Converting variable AC control to variable DC

Discussion in 'The Projects Forum' started by optimist55116, Dec 1, 2010.

  1. optimist55116

    Thread Starter New Member

    Nov 30, 2010
    I have been scratching my head for a couple of weeks on this problem and thought I would see if someone could nudge me in the right direction.

    I have radiant (in-floor) water heating in my house and need to control a mixing valve using an electronic actuator. The (Belimo) actuator uses 2-10vdc (approx 10mA) as a control voltage to position the mixing valve (from fully open to fully closed).

    I already have a rather expensive and complex heating control system designed to do the same thing, but by varying the speed of a circulation pump with 15 to 120 VAC

    Am I over-thinking the conversion of control voltage? Should I just pick some resistors to limit the current and split the voltage up. And then just send it through some bridge and filter circuit?

    Or should I use a transformer instead? I see a lot of circuits that are built to handle fluctuations in input voltage, but I need to keep those fluctuations.

    I am worried about things like spikes or more than the 10VDC getting passed through: these actuators cost about $300 and have a micro-controllers of their own.

    Thanks in advance.
  2. wayneh


    Sep 9, 2010
    So it's ok for the two controls to be in lock step together? Then a transformer+bridge rectifier+Zener diode is a workable approach. The Zener would give you protection against over-voltage.

    A wall wart rated to 9v DC might be a place to start. Are you sure your AC controller is really reducing voltage - while maintaining a sine wave - or is actually varying the duty cycle? This approach won't work with the latter.
  3. optimist55116

    Thread Starter New Member

    Nov 30, 2010
    Good question. (I hadn't thought about that...) I do remember the (Tekmar) heating control manual saying that the voltage varied from 0-120VAC, but that the boiler is cycled off at about 15VAC. Still it would be safe to either make a call or test it.

    Last night I did a little review of transformers. (It has been a long, long time since I've actually thought about this stuff.) Here's my dumb remaining question: Is there some lower voltage where the transformer starts behaving erratically.i.e., where the output voltage might vary independently of the input and send my mixing valve hunting back and forth?
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Transformers are very linear compared to semiconductors. You shouldn't have any issues with the ratio changing unless the primary side source cannot provide enough current for the load on the secondary side source, but as the discussion is about mA levels, that shouldn't be an issue.

    You do need to find out the frequency and waveform of the input, though. A PWM (Square Wave) Signal won't go through a transformer with any kind of accuracy. Sine waves go through just fine. The reason for this is that a square wave is very high frequency on the edges, and DC "in the middle", so to speak, whereas a sinewave is a continual single frequency.