Converting 24VAC Halogen SCR Dimmed Fixture to LED

Thread Starter

Tim Davis

Joined Jun 10, 2015
23
Hello, This forum looks like a great place.. I hope I can get some help. I have been reading alot but hoping some experts can chime in.

I have an "intelligent lighting fixture".. Ie. A robot with a lamp.. known as a "HES Trackspot" Used in nightclubs. The fixture is from 1992 with a simple macro size PCB. I am attempting to convert this unit to use high current energy efficient LED for less power and brighter colors. The issues I cannot resolve are

1) Fixture uses SCR for dimming, LED's mostly use PWM
2) Output voltage for lamp is 24VAC @ 250W, Input voltage for LED's is DC

So basically im trying to install this LED
http://www.luminus.com/products/CBM-360.html

This is the schematic for the old Fixture (Note page 14 shows the dimming circuit)
https://www.highend.com/pub/products/automated_luminaires/Trackspot/Schematic/Tspotr15.pdf
http://www.greatneckgames.com/images/audio-visual-lighting/large/trackspot.jpg (PIC)

So im guessing I need a constant voltage for the LED, an LED IC Driver for the dimming/power and some way to marry the information from the SCR circuit to the IC Driver.. all while using the Switching, Dimming of the original fixture. The original bulb was 250 Watt halogen and I believe the dimming for the bulb terminates with a decreased voltage to the bulb.

Hope this makes any sense!
 

Thread Starter

Tim Davis

Joined Jun 10, 2015
23
Hello.. Thanks for your reply. Yes we need higher color temperature. The link you provide says 250 Watt equal but the lumens are only 2500 Lumens. The lumens of the halogen lamp are 5600-7000. I have since purchased this LED http://www.ledquotations.com/three-...igh-power-80w-led-chip-integration-45mil.html which is actually only 1 inch across.

So my question now is I will purchase a seperate power supply for the LED as its the only option for a constant forward current to the LED. The power supply accepts 1 - 10V dc for dimming. So now I only need two things.

1) A way to trigger the LED to power on when the original lamp was supposed to strike
2) A way to hook into the SCR circuit (diagram above) to transform to 1-10V DC
 

Thread Starter

Tim Davis

Joined Jun 10, 2015
23
I just realized that the dimming is already wired into the final 24V out (to the halogen lamp).. I could take this 24V and convert it into 10-12V somehow. It would need to adjust by voltage so say 24VAC would give 12VDC but 12VAC (dimmed) would give 5VDC. My only DUMB question is the bulb was rated at 250Watts @ 24VAC (10AMPS) but the max input current of the dimmer is 0.5A
 

ebeowulf17

Joined Aug 12, 2014
3,307
Specs, model number on dimmer? I would expect your 1-10V dimming signal input to be high impedance and not draw significant current unless driven with way too much voltage, but that's just a guess. Need to see datasheet to really know.

As for 24V SCR to 1-10V signal conversion, it sounds to me like an ideal application for an op amp integrator circuit... but I'm still new at all this and have only read about them, never built one. You'd definitely want someone more experienced to confirm or deny my theory.
 

Alec_t

Joined Sep 17, 2013
14,280
The LED in post #3 has this spec:-
DC Forward Current(IF): 2400mA
DC Forward Voltage(VF): DC 28.0-36.0v

That means your existing 24VAC supply won't be directly usable to power the LED; an AC/DC adapter will be needed to give a DC supply > 36V. Do you already have one? If not, a dimmable LED ballast is called for.
It would be relatively simple to convert the existing dimmer output (phase-controlled 24VAC) to a 0-10VDC (or similar) control signal for the ballast.

Edit:
Here's how you could do the conversion.
Dimmer-to-DC.gif
R4 is a dummy lamp load to maintain the SCR current above the 50mA minimum holding current for much of the mains cycle. The graphs show the converter output for dimmer settings of 100% (yellow) and 50% (blue).
 

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rb3

Joined Jan 6, 2020
1
Hey I own some trackspots and was curious about doing a modification similar to this to convert them to LED. Was the project successful? If so, Can you share the build process and parts used?
 

MisterBill2

Joined Jan 23, 2018
18,167
I just realized that the dimming is already wired into the final 24V out (to the halogen lamp).. I could take this 24V and convert it into 10-12V somehow. It would need to adjust by voltage so say 24VAC would give 12VDC but 12VAC (dimmed) would give 5VDC. My only DUMB question is the bulb was rated at 250Watts @ 24VAC (10AMPS) but the max input current of the dimmer is 0.5A
If the LED lamp assembly you are considering already requires 24 to 36 volts then you might get away with just a rectifier and a bit of filtering. But the very first thing to do is ask the LED lamp manufacturer if they have a recommended circuit to drive their product in a way so that it provides the performance that they claim. MOST manufacturers will be happy to describe the circuit that works best with their product.
 

mewanchyna

Joined Apr 17, 2020
1
Hey I own some trackspots and was curious about doing a modification similar to this to convert them to LED. Was the project successful? If so, Can you share the build process and parts used?
I am also curious about this. I am currently repairing many old Trackspots as well.
Mark
 

Thread Starter

Tim Davis

Joined Jun 10, 2015
23
I am also curious about this. I am currently repairing many old Trackspots as well.
Mark


Hello I will use this reply to update everyone on this project. This is alot.

What I did Back in June 2015
I originally bought a 100W LED, cooling it was difficult, as there is enormous heat from a very small area, and there is no room in the TS for a large CPU heatsink. I ended up using a CPU water cooler which worked well. I even got the dimming working. The result was nice as an experiment, but not very practical. The LED needed its own constant current power supply which was another huge brick of weight which we would have to attach to the back of the enclosure somehow, then throw in the huge radiator for the water cooler and another attached 55mm fan attached to the front... No real way to mount the LCD either.

The result? Most of the light energy (as in the halogen) was wasted as heat, I use the wrong color temperature LED so all the colors of the dichroic filters was off, the output was pretty much identical in light output (without a meter). I determined it was not worth the effort.

Fast forward to Feb 2021
Things have changed considerably in the land of LED's and microprocessors (Audrino/Rasberry Pi).. I decide that we can now eliminate the entire PCB & Power Supply and build new PCB/ Power supply. Can also use a rasberry pi to control the stepper motors and sensors, and DMX. We can use a 3D Printer to print out a tube for the light to flow. It might look like this



But now the circuit is getting rather complicated. I would be using off the shelf parts with some soldering/circuits which would need:

5 x 24V Stepper Motor Drivers
1 x Constant Current LED Driver circuit capable of 24-36VDC
Power Circuit for Steppers
MCP3008 - ADC Chip circuit for measuring current of steppers and other analog inputs
4 x LED Driver Circuits (optional RGBW LED)
RS432 Circuit for DMX & associated Microchips
UART / Serial for DMX
Circuit for Determining the resistance of the pan & stepper stepper motors hitting an obstacle (the stop thingie.. for homing)
Circuit for the two optical sensors on Gobo & Color wheels
All the source code to accept DMX & Convert DMX to GPIO

Now this is all actually doable, but im afraid the resulting PCB won't be much smaller than the original when you throw all those components, chips, heatsinks, and stuff on it. And after deeper investigation, I see that some circuits (namely the Stepper Motor circuit) on the TS are superior to the Raspberry Pi board circuits out there today using the L6219 package. But with the skill of people here, it may be possible to get that final circuit down to something small, good and efficient.

So that leaves the choice of all of the above, or.. Find a way to mount a heatsink in the TS and leave everything else alone.. You would not get RGB Color, but you could probably use a smaller/lighter power supply (shave 15 lbs off the weight) and never have to change the bulb again.... But going the Pi / Audrino route means you get 1.6 million colors, high speed strobe, quieter operation, ability to add stuff like Wifi, Wireless DMX, and Phone control., ability to do complex real time color schemes, And of course far less energy usage

Im curious to hear peoples thoughts as I have all that stuff in my cart on Amazon.
 

