Page 1 of 2 12 LastLast
Results 1 to 10 of 13

Thread: PWM to discrete on\off

  1. #1
    Newbie
    Join Date
    Apr 2010
    Posts
    19

    PWM to discrete on\off

    Hello,

    So here is my problem; I want to power some accessories of the interior lights in my truck. The problem is that the interior lights donít just turn on and off...they turn on normally but when they turn off they fade out (dim). I suspect (but am not sure) that the lights are actually powered by a PWM signal (rather than a simple on\off) and when going out the BCM simply modifies the duty cycle of the PWM signal to affect the fade to black. This means that when I connect my accessories through a relay the relay simply starts buzzing when the lights are going out.

    So here is the question...How do I create a simple discrete on\off from the PWM signal?

    Thanks,
    Steve

  2. #2
    MySQL Error soundman98's Avatar
    Join Date
    Jan 2008
    Location
    on the border of northern IL/IN
    Posts
    5,820
    a diode and a capacitor should do the trick-- the capacitor will 'fill in the gaps' of the pwm signal, and the diode would be installed before the cap to make the cap only feed into the relay.

  3. #3
    Newbie
    Join Date
    Apr 2010
    Posts
    19
    Wouldnt there still be a chattering issue as the PWM signal fell through the treshold voltage of the relays coil?

  4. #4
    MySQL Error soundman98's Avatar
    Join Date
    Jan 2008
    Location
    on the border of northern IL/IN
    Posts
    5,820
    with a large enough cap, i don't think it should..

  5. #5
    Raw Wave
    Join Date
    Nov 2009
    Posts
    2,118
    Quote Originally Posted by sredmyer;1480517I want to power some accessories of[I
    f[/I] the interior lights...
    Maybe if you explain what it is...
    It is rare for "accessories" to only be powered whilst the dome is on and dimming.


    Furthermore, the dimmer might be analog.
    But you'd be better off using a transistor as a big cap could blow the PWM, and an analog dimmer will reduce the coil's voltage.

    Usually accessories (other than puddle lights etc) are triggered by the door switches etc that control the dome lights. (Hence it doesn't matter if the dome light is blown or manually turned off).

  6. #6
    Newbie
    Join Date
    Apr 2010
    Posts
    19
    Actually, the "accessories" are basicly puddle lamps. What I am trying to do is power the lights on the ends of my running boards. Powering them directly from the dome light wires apparently puts to much load on the PWM circuitry beacuse the lights on the running boards and the dome lights quit working at all when the running board lights are directly connected.

    Thanks,
    Steve

    BTW, the vehicle I am working on is a 2011 Ford Super Duty truck. The running boards are factory OEM boards that came from a different truck. The wire I am trying to use to power the board lights is the wire the Ford wiring diagram says is used to power them when they are factory installed. As to why it doesn't work, all I can figure is that the circuitry is beefier in trucks that actually had these boards from the factory??
    Last edited by sredmyer; 08-12-2012 at 09:48 AM.

  7. #7
    Raw Wave
    Join Date
    Nov 2009
    Posts
    2,118
    It's more likely that the running board version uses a different PWM, though most (Japanese) PWMs have a 10A capacity.


    But if it is PWM, and if the puddle/running-boards are to imitate the dome, then it should simply be a matter of inserting a buffer circuit - eg, a MOSFET, else transistor.

    If the PWM is 0V (GND) when off and PWMs +12V, then an N-Channel MOSFET can be used to ground PWM the puddles (ie, puddles to +12V & their GND end to the MOSFET).

    Else if some of the lamps are converted to LEDs, the original PWM might work.

  8. #8
    Newbie
    Join Date
    Apr 2010
    Posts
    19
    Although I can figure out some simple electronic stuff, when you start talking about MOSFET and transistors I get lost pretty quickly. Can you be more descriptive about the circuit you are envisioning?

    Thanks,
    Steve

  9. #9
    Newbie
    Join Date
    Apr 2010
    Posts
    19
    Another question, how can I tell whether the dimming is signal is an analog one or is in fact a PWM? When I measure the voltage, I measure ~12 volts when the door is opened. When the door is closed and after the delay (about 30 seconds) the voltage drops to ~8.5. The voltage stays there for 9 minutes and 30 seconds at which point it drops to ~0. The drop from 12 volts to 8.5 is somewhat gradual causing the dimming effect. However the drop from 8.5 to 0 is nearly instant. Having said all of that, how can I know whether the voltage is droping via analog control or if it is just an effect of my RMS meter reading a PWM signal with a decreasing duty cycle?

    Thanks for the help,
    Steve

  10. #10
    Maximum Bitrate Mickz's Avatar
    Join Date
    Sep 2009
    Location
    Australia
    Posts
    832
    You could try reading it with your meter on "AC".
    GA-Z77N-WIFI, i5-2400S, 8GB, Intel 520 128GB SSD, M4-ATX Modified, 2 Rev Cams, 2 Web Cams
    8" 16:9 TRANSFLECTIVE, Win8-64, Dual GPS RX and Garmin PC + Odyssey Nav, FM-DAB+, BB-Rec
    T-Screen HVAC control, custom microcontrollers, code and FE. CarPC Project

Page 1 of 2 12 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •