Sinewave over DC

Discussion in 'The Projects Forum' started by bastukee, May 6, 2010.

  1. bastukee

    Thread Starter New Member

    Jan 14, 2010
    I have a device with only 2 Wires. The topside unit for the device I'm trying to replace provides +24V and 0 to these wires. The device manipulates the +24V and creates a +2.5V -2.5V sinewave that rides on top of the +24V and returns to the topside unit.

    My current issue is that if I power the device with a regular +24V power supply, it won't return a sine wave. Would there be any reason a device could talk back on one power source but not the other? Maybe I'm missing something riding AC over DC 101?

    The topside unit has a +12V and -12V power supply inside...but externally I only see +24V and 0 when I meter it.

    Ideas, suggestions?
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    You can modulate a sinewave and carry a DC signal on board. Thats how new digital radio works.

    Your meter may be having a problem showing both.
  3. bastukee

    Thread Starter New Member

    Jan 14, 2010
    I use a Oscilloscope to see the wave, but it is none existent when powered by a power supply other than the original topside unit.

    Could it be that the +24 has to be "weak" for the device to be able to manipulate the DC source to create the sine-wave?
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    You probably need an inductor isolating the power supply regulator. Otherwise, the AC signal gets eliminated as the regulator action controls the output level to +24 volts (in the case of your +/- supply, to +12. In that case, the -12 volts is used as ground so the +12 appears as +24). The effect is about the same as a hash filter in a radio circuit.
  5. bastukee

    Thread Starter New Member

    Jan 14, 2010
    It works! I used a 10 MH on +24 and it looks great so far...
  6. SgtWookie


    Jul 17, 2007
    If the AC is either fixed or a range of frequencies, an LC tank circuit with the appropriate values will pass the DC, but leave the AC "untouched".

    If you provide an AC frequency or range of frequencies, it would be relatively easy to design such a tank.

    The introduction to LC tank circuits in our online E-book is here:
  7. bastukee

    Thread Starter New Member

    Jan 14, 2010
    I used the 10 MH to allow the sinewave to happen on a regulated power supply. Then I used a 100uF/10K resistor to pass the Sinewave into my circuit, I used a rectifier diode to make it 0-3V and I used a 7400 to buffer it "3 times" and I got a perfect TTL Low to High 0-5V DC.

    The idea was that I needed to read this frequency on a micro controller, which I now have achieved :)

    Thank you!