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Topic: "Fooling" constant-current control loops in LED driving? (Read 7 times) previous topic - next topic

petteri_t

Dear all,

Has someone tried to drive LEDs by switching the output of a constant-current supply (like a lab power supply or a constant-current LED driver) with the PWM from Arduino? I tried to switch the current from a benchtop lab power supply with the Sparkfun MOSFET Power Control Kit (https://www.sparkfun.com/products/10256), and clearly the constant-current mode did not like it and the output current did not stay stable.

I understand that the LED driving scheme could be designed from a scratch or simply use commercial LED drivers such as BuckPuck (http://www.ledsupply.com/buckpuck.php), XP Power (http://www.xppower.com/productSelection.php?groupid=100019&lang=EN) or Allegro (http://www.allegromicro.com/Products/Regulators-And-Lighting/LED-Drivers-For-Lighting/A6211.aspx) that allow dimming using a PWM dimming. However I would like to increase the dimming range by adding the switching stage in cascade with the already existing constant-current supply, and at some point modulate the constant current for sinusoidal output with the PWM.

I thought of doing a "brute force" non-elegant tryout circuit with NPN and PNP transistors so that the HIGH signal of PWM would switch on the NPN and turn on the light and during the LOW signal of PWM the PNP is turned on and the constant current can go through it so that the sum current is always the same independent of the duty cycle. I failed to find any example circuits really for such an idea so I was hoping that maybe some forum reader could help? Could LED be driven efficiently with Dual npn-pnp complementary bipolar transistors? Or are there more elegant solutions?

Thanks

dhenry

Quote
Sparkfun MOSFET Power Control Kit (https://www.sparkfun.com/products/10256), and clearly the constant-current mode did not like it and the output current did not stay stable.


You do realize that that particular kit is not a constant current driver, right? However, if you add a small npn + a resistor to it, you can easily make it into a constant current drive.

Alternatively, you can find a switching mode power supply and pwm its on/shutdown pin to pwm dim the leds.

petteri_t

#2
Oct 19, 2012, 12:31 am Last Edit: Oct 19, 2012, 12:38 am by petteri_t Reason: 1

You do realize that that particular kit is not a constant current driver, right? However, if you add a small npn + a resistor to it, you can easily make it into a constant current drive.


Yes I realized that it was not a constant-current driver but had that available and thought of trying it with the benchtop power supply (ISO-TECH DC Power Supply IPS 3303S, http://radionics.rs-online.com/web/p/bench-power-supplies/6842998/). Do you think there is a more elegant way to switch the output from constant current supply such as those from benchtop lab power supply or from commercial LED drivers

[edit] The basic circuit that I know just sets up the current with the R3 resistor (http://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/step2/Specs-Function/) but as I would already have the constant current source, I would like to tinker the circuit so that I could add the switching stage.

Boffin1

I am not sure what you are trying to do , but you could drive a simple constant current driver with the pwm to vary the brightness while limiting the max current to 20mA
With my mobile phone I can call people and talk to them -  how smart can you get ?

petteri_t

#4
Oct 19, 2012, 01:03 am Last Edit: Oct 19, 2012, 01:48 am by petteri_t Reason: 1

I am not sure what you are trying to do , but you could drive a simple constant current driver with the pwm to vary the brightness while limiting the max current to 20mA


Basically I just to want to increase the dynamic range of the light intensity. So if I would for example have a constant current supply (DC output) with 16-bit resolution, I could then the switch off then that output with 8/10-bit PWM of Arduino Uno or 16-bit PWM from Digilent Uno32 for example so I would in theory have 32-bit resolution for light intensity. So with that design idea I could replace independently the constant current and switching parts if better components become available. And my max current to be switched is at the moment 1 A.

So in other words the current sensing part of the constant current source would not probably in general like that the output current is switched ON and OFF and I was looking for ways to make the feedback loop to think that the current output from the constant current source is always the same even though the current passing through would not be the same, i.e. it would be determined by the PWM duty cycle outputted from Arduino. And when the PWM would be LOW the current would go through the PNP-transistor.

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