100W LED Cob bead

[Gerry]

Been trying out some 100W LED Cob beads with a view to possibly using them in Strand Minim Fs and 500W floods.

They appear to be nice and bright but haven't tried them inside a Minim or a flood yet.

They start turning on at about 24V DC and obtain full brightness at 30-32 V DC.

I have some 12-24V DC DMX decoders which output 0-24V DC according to the DMX control signal.

Can't find any up to 36 V DC DMX decoders, other than one with a very low, less than 1 amp, output. Need about 2-3 amp output.

Is there any way I can boost the output, from the decoders, to 36V DC?

I'm aware of the LM2596 type boosters but they give a settable fixed output for a variable input so as I increase the DMX signal to a DC output above about 4V DC the full 36V DC is output from the booster.

Cheers

Gerry

[J Pearce]

Someone did suggest that the 12-24V decoders might be happy on higher voltages - but may need extra regulation adding for the power supply that runs the logic components.

[adam2]

This may be more complex that it first seems, LED chips need a constant current power supply, or for dimming a variable but regulated current.

 

Attempts to use a constant voltage are likely to fail since a voltage even slightly too low will greatly reduce the light, and even slightly too high will destroy the LED by over temperature.

 

It is of course possible to connect a series resistance and use a voltage controlled power supply, but with high power LEDs this entails energy waste and extra heat output.

 

 

 

[timsabre]

LED dimming isn't done by varying the voltage, as Adam says you need a constant current power supply, you then use PWM to alter the duty cycle of the power supply to change the brightness. Most of the simple switcher type chips have a Dim/PWM input for this purpose. So for 50% brightness it's on for half the time, off for half the time, 25% it's on for a quarter and off for 3 quarters etc. If you do this above about 250Hz then the eye cannot see the pulsing.

 

You can buy constant current supplies with PWM input from people like Meanwell, then use a cheap DMX tape driver from Ebay to provide the PWM signal. You can't usually just PWM the output of a constant current supply (or any type of switcher) as they don't respond correctly to the load being connected and removed at 250Hz and tend to go bang or just fail to work in a less interesting way.

[sunray]

LED dimming isn't done by varying the voltage, as Adam says you need a constant current power supply, you then use PWM to alter the duty cycle of the power supply to change the brightness. Most of the simple switcher type chips have a Dim/PWM input for this purpose.  So for 50% brightness it's on for half the time, off for half the time, 25% it's on for a quarter and off for 3 quarters etc. If you do this above about 250Hz then the eye cannot see the pulsing.You can buy constant current supplies with PWM input from people like Meanwell, then use a cheap DMX tape driver from Ebay to provide the PWM signal. You can't usually just PWM the output of a constant current supply (or any type of switcher) as they don't respond correctly to the load being connected and removed at 250Hz and tend to go bang or just fail to work in a less interesting way.

 

I have used these <a href='http://www.ebay.co.uk/itm/3CH-DC12-24V-RGBW-DMX-512-Decoder-LED-Controller-RGB-LED-DMX512-Decoder-BX/122552065296?hash=item1c88ac1d10:g:2ZgAAOSwCkZZRKmq' class='bbc_url' title='External link' rel='nofollow external'>http://www.ebay.co.u...ZgAAOSwCkZZRKmq</a>  to replace the IR controllers in 50W RGB floods, the FET's are rated well in excess of 35V but the series resistor leading to the 5V regulator will need to be replaced.

For power supply I have used 20V transformer, bridge rectifier and 4700uf.

[timsabre]

Are you saying 20v transformer straight onto an LED with no control of current? That's not going to make the LED happy.

[sunray]

Are you saying 20v transformer straight onto an LED with no control of current? That's not going to make the LED happy.

I've only been looking at LED lighting over the last year and playing with a selection of ebay PWM decoders there has been no mention of CC PSU.

I struggle to see how CC would work with these boards where there are 3 or more channels dimming at different levels.

The Flood lamps I have converted had a driver module in them which appears to be CV(ish) PSU being around 36V off load and 32V all on.

 

I've had no problems with the set-up so far which has been used as house lights for 3 shows, 2 of which are murder mysteries with a 3 course meal and remain on with a dim orange while the play is on, so they have light to make notes, then full white(ish) for meals and the rest of the time. Even worse I run a pair of floods per controller.

The only issue being the Red needs 30ohms (also in the original controller) to restrict voltage.

 

What problems am I likely to encounter and how can I employ CC with these controllers?

[timsabre]

The thing with LEDs is they have a fixed forward voltage, which is about 2v for a red and 3v for other colours (per individual LED die). As you increase the power supply voltage, the LED attempts to hold the voltage to its 2 or 3v, drawing more and more current until it eventually burns out.

For small 20ma leds you can control the current using a resistor in series, and the resistor absorbs the extra voltage, but for power leds the resistor dissipates a silly amount of power (but some fixtures, probably including yours, still do it that way).

The best way is a constant current power supply which automatically varies its output voltage until it is providing the desired current - in an rgb fixture you would have 3 of these. If you look inside an led par can that is how it's usually done.

