Controlling 35W load from microcontroller

[mkelly09]

Hello,

A few months ago I posted a question about controlling the intensity of a car headlight from a microcontroller. I ended up using a PWM signal from a microcontroller to turn on/off an N-channel MOSFET, as shown in this circuit:

https://i.imgur.com/Ca8kJce.jpg

It ended up working great.. except for one major problem. I was planning on using the car headlight as a solar simulator to shine onto a solar cell. The issue is that the current generated out of the solar cell is also flickering at the same frequency that the transistor is turning on and off, which is a big problem for my application.

This method of controlling the intensity of the car headlight results in a flicker in the light source - one that is not noticeable to my eyes, but the solar cell picks it up quite well. I was able to increase the PWM frequency from 500Hz to 62.5KHz, but I still have noticeable flicker in the current being generated by the cell.


I need a new method to control the intensity of the light source from a microcontroller, one that does not cause any flicker in the light source. Does anyone have any ideas?

I was thinking of putting a large capacitor in parallel with the light source to smooth out the flicker, similar to a filter capacitor in a rectifier circuit. The problem is that when i simulate this in SPICE, I end up with a filter capacitor with extremely large values (on the order of 100s of mili farads!) in order to convert the waveform to DC. Also, i'm not sure what type of capacitor I would need.. the current through the light source is around 3 amps.

If anyone might be able to provide a suggestion or to steer me in the right direction, it would be greatly appreciated!

Thanks.

 

[FvM]

I doubt there can be significant intensity modulation of an incandescent lamp at 500 Hz PWM, and it's about to impossible at kHz frequency. More likely, there's some kind of electrical crosstalk in your circuit.

Nevertheless, if you want clean DC current, you can go either for linear regulator (requires a certain heatsink size) or a buck converter topology using a storage inductor and a diode. A filter capacitor can be used to further smooth the current.

 

[mkelly09]

I doubt there can be significant intensity modulation of an incandescent lamp at 500 Hz PWM, and it's about to impossible at kHz frequency. More likely, there's some kind of electrical crosstalk in your circuit.

This is very interesting. If this was indeed the case it would certainly save me a lot of time pursuing other options. Could you expand on why you doubt the intensity of the light source is being modulated?
I hooked up the circuit shown in the picture above and then shined the light source onto my solar cell. I then connected the output of the cell across a resistor and displayed the voltage drop across this resistor on an oscilloscope, and here is where I saw that the current of the cell was not constant. The sense circuit and control circuit were completely separated.

So you are suggesting that I take the signal out of the mosfet in the above circuit and use it as the input to a linear regulator or switching regulator to create a DC signal? (I of course would still need some way of controlling the intensity of the signal that gets fed to the light source with my microcontroller, so I assume that I would keep the original mosfet circuit)

Thank you very much for your reply!

 

[FvM]

Low voltage incandescent lamps have time constants in a 100 ms range, resulting in 1 to 5 Hz cut-off first order low-pass characteristic. Gives per mille range residual modulation with 500 Hz PWM. If your measurement circuit is disturbed by current variations in this order of magnitude, there's something wrong in your design. How about using integrating measurement methods with reasonable integration time, e.g. industry standard multimeters or low-speed ADCs.

A linear regulator would be preferably controlled by an analog setpoint or a DAC, but you can convert digital pwm with a suitable low pass into analog.