Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Tom Bourke]

Any one try this with the lower end Color Key?
One thought I had was an RC circuit in parallel with the main feed to each color bank.  Tune this network to offer some lag time to the light change.  I know I will lose the strobe ability but that is not really a problem to me.  

[Jon Evans]

I'm not sure what you mean by "ziper effect" but flicker is probably due to low-speed PWM dimming.  If you replaced the driver circuit with something that ran at a higher clock speed, you could probably minimize flicker effect (especially visible at high dims/low brightness)

[Tom Bourke]

The zipper effect (and flicker) is only visible when looking at video of the LEDs.  LEDs respond fast enough that you can see the change from one light level to the next when dimming.  You could also call it a stair effect.  What I have in mind is a capacitor in parallel with the LEDs to slow the rate of change.  

[Jon Evans]

On fixtures that use "real" LEDs (i.e. ones designed for illumination rather than indication, usually called "1 watt" or "3 watt" even though they don't use that amount of power), the dimming power supply must be a constant-current, not constant-voltage supply.  Bypassing the LED with a capacitor would probably not have the desired effect because of this.

The driver in question probably has a control-voltage or PWM dimming control that varies the output current to dim the LED.  If the fixture only uses one DMX channel for each color dimmer, that is only 255 steps of dimming, which is visible to video, apparently.

The solution seems to be to have a more intelligent controller in the fixture that either uses 2 DMX channels to get 16-bit dimming resolution, or has a (hopefully programmable) "ramp" between steps.  In this case, if the DMX input changed from 126 to 125, the microcontroller in the fixture would ramp the control voltage or PWM signal going to the particular LED driver in question from one value to another rather than stepping it.

Of course, I'm guessing at the problem here!

[Rob Timmerman]

Jonathan Evans wrote on Tue, 13 April 2010 16:46

On fixtures that use "real" LEDs (i.e. ones designed for illumination rather than indication, usually called "1 watt" or "3 watt" even though they don't use that amount of power), the dimming power supply must be a constant-current, not constant-voltage supply.  Bypassing the LED with a capacitor would probably not have the desired effect because of this. The driver in question probably has a control-voltage or PWM dimming control that varies the output current to dim the LED.  If the fixture only uses one DMX channel for each color dimmer, that is only 255 steps of dimming, which is visible to video, apparently. The solution seems to be to have a more intelligent controller in the fixture that either uses 2 DMX channels to get 16-bit dimming resolution, or has a (hopefully programmable) "ramp" between steps.  In this case, if the DMX input changed from 126 to 125, the microcontroller in the fixture would ramp the control voltage or PWM signal going to the particular LED driver in question from one value to another rather than stepping it. Of course, I'm guessing at the problem here!

ALL LEDs can be driven with constant current or constant voltage drivers.  Constant current drivers are more common as the amount of light and heat you get out of an LED is determined by the current through the LED, and the forward voltage will vary between LEDs.  Yes, a constant current driver can cost more than a constant voltage driver, but once you have enough LEDs to get useful light from the fixture, the extra cost of the CC driver is quite small.

Flicker and stair-stepping intensity are two distinct problems, with different solutions (although increasing the time constant of the LED circuit to mimic a filament source will deal with both).  Flicker is typically an artifact of the switching method and frequency used to drive the LEDs.  This is normally a video problem, and flicker shouldn't be visible to the naked eye unless the fixture design is atrocious.  Stair-stepping is an artifact of 8 bit control and high-brightness fixtures.  Better fixtures will smooth between steps, but there's only so much that can be done, especially given the logarithmic response of the human eye.  Moving to 16 bit control helps dramatically, provided you have a console and fixture that supports it.

Increasing the time constant of the LEDs in your fixture may be simple, or it may not.  It all depends on the driver circuit in use.