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PWM to Analog

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EEYORE

Mon Jul 19 2010, 09:03PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello all,
I am building a high power LED light for my reef tank and need some help. I want to use some driver boards I made a while ago meant for driving high power lasers via an op-amp and mosfet. A voltage divider using a voltage reference varies the current from zero up to max.

Now what I need to do is be able to use a micro controller to control this current controller so that I can have special effects and special control over the leds. I know that most the microcontrollers out there that reefers like to use, use PWM output. I need to hook up the controller I made (that uses a voltage to control current) right up to a PWM output.

Can I use something like a resistor and capacitor style filter?

Thanks!
Matt

Bjørn

Mon Jul 19 2010, 10:10PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Yes as long as you don't have any special requirements when it comes to bandwidth. Connect a capacitor between ground and the PWM output using a series resistor. Make sure the rest of your circuit does not place any load on the capacitor.

EEYORE

Mon Jul 19 2010, 10:33PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello,
There shouldnt be any load on the PWM output since it is controlling the voltage on the op-amp input pin. How should I calculate this RC filter? Im guessing it will depend on the PWM?

Anything in particular to consider about using this method? I have seen a few people using an op-amp to actually perform this task. Might I be able to just skip the RC filter since I am using this as an input to the op-amp inplace of its current voltage reference+divider method to get a variable (and stable) voltage?

Thanks!
Matt

Bjørn

Tue Jul 20 2010, 12:06AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

If you want a voltage you need a filter, a simple RC filer or something more advanced. It may work without and run control the LEDs by PWM or it might destroy the LEDs, it all depends.

The cut off frequency is 1/(2*PI*R*C), that is the frequency that will be halved in power after passing through the filter.

Pinky's Brain

Tue Jul 20 2010, 12:36AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Just PWM switching the LEDs directly is a lot more energy efficient of course.

EEYORE

Tue Jul 20 2010, 02:11AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Thanks guys...

I chose to avoid PWM as the main way to control the LEDs so that I could just use my on board voltage reference+divider to mix two different strings of LEDs. Blue (460nm) and white (6500k). I can dim the whites with one trimpot and as a result have a more blue looking tank. Or I could dim the blues for a more white looking tank. If I choose to use a microcontroller to do all this for me, then I can switch the op-amps inputs (its a dual op-amp) over to a microcontroller via the RC filters to turn the PWM into a voltage.

This can also double as a very high power laser diode driver.

Sound good?
Matt

UltraMagnus

Tue Jul 20 2010, 06:03AM

Registered Member #2875 Joined: Mon May 24 2010, 08:28AM
Location: England
Posts: 42

A lot of PIC microcontrollers have an analogue output, not sure if that may be easier

Mattski

Tue Jul 20 2010, 07:48AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Since you're not using this PWM to actually drive the LED's, and little current will flow to the opamp, you should make the cutoff frequency pretty low just to filter the signal better. I would put the filter cutoff a decade (factor of 10) or more below your switching frequency. There is a tradeoff in that if you need to be able to quickly change that filtered analog voltage, you need the cutoff frequency to be higher. But 50Hz is probably a good conservative upper bound, so for example 500Hz cutoff will give you tons of signal bandwidth while probably filtering it pretty good too.

edit: If you want to know where the filter theory comes from, take a look at

As Bjorn pointed out an RC filter is a first order filter with a cutoff of 1/(2*pi*R*C). At low frequencies, including 0Hz (for an unchanging DC output voltage), the voltage gain of the filter is 1: no gain and no loss. At frequencies above the cutoff frequency, the gain basically decreases a factor of 10 for every factor of 10 increase in frequency.

The PWM signal that you are filtering consists of the DC or average component that you want to be output as a constant voltage, plus sine waves at the switching frequency of the PWM and its harmonics. If you make a first order filter with a cutoff at 1/10 the switching frequency you will thus be attenuating the switching components by a factor of 10 or more.

lightlinked

Tue Jul 20 2010, 10:48AM

Registered Member #2087 Joined: Tue Apr 21 2009, 08:32AM
Location:
Posts: 115

wouldnt it be more ideal to use a high power led driver SMPS buck controller? it would be more efficient since it would be a constant current LED controller based on a switching design so you would not be dissipating power away as heat for current limiting as a linear design would.

radiotech

Tue Jul 20 2010, 03:40PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

First, does your micro have an output that can be configured as an analog output?

Can you add a photoresistive element to your current controllers input system of voltage dividers such that if a light shines on the element the resistance drops? A CdS cell does this. Then get the PWM output to light a While LED shining on the CdS cell. This will give you isolation. Connect a 0.47ufd capacitor across the CdS cell.

If the PWM output source 0.06 Amps-, use a 2 volt #49 lamp instead of the LED.

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