Topical Tip: High Brightness 1 watt LED Dimmer.

by Michael Stubbs-Race, Australia

LEDs are now widely used in microscopy, and although very efficient and compact, they are not the easiest to handle when it comes to dimming. The following sketch is of a circuit I use for just that purpose. N.B., this circuit is for 1 watt LEDís only.

The circuit described is made up from whatever I could find in the junk box. It may not be the ideal design, but I find it works quite well, nonetheless. It can be made up on any piece of Vero board, and should work from any DC supply from 9 volts to 30 volts. Values are probably not critical, and doubtless, other MOSFETs would work just as well.

A universal timer NE555 running around 35 Kc/s or so is set so that its pulse width is adjustable by means of the 10k potentiometer. The pulse output from this is used to switch the IRF520 MOSFET. The LM317 is arranged as a current regulator to prevent LED runaway.

The LM317 will need a heat sink, but the IRF520 wont. Other MOSFETs may, so it would pay to experiment if you wish to use an alternative device.

Comments to the author Michael Stubbs-Race are welcomed.


Circuit diagram


Above left: As can been seen from the pc picture, the LED dimmer takes up little space; mine is built into a project box with power supply and controls. The ribbon wire white/grey/mauve goes off to the pot' (brightness control). Top left is the LM317 current regulator which I find, would like a larger heatsink than the one shown here. Power is fed in at top right red/black. Centre bottom the NE555 timer. Bottom right MOSFET IRF520.

Above right: Close up shot of LED mounted on its heatsink. The LED is held in place with a piece of copper cut from a conduit strap flattened out and drilled. A washer(s) is placed betwixt copper strap and heatsink to level things up. I chose this method because it keeps the LED pressed onto the heatsink, there being no other fastening beyond perhaps some form of adhesive. The more expensive LEDs do have mounting hardware - well that's part of what you pay for!



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