Rory Posted December 13, 2005 Report Share Posted December 13, 2005 Hello all! After reading ghostface's post about LED lighting i have decided to make myself an LED rig for 'Moonlight' Anyway i used to consider myself quite the handy man when it came to electricity (back when i was 13) but these days i just get confused! So what im after is an LED 'Dimmer' so i can change the intensity of the LED's. Does anyone know how to do this in the cheapiest and simpliest way? Or does anyone have any schematics i can have? (I've google'ed it but i just confused myself even more!) Cheers Rory Quote Link to comment Share on other sites More sharing options...
suphew Posted December 14, 2005 Report Share Posted December 14, 2005 Been a while for me too, but I'm pretty sure you can just reduce the voltage to dim them, the standard old style LED's run at 7.2v if you ran two of them in series they should be half as bright. Or maybe my memory is crap and this wont work! lol Quote Link to comment Share on other sites More sharing options...
Ira Posted December 14, 2005 Report Share Posted December 14, 2005 Just splice a rheostat into the wire somewhere, that'll do it just fine. Just get a good one though, I fried one that should have been rated high enough, but apparently wasn't... Quote Link to comment Share on other sites More sharing options...
Rory Posted December 14, 2005 Author Report Share Posted December 14, 2005 Just splice a rheostat into the wire somewhere, that'll do it just fine. Just get a good one though, I fried one that should have been rated high enough, but apparently wasn't... Yeah thats what i thought i'd be using. However the problem for me arises when i have three wires (terminals) poking out the damn thing! How do i rig this up? Quote Link to comment Share on other sites More sharing options...
Ira Posted December 14, 2005 Report Share Posted December 14, 2005 You have a multimeter? Put it on the terminals and measure what happens to the resistance when you twist the rheostat. Then wire the thing up to the ones that do what you want. Quote Link to comment Share on other sites More sharing options...
Rory Posted December 14, 2005 Author Report Share Posted December 14, 2005 You have a multimeter? Put it on the terminals and measure what happens to the resistance when you twist the rheostat. Then wire the thing up to the ones that do what you want. Well i have no multimeter so i think ill get ahold of a rheostat and just wire it up in different ways and see what happens! Cheers Ira! Quote Link to comment Share on other sites More sharing options...
misnoma Posted December 14, 2005 Report Share Posted December 14, 2005 well you can't put more voltage in with it I would go for the center connector and one of the edge ones, you're most likely to have most success with that. Quote Link to comment Share on other sites More sharing options...
Warren Posted December 14, 2005 Report Share Posted December 14, 2005 LED's are current sensitive devices, not voltage sensitive. The intensity changes by altering the current through the LED. Different colours and types of LED have different forward state voltage drops. You need to know the forward state voltage drop and maximum continuous current rating of the LED before you can calculate the correct value potentiometer (rheostat). You'll also need a series resistor to limit the maximum current when the Pot (potentiometer) is wound to zero ohms. Stuff you need to know: 1. Forward voltage drop of the LED 2. Maximum Continuous Operating Current 3. Power Supply Voltage you're feeding it from Potentiometer: A variable resistor, usually with 3 terminals. With the pot shaft facing towards you and the 3 terminals facing downwards: The Center is the wiper. The right hand terminal is the side the resistance reduces when turned clockwise. The left hand terminal is the side the resistance increases when turned clockwise. We want the current to increase in the LED when the pot is turned clockwise so we use the center terminal and one on the right hand side (when the resistance reduces the current increases). Now the maths... V = Power Supply Voltage I= Maximum Led Current Vled = LED Forward State Voltage Drop N = Number of LED's in series R1 = Series Resistor Value R1 = (V - N*Vled) / I So if the PSU is 9V, 2 LED's with a forwards state voltage drop of 1.7V and maximum current of 20mA R1= (9-2*1.7) / 0.02 or 280 Ohms (use 270 Ohms as it's a readily available value). The pot value: The 280 ohm resistor sets the maximum current but we need to be able to reduce the current by increasing the total resistance in series with the LED's. To reduce the current and intensity to 1/10 of the maximum value you need a pot of 9 times the value (10-1) or 2520 Ohms to give you a total resistance of 10 times the original (the R1 value + the value of the pot). If you are going to put multiple LED's in parallel and want to dim them all at once: Calculate the resistor value R1 as before for the number of LED's you are putting in series. Each parallel string or single LED needs a R1 on it so when the pot is wound to zero ohms the correct current flows. The pot value will be the same as before divided by the number of LED strings. If 4 strings are paralleled then the pot is 630 Ohms. You now only need to check the power rating of the pot is high enough for the job. An approximate check is to take the maximum current you need, multiply it by 0.7, square it and multiply it by 0.3 x the pot value. So, for the 4 LED strings, the current is 20mA * 4 = 80mA and the pot is 630 Ohms. So the power in the pot worst case is 56mA^2 * 630 * 0.3 =0.056^2*189 =0.59 Watts So you'll need bigger than a 0.6W pot - in this case go for a 1 Watter... See the diagram attached: Quote Link to comment Share on other sites More sharing options...
Rory Posted December 14, 2005 Author Report Share Posted December 14, 2005 Fantastic warren! 6th for physics just hit me as soon as i read your post! Cheers for all the info its bound to be helpful! "attached diagram" didn't seem to show up Quote Link to comment Share on other sites More sharing options...
Warren Posted December 14, 2005 Report Share Posted December 14, 2005 Try now, put a link instead... Quote Link to comment Share on other sites More sharing options...
Rory Posted December 14, 2005 Author Report Share Posted December 14, 2005 COOL! That is just GREAT! Cheers! Rory :bounce: :bounce: :bounce: :lol: :bounce: :bounce: :bounce: Quote Link to comment Share on other sites More sharing options...
malawi_man Posted December 19, 2005 Report Share Posted December 19, 2005 anyone know if i could wire 12 of these - http://www.trademe.co.nz/structure/auct ... d=43034095 onto a 6v AC 450mA adaptor?? what size resistors would i need?? thanx Daniel Quote Link to comment Share on other sites More sharing options...
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