Quad channel high power LED driver
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Andrew Stone
Sep 28, 2011, 11:00:32 PM9/28/11
to toasted-circu...@googlegroups.com, Kevin McPherson
As some of you have seen, I made a quad channel high power LED driver add-on for the Lightuino boards. Thanks Kevin for reminding me that I haven't posted info about it yet.
Attached is a closeup.
Its made with header pins so you can plug it into a breadboard (as shown).
The way it works is it senses the voltage going through the big flat resistors (and then through your LEDs) on the bottom and it uses a quad-channel opamp to match that voltage to a drop across the variable resistor on the upper right. So if you turn the variable resistor, the circuit reacts by changing the CURRENT moving through the resistors. The extra voltage is dissipated in the 4 large transistors (which is why they are so big). The same thing occurs if you change the load (i.e. the LED) or change the LED input voltage. The end result is a constant-current circuit.
This circuit is not as efficient as some that you can buy online which essentially use small switched-mode power supplies per LED. So you want to use an external wall-wart transformer to make your input voltage to be just a bit above your high-power LEDs (for efficiency). Also make sure that the big transistors don't heat up too hot; if they do use a lower voltage, add a heat sink, or use 2 LEDs in series. But it is a lot cheaper per channel and is adjustable!
To pick your current (without blowing out your LED), please hook up a few low ohm resistors in parallel (or one high wattage) to simulate the LED. Then use your multimeter to measure the current flowing through them. Next turn the pot on the right until you are seeing the amperage you need!
How to hook it up:
There are 12 pins across the top in a group of 7 and then 5. In reading order the pins are:
Group 1:
1,2,3,4: LED control (on/off PWM). Connect Lightuino's outputs here.
5: 5v Connect 5V from the Lightuino here
6: GND connect GND here. DO NOT swap 5v and GND or you will blow out your board. (Sorry I should have put a diode in, but I did not realize the chip on the lower left would be so sensitive).
7. LED: LED high side input voltage. Actually you don't need to use this. You can connect your LEDs directly to the voltage source just like on the Lightuino. This is just for convenience.
Group 2:
8,9,10,11. LED cathodes (-). Connect the output or ground side of the LEDs here.
12. (and the unpopulated holes below pins 8,9,10,11). LED anodes. These are connected to pin 7, so if you choose to input a voltage there it will come out here. This would allow you to solder an 4x2 ribbon cable header onto the top of the board (for example) and get 4 pairs of LED anode/cathode wires. But that's not the normal way I make them...
When first trying it, leave 1,2,3,4 (the PWM) unconnected.
Now hook up 5&6 (5v and GND). Then hook your LED (or resistors if you are setting the current) up to your LED Vin (basically whatever you want, the transistors are good up to 50v if I remember correctly -- but for heat reasons, keep it within a few volts of what your LEDs need) and then to one of the LED cathodes (pins 8-11). Apply power and your LED should light!
Now you can hook the PWM channels up the the Lightuino. In code, the "sense" will be the opposite. So if you write a 8192 (MAX_BRIGHTNESS) value in the Lightuino, that will be OFF. And writing a 0 will be 100% on. I know, its inconvenient. But that's what code is for (and its easy to hide in a helper function)!
Advanced Note:
Note, you can actually use this circuit with an Arduino (or any uP) instead of a Lightuino. But there is a caveat: if you drive the signal HIGH that will turn it on with constant VOLTAGE. Like a normal low-side transistor switch. If you set the signal as an "input" (or high impedance) that will turn on the constant current feature, and setting it low will turn it off.
Full schematics and board design are available on request (MIT license -- go crazy!) and I'll check them into the code.google.com site where the Lightuino code is too.
Cheers!
Andrew
Attached is a closeup.
Its made with header pins so you can plug it into a breadboard (as shown).
The way it works is it senses the voltage going through the big flat resistors (and then through your LEDs) on the bottom and it uses a quad-channel opamp to match that voltage to a drop across the variable resistor on the upper right. So if you turn the variable resistor, the circuit reacts by changing the CURRENT moving through the resistors. The extra voltage is dissipated in the 4 large transistors (which is why they are so big). The same thing occurs if you change the load (i.e. the LED) or change the LED input voltage. The end result is a constant-current circuit.
This circuit is not as efficient as some that you can buy online which essentially use small switched-mode power supplies per LED. So you want to use an external wall-wart transformer to make your input voltage to be just a bit above your high-power LEDs (for efficiency). Also make sure that the big transistors don't heat up too hot; if they do use a lower voltage, add a heat sink, or use 2 LEDs in series. But it is a lot cheaper per channel and is adjustable!
To pick your current (without blowing out your LED), please hook up a few low ohm resistors in parallel (or one high wattage) to simulate the LED. Then use your multimeter to measure the current flowing through them. Next turn the pot on the right until you are seeing the amperage you need!
How to hook it up:
There are 12 pins across the top in a group of 7 and then 5. In reading order the pins are:
Group 1:
1,2,3,4: LED control (on/off PWM). Connect Lightuino's outputs here.
5: 5v Connect 5V from the Lightuino here
6: GND connect GND here. DO NOT swap 5v and GND or you will blow out your board. (Sorry I should have put a diode in, but I did not realize the chip on the lower left would be so sensitive).
7. LED: LED high side input voltage. Actually you don't need to use this. You can connect your LEDs directly to the voltage source just like on the Lightuino. This is just for convenience.
Group 2:
8,9,10,11. LED cathodes (-). Connect the output or ground side of the LEDs here.
12. (and the unpopulated holes below pins 8,9,10,11). LED anodes. These are connected to pin 7, so if you choose to input a voltage there it will come out here. This would allow you to solder an 4x2 ribbon cable header onto the top of the board (for example) and get 4 pairs of LED anode/cathode wires. But that's not the normal way I make them...
When first trying it, leave 1,2,3,4 (the PWM) unconnected.
Now hook up 5&6 (5v and GND). Then hook your LED (or resistors if you are setting the current) up to your LED Vin (basically whatever you want, the transistors are good up to 50v if I remember correctly -- but for heat reasons, keep it within a few volts of what your LEDs need) and then to one of the LED cathodes (pins 8-11). Apply power and your LED should light!
Now you can hook the PWM channels up the the Lightuino. In code, the "sense" will be the opposite. So if you write a 8192 (MAX_BRIGHTNESS) value in the Lightuino, that will be OFF. And writing a 0 will be 100% on. I know, its inconvenient. But that's what code is for (and its easy to hide in a helper function)!
Advanced Note:
Note, you can actually use this circuit with an Arduino (or any uP) instead of a Lightuino. But there is a caveat: if you drive the signal HIGH that will turn it on with constant VOLTAGE. Like a normal low-side transistor switch. If you set the signal as an "input" (or high impedance) that will turn on the constant current feature, and setting it low will turn it off.
Full schematics and board design are available on request (MIT license -- go crazy!) and I'll check them into the code.google.com site where the Lightuino code is too.
Cheers!
Andrew
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