EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: drummerdimitri on February 19, 2015, 01:10:46 am
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Hello,
I was wondering if it would be possible to power electronics using a very high 90+ % duty cycle square wave as PWM instead of DC voltage.
I know my flashlight has PWM modes to reduce brightness but can it be used as a DC power supply for example?
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For dumb loads like LEDs and motors it's fine as long as your PWM frequency is high enough to not cause issues.
If you want to make a voltage rail for ICs then it needs to be more stable (each pulse could reset the device). You can't just dump the PWM into a capacitor -- it'll charge up to full voltage but pull higher current during the "on" times to make up for the difference. There is probably a really nice solution to this I don't know of :)
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For dumb loads like LEDs and motors it's fine as long as your PWM frequency is high enough to not cause issues.
If you want to make a voltage rail for ICs then it needs to be more stable (each pulse could reset the device). You can't just dump the PWM into a capacitor -- it'll charge up to full voltage but pull higher current during the "on" times to make up for the difference. There is probably a really nice solution to this I don't know of :)
You could probably use the PWM as part of a SMPS, but by then the cost of the extra components is a lot more than just a simple linear regulator.
Why exactly do you want to do this? Just to get a variable power supply? If you don't need more than 1A or so, just use an LM317, or any of the endless variety of adjustable linear regs.
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you could try a low pass filter to smooth out the PWM, I have used that from a MCU to a opamp with good results before. There are some more complex filters but then it starts to get expensive.
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Are you asking whether you can directly connect a PWM signal to a device multiply the supply voltage by the duty cycle %?
While you can make the energy delivered from a PWM equal to the energy delivered from a DC voltage often this isn't possible.
This is just related reading stuff, don't worry about it if your not interested in the theory:
If you would like to use it as a DC power supply, you could do the normal ladder network style LC filters there:
http://en.wikipedia.org/wiki/Electronic_filter_topology (http://en.wikipedia.org/wiki/Electronic_filter_topology)
A buck convertor uses an LC circuit for just this purpose and many buck convertors infact use PWM controllers:
Buck Convertor WIKI http://en.wikipedia.org/wiki/Buck_converter (http://en.wikipedia.org/wiki/Buck_converter)
Example PWM BUCK http://www.ti.com/lit/ds/symlink/tps5110.pdf (http://www.ti.com/lit/ds/symlink/tps5110.pdf)
If you simply want to use a PWM output as a DAC none of what I have said above is really necessary:
Connect a simple RC low pass filter (go for a highish value of R, let the micro max current output dictate this value) to the output and level shift + amplify with an Op-Amp. This may or may not be acceptable in your application.
Many times 100% duty cycle is acceptable, I'm guessing your worried about power dissipation. The power dissipated is very heavily determined by the load and the supply voltage for the PWM waveform.
What kind of load are you planning to connect a PWM waveform to?