Old disco lighting - LED conversion

[Nakatomi]

So long as you don't need a back reflector to focus a beam there's quite a lot you can get away with I expect. But if you do, where would you even start working out what lenses to use, let alone try to source them?

[dicky]

Right-O here we go.

I've ordered some 100W RGB LEDs today (got a better deal ordering more than one) however they have to come from China so probably willl be a couple of weeks or so but they are cheap enough - about a fiver each.

I'm gonna have a go at that mystery/InvincibLED/Lex/ whatever it is fixture I have..... and try to upgrade it from 10W-ish (I actually think it is 3 x 3W).... to 100W RGB LED

Will post here as things go along, or maybe not.. as the case may be..... but I will give it my best shot anyway

Rich

[musicology]

Blimey is that all they cost? We are being seriously ripped off for the new led fixtures. Reminds me of all the years we were being overcharged for cd's!

[funktions]

Right-O here we go.

I've ordered some 100W RGB LEDs today (got a better deal ordering more than one) however they have to come from China so probably willl be a couple of weeks or so but they are cheap enough - about a fiver each.

I'm gonna have a go at that mystery/InvincibLED/Lex/ whatever it is fixture I have..... and try to upgrade it from 10W-ish (I actually think it is 3 x 3W).... to 100W RGB LED

Will post here as things go along, or maybe not.. as the case may be..... but I will give it my best shot anyway

Rich

stupidly last night I also ordered some bits, see what someone has started, and there I was looking to spend some on a nice shiney behringer .. only ordered enough bits to do one. just in case lol

[dicky]

I have also ordered a 30W RGB LED as I think I could connect this in direct replacement of the 3W RGB on the fixture I want to try upgreade (that mystery LED light I posted a few weeks back)

If you look at this listing http://www.ebay.co.uk/itm/1W-10W-20W...item4864dee330 and select the different wattage LEDs then look at the pictures you will see that the 30W LED is basically 10x 1W LEDs in series (per colour) and the 50W (they call it but it must be 60W) is two series chains of 10x1W in parallel and then the 90W is 3 series chains of 10x1W in parallel (per colour)

So you get 30W RGB = 10W per colour, 60W RGB = 20W per colour and 90W RGB = 30W per colour

The upshot of all this - well the original fitting has 3W per colour (which is series chain of 3x1W LEDS per colour) with a 12V supply, a 5 Watt series resistor (value varies per colour as the forward voltage drop for three Red LEDs in series is considerably less than Blue or Green) and a IRFZ44N powerFET to turn the colour on or off. http://www.irf.com/product-info/data...ta/irfz44n.pdf

So ohms law and all that stuff tells us if we fit a 30W LED instead of a 3W, then we need to increase the supply voltage from 12V to around 36V as we now have 10 LEDs in series per colour, but the current drawn is the same as for a 3W LED This means the existing output devices can still handle the higherpower LED

The only caveat with this that Ican see is that the difference in forward voltage drop between the Red and the Green/Blue (which are quite similar to each other) LED chains is now three times larger or thereabouts.

So for 3 Green or Blue LEDs 1W in series you probably have a forward voltage of around 10 Volts. For 3x 1W Red its probably about 6.6V so the difference is around 3.5 V extra being dropped by the Red series resistor which means it runs hotter (more wattage) than the Blue and Green ones.

However for 10x 1w LEDS in a chain its gonna have a forward voltage drop of around 33V or so for the Blue and Green but only 22V for the Red. Now here we start to appreciate the crux of the matter. This extra voltage needs to be dropped by the series resistor and this means changing the Red series resistor for a higher value but it also means it is gonna dissipate around 3x the wattage of the existing one. So to be safe I probably need a 20W resistor of a suitable ohm value (which I would make from using 2x 10W resistors in parallel as they are easily available) and I need to make sure the lamp can dissipate the extra heat so probably need to fit a small fan in the rear of the casing. And of course a large heatsink/fan on the 30W LED

To increase to 90W (30W per colour) needs the same 36V(ish) power supply but the current is now 3 times higher also. From the IRFZ44N datasheet the output devices would handle this OK, though probably need to be removed from the PCB and mounted where I can attach a heatsink then wired back to the PCB with extending wires. Same applies to the series resistors as they would also dissipate 3x the wattage as heat.

I'll get all this set up on my test bench once the LEDs arrive so I can work out the resistor values I need to fit but from a bit of research and reasoned thinking I suspect upgrading a 3W fixture to 30W is generally quite simple - just a matter of fitting an additional 36V power supply for the LED (the existing circuitry can run from its supply as it does now) checking that the output FETs can handle the higher voltage and tinkering around with the series resitors and cooling arrangements.

To upgrade to 90W requires more modifications to the output stage due to the 3x greater current and the wattage dissipated in the REd LEDs series resistor in particular statrs to get a bit silly

That's my thoughts on it anyway

Rich

[DJ Jules]

Rather than messing about with the resistors it might be worth considering using a single constant current driver to create a new regulated supply and then switch that supply instead using the FET's?

You'd have considerably lower heat output inside the fixture?

I didn't realise that the forward voltage across the red LED's was less than green/blue - interesting to know! Is this the same in tape strips, etc?

Julian

[dicky]

Yes, if they have the same number of devices per chain as the green and blue as a Red LED has a forward voltage of about 2V and a Blue or Green about 3V

Problem with using a constant current source is that all the Anodes are connected together in the device so you need to use one supply voltage that has to be at least 33V..... so you have this problem of 11V to get rid off on the Red chain. Also the current is not constant, it depends on what combination of colours are lit

This article will give you a lot of useful info Jules - a lot of what I posted comes from here http://www.tbideas.com/blog/build-an...power-rgb-led/

Rich

[funktions]

just had the last part delivered today, BUT due to a slight eye issue I might have to put it on hold for a few days. I have a feeling that there will be "just" enough space

[dicky]

I only ordered the 30W RGB LED today so it seems like you are gonna get there before me mate

[Nakatomi]

Yes, if they have the same number of devices per chain as the green and blue as a Red LED has a forward voltage of about 2V and a Blue or Green about 3V

Problem with using a constant current source is that all the Anodes are connected together in the device so you need to use one supply voltage that has to be at least 33V..... so you have this problem of 11V to get rid off on the Red chain. Also the current is not constant, it depends on what combination of colours are lit

This article will give you a lot of useful info Jules - a lot of what I posted comes from here http://www.tbideas.com/blog/build-an...power-rgb-led/

Rich

It'd be a constant current source per LED colour not overall wouldn't it? So you'd keep the anodes commoned but just limit the current to the R,G & B cathodes separately I reckon.

Some solutions look like they set the current limits in software by adjusting the PWM ratios.. not really what you want for a retrofittable solution.