[gyro_gearloose]

The problem I’ve found with many of the off the shelf drivers is that even when they do allow some form of dimming (DALI, PWM, 0-10V, and, rarely, DMX) the driver doesn’t seem to dim smoothly to 0%. The drivers often only dim to 1-5% before either snapping off themselves, or requiring the mains power to the driver to be cut off. Not very useful for theatrical dimming!

[boatman]

The problem I’ve found with many of the off the shelf drivers is that even when they do allow some form of dimming (DALI, PWM, 0-10V, and, rarely, DMX) the driver doesn’t seem to dim smoothly to 0%. The drivers often only dim to 1-5% before either snapping off themselves, or requiring the mains power to the driver to be cut off. Not very useful for theatrical dimming!

 

The problem with cheap LED controllers is that they are only 8-bits and any LED run at a PWM ratio of 1/256 is still quite bright. You need to translate the 8-bit DMX value into a 16-bit PWM value using a square law like this (((DmxValue + 1) ^ 2) - 1). That will translate 0-255 into 0-65535 and give you a proper dimming curve.

[timsabre]

The problem with cheap LED controllers is that they are only 8-bits and any LED run at a PWM ratio of 1/256 is still quite bright. You need to translate the 8-bit DMX value into a 16-bit PWM value using a square law like this (((DmxValue + 1) ^ 2) - 1). That will translate 0-255 into 0-65535 and give you a proper dimming curve.

 

This would be ideal but very few drivers will run or switch fast enough to do 16 bit PWM. You generally have to fudge it by messing around with the bit periods so that the lower-order bits are very short and the higher order bits are long, which gives you smaller dimming steps at the bottom.

[sunray]

The thing with LEDs is they have a fixed forward voltage, which is about 2v for a red and 3v for other colours (per individual LED die). As you increase the power supply voltage, the LED attempts to hold the voltage to its 2 or 3v, drawing more and more current until it eventually burns out.For small 20ma leds you can control the current using a resistor in series, and the resistor absorbs the extra voltage, but for power leds the resistor dissipates a silly amount of power (but some fixtures, probably including yours, still do it that way).The best way is a constant current power supply which automatically varies its output voltage until it is providing the desired current - in an rgb fixture you would have 3 of these. If you look inside an led par can that is how it's usually done.

 

The original IR controllers really were a SMPS running at 36 to 32V and 3 FETs, very obviously cheap and cheerful.

My only other experience of this sort of thing has been repairing 155LED eurolites which run 3 10mm LEDs in series with a resistor.

 

Are there any 'off the shelf' controllers ready to drop straight into LED conversions?

[timsabre]

The original IR controllers really were a SMPS running at 36 to 32V and 3 FETs, very obviously cheap and cheerful.

My only other experience of this sort of thing has been repairing 155LED eurolites which run 3 10mm LEDs in series with a resistor.

Are there any 'off the shelf' controllers ready to drop straight into LED conversions?

 

SMPS is a lot better than your transformer though as the output is very regulated. If you can set the SMPS to near the forward voltage of the LEDs, then put in a series resistor on the reds to handle the lower voltage, then you can get away with it, it's a horrible way to do it and prone to overdriving the LEDs. With your transformer method you risk seriously overdriving the LEDs which will result in them failing well before their rated life.

 

Eldoled do some nice drivers for RGB LED conversions. For single colour, Meanwell also.

[alistermorton]

The problem with cheap LED controllers is that they are only 8-bits and any LED run at a PWM ratio of 1/256 is still quite bright. You need to translate the 8-bit DMX value into a 16-bit PWM value using a square law like this (((DmxValue + 1) ^ 2) - 1). That will translate 0-255 into 0-65535 and give you a proper dimming curve.

 

This would be ideal but very few drivers will run or switch fast enough to do 16 bit PWM. You generally have to fudge it by messing around with the bit periods so that the lower-order bits are very short and the higher order bits are long, which gives you smaller dimming steps at the bottom.

 

I wonder if there's a happy medium = 10 or 12 bit dimming perhaps. 10 bit gives you 1024 steps (so a switch frequency of about 1kHz?) and 12 bit 4096 - is ~4kHz too fast for most switchers?

 

EDIT: Also - the RC4 dmx2dim (for example) does very smooth dimming of LEDs - much better than the cheap dimmers you get on eBay, yet it's being fed the same DMX data. I'm guessing there's some additional slew rate limiting going on with the pulse width transitions.

[timsabre]

I wonder if there's a happy medium = 10 or 12 bit dimming perhaps. 10 bit gives you 1024 steps (so a switch frequency of about 1kHz?) and 12 bit 4096 - is ~4kHz too fast for most switchers?

 

No, at 10 bit, say the PWM is running at 200Hz which is about the slowest you can get away with, each bit is 4.8 microseconds wide.

At 12 bit, each bit is 1.22us. Most switching drivers run at 500KHz to 1MHz and you need a few cycles of the switcher for each pwm pulse to keep the thing stable and non flickery - so 10 bits is about the best you can do.

 

If you are just switching DC to LED tape like the RC4, rather than a constant current drive, you can go quite a bit faster.