Incandescent Dimmer

Discussion in 'The Projects Forum' started by tmartin, Jun 29, 2011.

  1. tmartin

    Thread Starter New Member

    Jun 26, 2011
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    0
    Hi All,

    I am building a Wireless Low Voltage Dimmer for a theatre project.

    Essentially, we have a DMX (RS485) signal going into an Xbee Pro transmitter, and getting sent to another receiver.

    On the Receiving end I have another Xbee Pro Module on a Parallax Propeller Board to receive and split out DMX. The DMX then goes to an Opto Isolater, and to an output XLR connector and the to the dimming processor. The Opto Isolator Chip has its own isolated power from the prop to a power isolator then to the OptoIso. I have designed this so the Prop gets its 3.3v/5v from its own power supply or 9v Batteries, and the Dimmed Load gets power from a 12v source like a Gel Cell Battery.

    Inside the receiver I am also trying to put together a 4 circuit low voltage high wattage dimmer system with PWM.

    So the Prop is controlling 4 150W FETs (Digi Key P/N: 497-7535-5-ND) I am assuming that the 2.5v Vgs is the PWM signal from the processor?

    What else do I need for this circuit? Someone else that I consulted mentioned a series resistor on the gate to control it opening.

    Prop->(Control) to FET
    Feed V -> 30A Fuse -> FET

    FET->Terminal Blocks->Load
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,061
    Yes, the PWM signal goes to the gate. Sometimes a resistor is used to limit the current to the gate, which is a bit like an un-charged capacitor, so a brief "short". The resistor protects both the gate and the PWM source from this brief condition. Sounds good, but it's usually not necessary. On the other hand, a resistor from gate to ground is a good idea to protect from the controller going goofy. The resistor will turn off the FET if the controller isn't sending a signal.
     
  3. tmartin

    Thread Starter New Member

    Jun 26, 2011
    5
    0
    Right. So all I need for the PWM Dimming is the Processor, and FET? I can't figure out what sized resistors to use on the gate.
     
  4. tmartin

    Thread Starter New Member

    Jun 26, 2011
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    0
    Again, If someone could help me with this. We are looking at approximately 50w @12v load.

    Feed V to FET Source Pin> 3.3v PWM (From Prop) to FET Gate Pin > Load to FET Drain Pin

    + a 100Ohm resistor from Gate to Ground, which in reality doesnt exist because there is no ground on the battery supply.

    + a 1Kohm resistor from the propeller to the gate pin?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Sorry you haven't been getting much of a response. Seems that recently we have a lot of folks with questions and not many folks with answers.

    Here is the datasheet for your MOSFETs:
    http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00002217.pdf

    The MOSFETs you have selected have a threshold Vgs of 2.5v, documented on the datasheet as Vgs(th). This means that below 2.5v, the MOSFET will be considered turned off, but above 2.5v it is just beginning to conduct at 250uA drain current, or 0.25mA.
    You really need to look at the Rds(on) specifications. There, Rds(on) is specified for Vgs=5v and Vgs=10v. You really need to get Vgs up to at least 5v in order for the MOSFET to be considered turned on.

    The propeller I/O pins have a limited source/sink current ability; probably around +/-20mA. If you're using 3.3v for the propeller's Vcc/Vdd, that won't be enough to turn on the MOSFETs. Even if you're using 5v for Vcc/Vdd, the turn-on and turn-off times will be rather slow, and you will be putting stress on the propeller's I/O pins.

    You can use a gate driver IC to turn the MOSFETs on/off more quickly. You might look at using a Microchip TC4467, TC4468 or TC4469. These are quad low-side gate drivers capable of 1.2A peak drive current; they'll turn the MOSFETs on and off much more quickly than your uC could alone. They need a supply voltage from 4.5v up to 20v; you could operate them directly from a 12v lead-acid battery. They'll recognize your uC's logic input level; anything >=2.5v is considered a logic high.
    Datasheet: http://ww1.microchip.com/downloads/en/devicedoc/21425b.pdf

    You don't mention what PWM frequency that you are considering. I'll suggest that with incandescent lamps, anything >= 200Hz or 5mS will be plenty fast enough.

    A 22 Ohm resistor between the gate driver and the MOSFET gate will dampen the tendency to "ring", while not slowing the charge/discharge times too much. You can add a 10k resistor from each gate to the source terminals to keep the MOSFETs turned off in case the gate drive circuitry fails.
     
  6. tmartin

    Thread Starter New Member

    Jun 26, 2011
    5
    0
    So now If i use a gate driver, i still send my signal from the prop IO pin to the gate driver input, as well as supply voltage to the gate driver, than the gate driver output to the Fet Gate? And since they are quad drivers, one will do for all 4 FETs?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, you have the idea.
     
  8. tmartin

    Thread Starter New Member

    Jun 26, 2011
    5
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    when you say Source Terminal for the 10k resistor, is that just bridging the gate pin to the source pin?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yes, connect a 10k resistor from the gate to the source.
     
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