Thread Starter

Tim Davis

Joined Jun 10, 2015
23
The LED in post #3 has this spec:-
DC Forward Current(IF): 2400mA
DC Forward Voltage(VF): DC 28.0-36.0v

That means your existing 24VAC supply won't be directly usable to power the LED; an AC/DC adapter will be needed to give a DC supply > 36V. Do you already have one? If not, a dimmable LED ballast is called for.
It would be relatively simple to convert the existing dimmer output (phase-controlled 24VAC) to a 0-10VDC (or similar) control signal for the ballast.

Edit:
Here's how you could do the conversion.
View attachment 87133
R4 is a dummy lamp load to maintain the SCR current above the 50mA minimum holding current for much of the mains cycle. The graphs show the converter output for dimmer settings of 100% (yellow) and 50% (blue).
Hello Alex . I didn't even see this in 2015 but am digging back into this project. I have taken your schematic and duplicated it in easyeda but I'm confused as to what it does. Does it just convert the 24VAC in to DC 1-10v? If so few questions..

1) Can we do 5 volts instead of 10?
2) I would need another power supply for the LED which I'm trying to avoid, I'm trying to use the 24VAC to power the LED via conversion circuit.. with dimming... is this possible? If not can I get some power from the board some other way? Because the power supply can do 350 watts I should be able to get some juice out of it
3) is the ground used for the DC output? And what does the arrow pointing from capacitor for?

Thanks!

For everyone else.. I read that color combination is very difficult with the sharp gobos and would be easier to just use the color wheel and white LED
 

Alec_t

Joined Sep 17, 2013
14,280
Does it just convert the 24VAC in to DC 1-10v?
Yes.
1) Can we do 5 volts instead of 10?
Just crank the pot down. But if you only want a 24VAC to 0-5VDC converter for a low current signal as a control input, that circuit is not ideal. You'd be better off with something like this :
24VACto5VDC.jpg
2) ..... I'm trying to use the 24VAC to power the LED via conversion circuit.. with dimming... is this possible?
Not with my converter circuit. You would need a proper LED driver module to suit the actual LED you are using. LEDs are current-driven devices, not voltage-driven.
If your LED has a forward voltage greater than the rectified and smoothed output of your 24VAC supply (i.e. about 33V assuming little ripple) then the driver module would need to be a boost converter with a controllable current.
3) is the ground used for the DC output? And what does the arrow pointing from capacitor for?
Yes. The arrow merely denotes an output.
 

Thread Starter

Tim Davis

Joined Jun 10, 2015
23
If the LED lamp assembly you are considering already requires 24 to 36 volts then you might get away with just a rectifier and a bit of filtering. But the very first thing to do is ask the LED lamp manufacturer if they have a recommended circuit to drive their product in a way so that it provides the performance that they claim. MOST manufacturers will be happy to describe the circuit that works best with their product.
I have since changed the LED to 30-34V 3A 100W .. manufacturer recommends any PSU with constant current of 3A and it will draw the appropriate voltage. I tried rectifier circuit, with multimeter at 5% brightness I got about 1.5 volts DC I tried it at 50% got like 17 volts. put multimeter to measure current.. diodes fried with nice cloud of smoke.. luckily no damage to fixture.

At 50%+ brightness, a rectifier would probably suffice on its own with a constant current boost convertor.. assuming we find a way to limit the output as we don't want 250 watts input. but The main thing I can't understand is.. the unit only outputs 0.5-1VAC at 5% brightness and I need 30 volts DC and I can't use ohms law calculator because I don't know how many amps it puts out at 1 volt AC..

Is it possible to boost 1VAC to 30VDC?
 

Alec_t

Joined Sep 17, 2013
14,280
The main thing I can't understand is.. the unit only outputs 0.5-1VAC at 5% brightness
5% of 24VAC rms is 1.2VAC rms. However, a conventional multimeter won't give reliable AC voltage measurements unless the waveform being measured is a true sinewave. A phase-controlled dimmer produces a chopped sinewave, not a true sinewave.
Is it possible to boost 1VAC to 30VDC?
A transformer could do that, but if you want 3A out you would have to put at least 30 x 3A = 90A in. That's a pretty chunky transformer.
 

