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Topic: Controlling 50 x 3 watt LEDS (Read 579 times) previous topic - next topic

Photosbykev

Aug 20, 2018, 04:21 pm Last Edit: Aug 20, 2018, 04:27 pm by Photosbykev
Good morning forum,

I'm just starting to put together a lighting dome for some Reflectance Transformation Imaging (RTI) photography. Basically the process take loads of photographs of a target illuminated with light sources in difference positions and then uses software to analyse the image details.

The dome will house between 50-60 3 watt LEDs, I've chosen Bridgelux 6500k Daylight White: Forward Voltage: 3.2V-3.6V Forward Current: 600-750mA Output: 240 lumens for the diffuse light they output and the uniform colour temperature.

The dome will be portable so ideally the system will run from a 12v DC supply.

The photographic process is to turn on one LED, take a photograph (2-3 seconds) and turn the LED off, then select the next LED, turn it on, take a photograph and turn the LED off etc etc.

So you can see although there could be 60x3 watt LEDS in the dome only ONE led will be on at any one time. If possible I should be able to control the lighting intensity of all the LEDs to allow for bright or dark targets being photographed, maybe PWM?

I'm planning on using a Mega board to control the LED sequencing and ON/OFF duration and also trigger the camera at the correct time via an optoisolator.

and finally to the question(s) :) What is the current favoured way to control all the LEDs individually, individually wired via a constant current device? an array? are there shields out there that would handle this 'high powered' LED and switching between them?

To my simple mind I could use one constant current device and then switch the output to the selected LED rather than have multiple drivers?

best regards
Kev


Wawa

Some thoughts.
3.2V-3.6V @ 600-750mA is 1.92-2.7watt (3 Chinese watts?).
2.5watt needs cooling, so a star base?

12volt supply to ~3.3volt LED is a big drop.
Linear regulation at 750mA would produce a lot of heat.
You might have to use pre-regulation in the form of a 12>5volt buck converter.
That 5volt could also power the Arduino.

PWM is turning the LED 100% on and off, with varying duty cycle.
You might see that as a dimmer light, but a fast camera sees this as a flash with 100% intensity.
Might be better to use linear dimming.

I am thinking: high-side linear constant current source, with the current controlled by the Arduino, connected to the anodes of all the LEDs, and each LED individually switched to ground.

I don't know of any chip that can switch 750mA, so I think you have to use 50-60 logic level mosfets.
Shift registers or I2C port expanders and a Nano could be better/easier than a Mega.
Leo..

Photosbykev

#2
Aug 21, 2018, 05:26 pm Last Edit: Aug 21, 2018, 05:35 pm by Photosbykev Reason: correction on one sentence
Thank you for your input Leo, I've replied where I can inside your quoted text

Some thoughts.
3.2V-3.6V @ 600-750mA is 1.92-2.7watt (3 Chinese watts?).
2.5watt needs cooling, so a star base?

Yes, all the leds are mounted on star bases and would be stuck to the acrylic dome with silicon adhesive, maximum illuminated period for any one LED would be < 5 seconds and then off for 2 -3  minutes so I though additional cooling wouldn't be necessary.


12volt supply to ~3.3volt LED is a big drop.
Linear regulation at 750mA would produce a lot of heat.
You might have to use pre-regulation in the form of a 12>5volt buck converter.
That 5volt could also power the Arduino.

I was thinking 12v purely because I have a range of 12v batteries I already use for photography, no issue with adding in a dc-dc regulator to drop the supply voltage to the LED driver and probably a 12v - 9v regulator for the arduino


PWM is turning the LED 100% on and off, with varying duty cycle.
You might see that as a dimmer light, but a fast camera sees this as a flash with 100% intensity.
Might be better to use linear dimming.

Ok, I can understand that, the shutter speed is likely to be 2-3 seconds when the target is illuminated by a single 3 watt LED, adjusting the LED supply current is probably a better option than PWM but the dimming would need to be constant for all the LEDs which is why I was thinking of using a single LED driver. An alternative would be to use 1/2 second on 1/2 off 1/2 second on for x number of times to limit the total light output (a very crude PWM but probably adequate and easily coded)

I am thinking: high-side linear constant current source, with the current controlled by the Arduino, connected to the anodes of all the LEDs, and each LED individually switched to ground.

What arduino controlled TTL device would switch the current on the anode side?

That's the sort of think I'm visualising, using multiple LED drivers doesn't make too much sense as only one LED will be lit up at any one time

I don't know of any chip that can switch 750mA, so I think you have to use 50-60 logic level mosfets.
Shift registers or I2C port expanders and a Nano could be better/easier than a Mega.

Can you suggest a suitable Mosfet? Could I drop the number of wires by using an 8 x 8 array with Mosfet switches on the anode and cathode sides of the array and all the anode side switches connected to the same constant current source?

I've only select a Mega because I have handfuls of them for various camera projects :)

Leo..

best regards
Kev

Wawa

#3
Aug 21, 2018, 10:54 pm Last Edit: Aug 21, 2018, 11:24 pm by Wawa
no issue with adding in a dc-dc regulator to drop the supply voltage to the LED driver and probably a 12v - 9v regulator for the arduino
LED (with current limiting) and Arduino can both be powered from 5volt.
Connect directly to the 5volt pin of the Arduino.

Thought about using an 8*8 matrix, with 8 high-side switches and 8 low-side CC sinks.
Would be a balancing act between Arduino pin current and supply headroom.
Leo..



DrAzzy

#4
Aug 22, 2018, 08:11 am Last Edit: Aug 22, 2018, 08:13 am by DrAzzy
I would put a 750mA constant current regulator (AMC7140 or 2xAMC7135 in parallel) on the low side, a fet on the low side of each LED, with the source connected to the CC reg and the drain connected to the LED; lots of cheap FETs are available in SOT-23 that would be fine for this, and drive the FETs with a string of shift registers.

Since the fet source won't be at ground, but a couple of volts above it (5v - LED Vf), you just need fets rated for <2.5v gate drive.
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Wawa

You could make an 8*8 matrix with eight of those AMC7140 chips low side, and eight p-channel fets high-side.
Leo..

Photosbykev

Thank you for all the input folks, i'm slowly getting a handle on what will be the best way of addressing this challenge :)

Kev

Unsigned_Arduino

  You could use a relay to switch the light on and off.
Do not touch the String class.

Ever.

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