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I want to drive an LED array from mains power. The wall switch has a dimmer on it, I'd like the intensity of the LEDs to reflect the relative position of the dimmer, so as I dim the switch, the intensity of the LED array drops.

Because I already have an 8V 12W Halogen bulb system in place, I want to replace it with as few parts as possible to convert it to LED.

I'd like to do this as simple as possible. I know there are more sophisticated circuits that could utilize MCU or other clock driven PWM, but I am wondering if there is a more simple solution I am overlooking.

My current plan was to use a step down transformer / rectifier circuit. I would calculate the maximum DC voltage available when the dimmer was turned all the way off, apply the correct resistors to limit the maximum current in this "max voltage" scenario, and then use a simple constant voltage regulator circuit to control the intensity of the LEDs as I dimmed the circuit.

Would this work? Do I want to keep a constant voltage and have the current variable as I dim the switch? Is there a cheaper/better/easier way to do this than my current plan?

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    \$\begingroup\$ You want to use PWM for this. You really, really do. \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 3 '15 at 4:54
  • \$\begingroup\$ What are the voltage and current requirements of the array? \$\endgroup\$ – EM Fields Feb 3 '15 at 14:09
  • \$\begingroup\$ Are you trying to keep the existing wall dimmer or replace it with something? For some reason this question is attracting poor answers .. \$\endgroup\$ – pjc50 Feb 3 '15 at 17:19
  • \$\begingroup\$ @pjc50 question updated, I am trying to swap out with as little effort as possible an existing Halogen system. \$\endgroup\$ – Geremy Feb 3 '15 at 17:46
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Wikipedia has some good examples about dimming. For as simple solution as possible (by electrical terms) I would use a transformer, full wave DC bridge and a thyristor. Again, wikipedia has a great picture of that

thyristor dimmer

Note that thyristor dimmers usually make some radio frequency noise but as I see it, this is one of the simplest AC dimmers.

You will find more info about different dimmers on http://en.m.wikipedia.org/wiki/Light_dimmer

And although you would like to keep out from MCUs, at least think about them. If the thyristor-stuff goes over your head, try to use MCUs. You will find them to be very useful for your future projects.

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Here you can look for better understanding:

http://powerelectronics.com/lighting/dimming-techniques-switched-mode-led-drivers http://www.allledlighting.com/author.asp?section_id=455&doc_id=559431

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  • \$\begingroup\$ It would help if you edited some useful information into your answers. Link only answers are discouraged. \$\endgroup\$ – David Feb 3 '15 at 8:24
  • \$\begingroup\$ @David: Sorry for that, and thank you very much for your direction. \$\endgroup\$ – user3535403 Feb 3 '15 at 10:14
  • \$\begingroup\$ @Olltsu: You can go for PWM option. If you use traic based Dimming for LED then it may be flicker. you have to use proper firing angle for dimming the LED. \$\endgroup\$ – user3535403 Feb 3 '15 at 10:16
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I'll take a stab. Cheap and quick, definitely not what I would do, as I am not a power electronics pro, nor an electrician by any means. If you use this you are accepting inordinate amounts of risk.

schematic

simulate this circuit – Schematic created using CircuitLab

If you're up for something simple, just source an ACDC supply module. Something like this might do it.

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  • \$\begingroup\$ If you're not sure of your answer, then why answer? \$\endgroup\$ – immibis Feb 3 '15 at 6:32
  • \$\begingroup\$ for the commentary. why does anyone do anything? \$\endgroup\$ – NateFisher Feb 3 '15 at 6:33
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In my opinion, the wall switch dimmer is already implementing some sort of thyristor-like circuit to provide less power to the ceiling/wall mains. This would mean that the AC power that you want to rectify isn't pure sinusoidal. Rectifying and filtering it would result in a variable DC voltage, so I wouldn't suggest adding a regulator at any point.

LED intensity is driven by current, not voltage. The voltage drop of an LED array is constant and fixed by the setup of the array itself. If you want to control the intensity you should either control the current or use a PWM-based circuit. I'd definitely go for the PWM option, but you'll probably need access to a non-regulated power source as well. If you had a near one, you could use the variable voltage provided by the wall switch to control the duty cycle of a PWM circuit which drives the LED array but is powered from the non-regulated mains. This isn't a quite "simple" solution but I wouldn't call it "sophisticated" either.

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You could investigate using a potentiometer (use a variable resistor for instance) to control the timing of a 555 timer IC. The output of the 555 would control the Gate of a FET, which would control the gate of another FET. The second FET would switch the supply on and off to the LED's and allow you to dim from a DC supply, which you should reasonably get from a rectified AC waveform. Sorry I understand this is not the simple idea you was after but it is a simplification of an industry standard way of PWM LED's.

There are plenty of 555 timer circuits on the web and information on how FET's (Field-effect Transistors) work.

You would need a PNP FET fed by a NPN and the theory would be that if the Gate of the PNP was connected to the DC ground via the NPN being on, then DC current would flow to the LED's. To turn on the NPN you would apply DC voltage (derived from your DC Source via the 555 output) to the NPN gate and this would turn on the NPN and make it conduct. Ideally you would have a high value resistor to ground on the gate of the NPN FET (4k7) and also need a resistor between the source and gate of the PNP (4K7), to ensure it turns off, when the NPN is not conducting.

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