High Efficiency LED dimmer circuit

[cprobertson1]

In 10 days time I depart to do the West Highland Way (a 95mi / 153..ish km hike along the West coast of Scotland) and I found myself in need to a lantern for inside my tent!

Now I could just buy one, but that's no fun - so I've decided to rig one up using my 3D printer for the casing and a simple LED circuit inside it.

But... single-brightness LEDs are too boring - so I decided to make it dimmable (and with any luck more efficient so the batteries last longer)



What is the best way to go about making a high efficiency LED dimmer driver? These are my ideas so far:

- Shoving a pot in front of the LED seems too easy - would I be right in saying it wastes a lot of energy in the form of heat?
- Would setting up a 555 timer using a potentiometer to control the duty cycle work? [PWM]
- Setting up an oscillator with the LED on the output: or is that more trouble than it's worth? [PWM]

Any help would be great! Thanks in advance!

[cprobertson1]

Nobody? Mayhaps I should have posted in the Projects Forum?

[DaveW]

A pot will waste energy as heat-how much depends on the difference between your battery voltage and the forward voltage of the LED. As you'll want some form of resistance in series with the LED in order to keep the current roughly constant, this is generally inefficient.
Switching power supplies are more efficient and work in constant current mode. 555's use a fair amount of energy by themselves. Much better to go for an off the shelf IC.
Assuming that your battery voltage is greater than your LED forward voltage, below would be a good place to start,

http://www.linear.com/products/step-down_(buck)_led_drivers

[pyrohaz]

I designed a circuit for my A Level electronics CW that uses a one dual opamp package, a few resistors and a pot to create a pulse width modulated signal. This signal can then drive a current source that can drive the LED. I've drawn it out on my lunch break, its not the best schematic so do excuse the quality! The current source ensures that the LED current was just < 20mA (The sim states 14mA). The weird resistor network on the inverting input of the second opamp is meant to be a potentiometer!

[rs20]

Step 1: Use an LED buck driver chip as suggested by DaveW, so that you're not wasting any heat in dropper or emitter resistors.
Step 2: Either mod the feedback path to make the LED current selectable (preferable), or as a fallback use a 555 to drive the "PWM" input that LED buck driver chips seem to invariably have.

[ceamiclover]

On a similar note:
Can I use a controller with PWM to optotriac to triac to dim a 120VAC 11W LED?

[Zero999]

On a similar note:
Can I use a controller with PWM to optotriac to triac to dim a 120VAC 11W LED?

No you can't do PWM with an opto-TRIAC. You can do leading edge phase control only.

If your LED doesn't support phase control dimming then you need to buy one which does.

[ceamiclover]

Are optotriacs not fast enough?

what is a simple and cheap method of dimming?  I was planning on doing PWM from and msp430.

[Zero999]

Are optotriacs not fast enough?

what is a simple and cheap method of dimming?  I was planning on doing PWM from and msp430.

It's nothing to do with speed. Once a TRIAC is turned on (whether it be optically or electrically triggered), it won't turn off again until the current flowing through it falls to near zero.

[Jeff1946]

Use the CMOS version of Dave's favorite chip: LMC555.  Specs show it uses less than 1 mw and will operate down to 1.5 V.  Yes you would need a current limiting resistor which wastes power but would be a simple circuit.

[Seekonk]

If a short period of time before you leave, you have to use what you have.  And just what is your available battery  voltage.   One group of chips forgotten about are switchers like TL494 found in many old PC power supplies.

[Zero999]

what is a simple and cheap method of dimming?  I was planning on doing PWM from and msp430.

In the case of a mains powered LED. It has to support dimming.

Phase control might work but trailing edge is better than leading edge and most cheap dimmers are leading edge.

I have some mains powered LEDs which support dimming. They even work with cheap leading edge dimmers but another problem is such dimmers normally have a minimum load of 40W so a small incandescent lamp needs to be connected in parallel with the LED.

You can easily make a cheap leading edge dimmer which will work at very low load currents but it will need both a live and neutral connection and can't just be connected in series with the load.

It is possible to make a trailing edge dimmer which is connected in series with the load but it's more complicated.