120V Variable Resistor/Digital Pot needed

Discussion in 'General Electronics Chat' started by cshoulder, Nov 15, 2010.

  1. cshoulder

    Thread Starter New Member

    Nov 15, 2010
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    Alright, I've been working on a project for some time now and I am having issues.

    The idea is this, I have an output signal from a microcontroller that needs to control an AC load via voltage. The load is pretty picky and a rather new technology. It works from 0-120VAC @ 60 Hz 1-3 mA.

    So basically I have the need for a digital pot that can handle 120VAC with a 0-5V input signal or some similar device to control the load. If anyone has any other ideas It'd be much appreciated.

    I've spent quite a bit of time looking for something like this without much luck unless I want to glue a LED to a photocell that can handle the power.

    And maybe someone here can answer this question as well. I have enclosed a picture of an output when sending PWM signal to a solid state relay. I would "think" that the output would show a square wave on the negative half cycle as well, but as you can see from the pic that is not what is occuring...Any ideas?

    Microcontroller = Arduino Duemilanove
    Relay = Crydom D5D10 Solid State Relay

    [​IMG]
     
    Last edited: Nov 15, 2010
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    If the load is just on/off, a relay would work.
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Why not use a Triac "chopper" circuit?
     
  4. cshoulder

    Thread Starter New Member

    Nov 15, 2010
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    Its not just an on/off load, though I wish it were. It's actually SPD(smart glass) film that varies light/heat transmission properties based on the voltage applied. I really need a full range of operation for the system to work properly. The issue is that the SPD film is so hard to come by that the only place I was able to get some only had AC operated film. The requirements of the project mean that it needs a DC control voltage from the microcontroller and I need to at least have several operating points for the AC load if not the entire analog range.
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Do you have a datasheet for the glass?

    --ETA: Have you tried simply using an AC Light Dimmer on it? It's still a chopper, but the effective voltage would vary.
     
  6. cshoulder

    Thread Starter New Member

    Nov 15, 2010
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  7. eblc1388

    Senior Member

    Nov 28, 2008
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    The response time of the film is from 1~5 seconds. That means a TRIAC control to varies the AC power is suitable but you will need isolation between MCU and load.

    You can control the amount of heat via phase control or cycle skipping.

    A zero crossing detector as input to MCU and an opto-TRIAC like MOC3021, which has about 100mA load drive capability would fit the bill nicely.

    Also the LED+CDS(LDR) is a totally acceptable solution, provided that the pane area is small, i.e. less than a few square feet.
     
  8. cshoulder

    Thread Starter New Member

    Nov 15, 2010
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    The film size is approximately 8.5 x 11 inches. And thanks, I'll start looking into this today!
     
  9. tom66

    Senior Member

    May 9, 2009
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    For isolation consider an opto-triac.

    How are you generating the AC signal? Is it coming from mains voltage?

    Your solid state relay has a diode in reverse from the source to the drain. This will conduct in reverse. The "body diode", as it is known, is due to a limitation in MOSFET technology, but it makes the fets more powerful, IIRC. Either way, you can't switch an AC signal with a MOSFET.
     
  10. cshoulder

    Thread Starter New Member

    Nov 15, 2010
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    That makes a lot of sense for the solid-state relay.

    I have already tested the film using main's voltage; however, for demo purposes we may use an inverter circuit.

    The AC signal from the picture at the top is from a function generator.

    Also, another question about the Triac...every application I've seen online has been using it similarly to a relay for simple on/off current switching. Does changing the input voltage allow less current or is it simply a on/off switch at the threshold?

    I also am not sure why I would need a zero crossing detector in my application. I simply need to be able to adjust the voltage the load receives with a control signal from the MCU. The MCU inputs and sensors are already working, I'm having an issue with the interface between the DC output and the AC load.
     
  11. eblc1388

    Senior Member

    Nov 28, 2008
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    To understand why a zero crossing detector is needed, you would need to Google the phrases "triac phase control".
     
  12. cshoulder

    Thread Starter New Member

    Nov 15, 2010
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    Ok, now i see what you mean! Interesting, I'll need to do a lot more reading now as we never touched on anything near this in my courses =x Thanks for the input.
     
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