Question

I am using Rpi GPIO 18 PWM control to dim a 7W AC LED lamp.

(A) Why there is a weird dip at duty cycle 85%?

(B) Why dimming not linear?

(C) Why use PWM frequency 5kHz?

(D) Why limit duty cycle to 86%?

(E) Why LED not completely switch off at duty cycle 0%

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Answer

(1) Short Answers

(A) There is no reason for a dip. Your program might be buggy. Try my bug free programs (Refs 5, 6).

(B) Dimming not linear because your eyes are logarithmic, which is non linear.

(C) There is no reason to use PWM frequency. AC mains frequency is 50/60Hz. So a “sampling” frequency 10 times of that should be flickering free. Try 1 kHz and try and error a pleasant frequency for your eyes only.

(D) There is no reason for a 86% duty cycle limit. Try and error higher. Nothing will explode.

(E) Perhaps there is sort of a “dead band” at AC mains voltage cross over. As soon as zero cross is detected, it take a little bit of time to tell the triac to cut off off current (just guessing, not very sure).

(2) Setup Mains AC Current Dimmer for Testing and Calibration

I would suggest to test and calibrate the dimmer in two steps:

(A) Use a type A (logarithmic) potentiometer.

(B) Use a python program to setup Rpi GPIO in PWM mode to control the dimmer.

Notes

Actually you can by hand use a jumper wire to connect Vcnt terminal to 3V3, Ground to see if dimmer is completely on and off. You can also use a cheaper square wave generator such as NE555 to control the dimmer. Using a digital pot is better than GPIO PWM because digital pot memorizes its setting.

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References

(1) MPDM V4.1 Documentation

(2) Universal AC MAINS Dimmer MPDMv4.1 Catalog- US$17

(3) Universal AC MAINS Dimmer MPDMv4.1 Operation

(4) MPDM Triac Operation

(5) Rpi PWM GPIO pin #18 python program Example 1- tlfong01

(6) Rpi PWM GPIO pin #18 python program Example 2 – tlfong01

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Appendices

Appendix A – WARNING!! You will play with LIVE MAINS!! Deadly zone!! – NetEvo

WARNING !!! If you don’t have any experience and are not qualified for working with MAINS power I will not encourage you to play around!.

The author takes no responsibility for any injury or death resulting, directly or indirectly, from your inability to appreciate the hazards of household mains voltages.

The circuit diagrams are as accurately as possible, but are offered with no guarantees whatsoever.

There is no guarantee that this design meets any Rules which may be in force in your country so please check before your local rules/regulations.

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Appendix B – MPDM v4.1 Universal AC Current Dimmer

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Appendix C – Zero detection and triac switching schematics

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Appendix D – MPDM Traic Operation Explained – NetEvo

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*Appendix E – Dimmer Testing Program (Version 3 ESP8266 LUA Version)*

References

http://esp8266-projects.org/2015/05/mpdmv3-wifi-mains-power-dimmer-switch-html/

https://www.youtube.com/watch?time_continue=6&v=kw8u4zBOIRs

https://www.youtube.com/watch?v=mQ5bhTV9Uu8

-- *** Define used GPIO pin ***

outpin=7                                          -- Select Triac Command pin - GPIO13 

gpio.mode(outpin,gpio.OUTPUT)

gpio.write(outpin,gpio.LOW)                       -- Triac OFF

inpin=6                                           -- Zero crossing detector input - GPIO12  

gpio.mode(inpin, gpio.INT, gpio.PULLUP)           -- attach interrupt to ZCD


-- *** Zero Cross ***

function zero_cross()
  dt = 76*dim
  --print("Zero cross detected!") 
  stat = "ON" 
  tmr.delay(dt)                    -- Firing delay time calculated above 
  gpio.write(outpin,gpio.HIGH)     -- Triac ON - Zero cross detected
  tmr.delay(100)                   -- Triac ON - Propagation time  
  gpio.write(outpin,gpio.LOW)      -- Triac OFF - let's be sure it's OFF before next cycle 
  tmr.wdclr()
  return stat
end


-- *** Fading function for testing mode ***

function fading()
  if(dim_up==1) then dim=dim+1 
     else dim=dim-1
   end

   if(dim < 10) then dim_up=1 dim=10
      else if (dim > 120 ) then dim_up=0 dim=120
    end
    end  

    print("Dimmer level : " .. dim)
    print("Fading mode  : " .. dim_up)
    tmr.wdclr()
    end

-- *** Main ***

dim = 120                                         -- Dimmer level - smaller value is brighter

dim_up=0                                          -- Fading direction - for test  run

gpio.trig(inpin, "up", zero_cross)                  -- ZCD interrupt attached - trigger on falling edge

tmr.alarm(0, 100, 1, function() fading() end)     -- timer for testing mode

-- *** Setup/Testing Notes ***

-- For testing, just save the code on ESP as ‘dimmer.lua‘, restart ESP and run:   

--  dofile("dimmer.lua")  -- Start the Dimmer Testing  mode

-- If you want the MPDMv3 software to start automatically when your CBDB module starts or reboots, then you neet to create and add some lines in your ‘init.lua‘ file:

--   dofile("dimmer.lua")  -- Start the Dimmer Testing mode

End of Answer