Thread Starter

Tim Davis

Joined Jun 10, 2015
23
5% of 24VAC rms is 1.2VAC rms. However, a conventional multimeter won't give reliable AC voltage measurements unless the waveform being measured is a true sinewave. A phase-controlled dimmer produces a chopped sinewave, not a true sinewave.

A transformer could do that, but if you want 3A out you would have to put at least 30 x 3A = 90A in. That's a pretty chunky transformer.
Since the fixture already has a very chunky transformer weighing in at over 15 lbs I dont want to add any more weight.

I will abandon trying to get the required juice from the lamp output. And use @Alec_t circuit for PWM signal to LED Driver board which can accept any DC from 8-40V input. I will tap directly into the existing fixture 24VAC mains output (fused of course) which I measured to be actually 28V.

Can I just rectify this and feed that directly into LED driver board which accepts wide range of DC input voltage and is rated at 6A 150W?
 

MisterBill2

Joined Jan 23, 2018
18,167
The diodes fried because the 1N4000 series are one amp diodes. If the LED is intended to be powered by 30 to 34 volts at 3 amps, you should get a good amount of light using that same 24 volt transformer and a single diode in series. But you will need to have a diode rated for 5 amps so as to have an adequate safety factor. At that point you can evaluate the light output and see if you really want a lot more light. With no filter capacitor the power will not be excessive. If you use a full bridge with 4 diodes then the peak will be a bit over 34 volts and so there might be a problem. The average power would still be within limits bit not the peak power.

And now a question about that new 100 watt light: Is the LED connection isolated from the metal portion of the fixture? There are some useful options available if the LED circuit is well isolated from the hardware.
 

Thread Starter

Tim Davis

Joined Jun 10, 2015
23
The diodes fried because the 1N4000 series are one amp diodes. If the LED is intended to be powered by 30 to 34 volts at 3 amps, you should get a good amount of light using that same 24 volt transformer and a single diode in series. But you will need to have a diode rated for 5 amps so as to have an adequate safety factor. At that point you can evaluate the light output and see if you really want a lot more light. With no filter capacitor the power will not be excessive.
Wow it worked! After all this, one single diode!. Thanks Mr Bill! But it flickers like a high frequency flicker, it kicks on at about 8% brightness (about the same as the halogen), At 20% draws 0.14A, 40% @ - 0.7A, 50% @ 1A and I don't have 5A Diode yet so i kept it under 1A. Voltage went from 15.5 down to 8.7 volts at 50% and seems to increase current, and lower voltage as it warms up.

But I would like to squeeze more watts out of it, if 50% is 1Amp, then maybe 100% is only 2A, some reviewers suggest 3A and 31.5V Max, but I suspect the limitation is mostly thermal. I would like to consume at least 88 watts @ 3A . but other than the flickering, I can't believe it just works.

If you use a full bridge with 4 diodes then the peak will be a bit over 34 volts and so there might be a problem. The average power would still be within limits bit not the peak power.
Im ok with peaks above the max as, it's all about the thermal management, and this device is already designed to hold a mini sun. Also, because this is an intelligent fixture, we can simply dim them from the maximum brightness in software. I also may add an auto shutoff feature, Would this be four 5A diodes with no capacitor?

And now a question about that new 100 watt light: Is the LED connection isolated from the metal portion of the fixture? There are some useful options available if the LED circuit is well isolated from the hardware.
Well, I am designing the circuit (which was just reduced to a single diode), so I could put isolation into the final design through octo couplers, physical barriers, or electrical tape. The LED has a metal base, mounted to a heat sink, that is mounted to the frame of the unit that shares it's thermal load with other transistors, and an air duct. But the LED leads are technically isolated, but there would only be one power source (the big 24V transformer)

This is the new LED
https://www.amazon.com/Chanzon-3000K-3500K-Intensity-Components-Lighting/dp/B01DBZHSC8/

Any thoughts on the flickering? Its possible it's the LED but I have another one coming this afternoon, actually those numbers above are from the LED from 6 years ago, I now have warm white coming today, I will see if the numbers change, but I doubt it, all these LEDs are 1W arrays of 100.
 
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