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Using a Wall Dimmer on LED Lightrope

I'm dimming a 9ft piece of manufactured LED lightrope with an ordinary household wall dimmer.

The nice thing about using a wall dimmer is that it doesn't get hot, it's cheap, easy to get and no lightrope mods are needed. The lightrope has an internal diode and internal resistors to run off

120VAC.

With the dimmer full on, I don't notice any flicker. However, flicker seems more noticeable (but still subtle) when the lightrope is dimmed. I suspect some combo of LED curve + dimmer waveform + eye persistence time to account for the flicker.

So.... how to get rid of the flicker and still use an ordinary wall dimmer?

Could I put the dimmer in series with a bridge rectifier? That'll put the flicker at 120Hz..

Or... Maybe design a more appropriate dimmer?

D from BC

Reply to
D from BC
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That's the best way to go. My suggestion would be to run it from DC, pulse-width modulated via a high current MOSFET or IGFET switch at (for example) 1KHz. That should be reasonably cheap, & won't flicker

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Reply to
Lionel

Mmm.... 170Vpk pwm on 40ft of lightrope (~300mA)...Isn't that make a big EMI transmitter? D from BC

Reply to
D from BC

It certainly can be, yes. The trick is to put a cap after the switch output to remove the harmonics.

--
   W  "Some people are alive only because it is illegal to kill them."
 . | ,. w ,      
  \\|/  \\|/              Perna condita delenda est
---^----^---------------------------------------------------------------
Reply to
Lionel

Going back to the original plan of a bridge rectifier - it may not work because then the average power through the light-rope is doubled (full wave not half wave), and it may overheat. You could compensate for that by setting an upper limit on the dimmer, or rewiring the series/parallelness of the lights in the tube.

Another alternative is to put a large capacitor in parallel with the lightrope, as while the lightrope will still get dimmer through the "off" parts of the cycle, it won't be fully off and that looks better to the eye.

Reply to
omattos

Ahh..good point about the average power... I didn't think about that..The lightrope has probably been designed to be max brightness (with long life) with half wave.. (The lightrope has that internal diode + LED's are diodes.) I'll compensate with full wave..Thanks.

Seems a bit scary to put a filter cap in parallel with the lightrope. Let's say power it's switched on right at the sine peak (~170V)...That's a big inrush current spike. Wouldn't that blow the dimmer(triac) + bridge rectifier combo? D from BC

Reply to
D from BC

put a bridge rectifier and an inductor (fluorescent lamp ballast? or three?) after the dimmer.

======= ----(D)---*-(+)--^^^^^---- X LOAD ----------*-(-)-----------

is it not currently at 120Hz ?

probably the best way.

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Bye.
   Jasen
Reply to
jasen

Or use a transformer with a lower output voltage and a DC filter.

greg

Reply to
GregS

IIRC a (triac based) wall dimmer doesn't rectify the line voltage. Maybe call it 4 quadrant phase dependent switching.

Why the inline inductor after the dimmer? To reduce EMI? The triac suddenly conducts at phase>90deg (and >270) across the inductor and the light rope. Wouldn't the inductor then return with a big nasty spike which will stress the dimmer and the lightrope?? D from BC

Reply to
D from BC

Nooooo! :P Not variable, bulky, can get hot, expensive compared to wall dimmer, needs to ordered and needs a box. However, a linear supply will have no flicker. D from BC

Reply to
D from BC

I was still using a dimmer. I have made variable AC lamp supplies using the transformer and dimmer, but I don't remember if it was AC or DC.

greg

Reply to
GregS

...

an attempt to reduce flicker. the load is basically a fixed voltage drop

No, inductors don't send spikes to triacs - triacs turn off when the current stops. You do get a step however if the current isn't in phase with the voltage.

The bridge recitifier howver should stop the current through the triac at 0V,

Hmm maybe it needs a 1 ohm series resistor just to make sure, because the bridge will be forwards biased by ithe inductor abd so looking like no resistance to the dimmer as voltage crosses 0.

Bye. Jasen

Reply to
jasen

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