT?

Originally posted by Saaby:
Ok well I think first on the list is to try a linear pot instead of the log pot...I don't need perfectly smooth dimming but smoother than it is (Maybe I can post a very small mpeg file) would be nice...

Tom, maybe I've said this already...but glad to see you've got an Arc on it's way

Click to expand...

<font size="2" face="Verdana, Arial">Thanks! I get the feeling that everyone driving a brown truck has been playing with it on the way from CA. I'm hoping tomorrow...

Well, other than having an overkill amp rating for your application (3A for the IRF 510), I have found this circuit works very smoothly. It will dim 20W MR-16 bulbs seamlessly with a multi-turn potentiometer (of course, the mosfet gets hot enough to fry ones fingers @ the 1.8 amp load), as the gate on the mosfet is free of the mechanical restrictions that potentiometers posess (carbon traces, wires, wiper and the like).

Dan

Ok well I think first on the list is to try a linear pot instead of the log pot...I don't need perfectly smooth dimming but smoother than it is (Maybe I can post a very small mpeg file) would be nice...

Tom, maybe I've said this already...but glad to see you've got an Arc on it's way

Originally posted by Saaby:
Looks nice, but answer me this--what's the advantage of the Mofset over a plain ole POT?

Click to expand...

<font size="2" face="Verdana, Arial">The FET can handle the current, the pot cannot.
Pots are not power devices, they're not meant to be used as current limiters.
In any case, using a pot as a dimmer is inefficient. PWM would be a much better and controllable method of dimming an LED.

PWM with a regulator doesn't seem to work, did I meantion this light uses a dat2zip madmax board? I believe McGozmo is working on a light that uses both but as of yet PWM has not sucessfully been combined with regulation.

Saaby's light is using the MadMAX board, which is a DC-DC converter for running LEDs based upon some of the MAXIM ICs. It already has a transistor in it, which is being operated in a PWM fashion, however the circuit also includes energy storage (in the form of an inductor and a capacitor) so that the output is smooth DC.

The adjustment pot should _not_ be in series with the LEDs carrying power; instead it is connected to the MadMAX board, changing the output voltage of the board.

I suspect that the problem is that the MadMAX is a _voltage_ regulated board, and that the voltage difference between bright and dim is probably quite small. I suspect that what you will want to do is come up with a set of _fixed_ resistors that are close to the desired value, combined with a pot to permit you some adjustment range.

The pot on the MadMAX board is set up as a three terminal voltage divider. One of the fixed terminals of the pot is connected to output, the other is connected to ground, and the slider is connected to Vsns. What you will want to do is to have two fixed resistors added to the 'ends' of the pot, so that the voltage adjustment range is made much smaller than the full possible range. Imagine a 10K pot connected between Ground and Vout. The slider can go from 0V to Vout. Now consider a 1K pot connected between a 5K resistor and a 9K resistor. The slider can go from 5*Vout/15 to 6*Vout/15, or about 0.33 to 0.4 times Vout. This will make for much more precise adjustment over a much narrower range. Now you just need to select fixed resistors that let you cover the desired range.

dat2zip would probably be able to suggest the proper values.

-Jon

Isn't there already a potentiometer off of the FB pin on that board? If so you may try replacing it with something more ergonomically correct (within the proper resistance range, of course). This maybe comparing apples to oranges, but I have modded LM2621 eval. boards (boost/voltage regulator) by replacing the Rf2 resistor (FB to ground) with a pot. In this way I have achieved a fair degree of adjustability (roughly 2 volts). You best consult dat2zip and the previous Madmax threads before proceeding in this manor.

Dan

My suggestion is two fold.

There were many suggestions in Saabys original post about using a potentiometer. One being part of the FB pin to adjust the output voltage which in turn will increase/decrease the LED brightness.

The second was to insert a potentiomenter in series with the LEDs. This potentiometer should be a linear potentiomenter and 0.25 or 0.5W style. The one I saw in a picture seems to be the proper size and power rating judging by the type and style in the photo.

You only really need one or the other.

You need to decide on how you want to adjust the brightness.

The Potentiomenter that is part is the circuit needs to have leads or wires as short as possible to not affect the performance of the MADMAX operation and efficiency. Thus, this potentiomenter if used must be located with the MADMAX board and cannot be used in a remote location.

On the other hand, the one in series with the LED can be used with long wires and has no impact on the functionality of the MADMAX circuit. This series potentiometer can be located as far away within reasonable limits.

I would set it up this way. Short the one in series with the LEDs temporarily. Adjust the one on the MADMAX board for your maximum brightness setting.

After that, leave the one on the board alone and remove the jumper and just use the one in series with the LEDs to adjust dim the brightness.

The one on the board can be replaced later with a resistor value of equal resistance the potentiomenter on the board is set to.

WayneY

I concur with dat2zip in regards with remoting the potentiometer on the FB pin (sorry, way short on sleep.....stupid helicopter game). On the boards I modded, the pot is less than 1/4" away and I only use them for trimming the circuit. Since you're only driving a small number of discrete leds, your potentiometer should suffice. The mosfet circuit is just away to get a little smoother operation with very little additional cost and effort.

Dan