PWM through a V Reg

Discussion in 'General Electronics Chat' started by DC_Kid, Jul 8, 2015.

  1. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    Q, i am looking at doing PWM through a DC regulator. i dont know the PWM freq just yet, but what are the pitfalls?

    crude example, power supplies are good 15vdc, light fixture bulb is rated 12vdc. if i use a "LM" style volt reg at the light fixture does the reg work ok with PWM or would i expect to see switching heating problems or failure to regulate at low % PWM ? i guess i could also clamp it using zener instead of reg, but just wanted to know more about PWM through a vreg.
     
    Last edited: Jul 8, 2015
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    You need a PWM capable regulator. A standard Reg won't PWM
     
  3. k7elp60

    Senior Member

    Nov 4, 2008
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    I think if instead of PWM, put a potentiometer to adjust the output voltage of the regulator that will adjust the brightness of the bulb. Most incandescant light bulb dimmers on ac use a triac and control circuitry to adjust the average voltage to the bulb. So adjusting the dc voltage to the bulb will adjust the brightness, plus you only have to add two resistors instead of a PWM circuit to control the output voltag.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    It is relatively simple if your regulator has an enable input pin: just wiggle that pin to ground or let float. Some just need a small voltage to turn on, a volt or so, so you should find many regulators you may drive with the logic signal directly.

    "Way back when" I made a 12V PWM system to run some model trains using a LM317 as the high current pass element. I added an NPN transistor driven from a '555 that could short out the bottom voltage adjust resistor. This gives you a PWM from anout 1.2V to whatever you set it for. If the constant 1.2V is a problem you can add a pair of rectfier diodes in series.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Even a standard LM317 will settle at 500Hz or so if the output capacitor is small or not there. Use an NPN to short the lower voltage setting resistor. That will cause the reg output to jump from ~1.2V to whatever it is programmed for...
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    There have been various inclusions in manufacturers appnotes over the years, of incorporating a linear regulator into a switching regulator that is loosely based on the hysteretic topology.

    Just recently I was browsing magazine articles and stumbled on a circuit similar to the standard appnote example of a 3-terminal regulator with added external bypass transistor. The usual arrangement has a PNP transistor with a low resistance B/E shunt, the shunt is in series with the chip input so as the current draw increases the resistor drops approaching 0.7V, the transistor starts to bias on. In the entirely linear version, the PNP collector current supplements the current from the chip output pin - in the switching version the collector is coupled to the output via a buck inductor. The buck inductor has the usual catch diode at the junction with the collector. In parallel with the catch diode is a potential divider - the chip ground pin goes to the middle of that.

    The PDF has printing disabled, so I can't use Cute pdf to generate a smaller PDF of just that page - anyone know how to crack PDF permissions?
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    If your bulb is 12V and your PWM is 15V, just keep your duty cycle below 75% and you will average 12 volts into the bulb. You can add a capacitor across the bulb inputs if your Pwm frequency is very low but it should not be needed for anything above 60 Hz so, in most every case, the capacitor is not needed.
     
  8. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    technically its still 15v across the load and the current that comes with when its 15vdc and PWM. the reg would be remote mounted at the fixture and there is no 3rd control wire to PWM the reg. the PWM is all digital control (wifi w/ remote and phone app), etc. the bulbs can tolerate a small Vdrop due to wire resistance. at 100% duty cycle the bulbs will be right around 11.4v. was just curious about PWM through a reg where the src V to the reg was under PWM.
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Use print screen and MS paint to crop. That's what I do.
     
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