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Voltage Dependent Resistor

Discussion in 'General Electronics Discussion' started by Y2KEDDIE, Nov 9, 2012.

  1. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    I' ve seen VDR's used in televison deguassing circuits and now in a power supply.

    It is wired in series with the AC line feding a transformer which feeds a Bridge rectifier.

    How is the VDR sized in the powersupply cicuit. Can it be replaced with a slo-blow fuse?
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hi there :)

    VDR's are not quite the correct term

    in a degaussing circuit of a CRT type TV the device is called a PTC
    These devices, sometimes 2 pin, sometimes 3 pin, start at low resistance and over several seconds of power applied go very high resistance
    This allows mains current to briefly flow through the degaussing coil and demagnetise the screen each time the TV is turned on.

    The device in a power supply between the mains input and the rectifier is called a MOV ... Metal Oxide Varistor
    It operates very differently to a PTC. A MOV will measure low resistance and will allow current to flow indefinately, as long as the Voltage is within its manufacturered tolerance range.
    When the Voltage spikes ( say with a surge) this will also cause the current to spike. This causes the MOV's resistance to increase greatly and limit current flow to the rest of the power supply from the mains supply.

    MOV's come in a wide range of voltage and current specifications, so therefore you need to figure out how much current will normally be flowing ( including any peak demands) and select a MOV specification to suit

    cheers
    Dave
     
  3. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    More on VDR, MOV's, thermistors

    I have a kit called Globar resistors for deqauss circuits. FRTV5 There are two "resistors" consisting of a thermistor and a varistor. One is black coated the size of a quarter, the other is smaller than a dime, silver in color. Which is the thermistor, and whch is the varistor.

    Is the varistor (black device?) the same as a MOV.

    My application is the input to a bridge rectifier in a power supply.
    I tried the black device and it seems to work (allows current to flow). I have different sizes of this device robbed from TV deguass circuit. The smaller ones get hot, the bigger ones run cooler. With no markings, I'm not sure if I sized it right. I figure if it runs cool continously it must be working correctly.

    Any more insight?

    Thanks!

    Eddie
     
    Last edited: Nov 9, 2012
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    without knowing what thos specific parts are , no idea.

    but dont fool yourself, it may be letting current flow, BUT and I really emphasise that
    BUT your PSU may not be getting any protection if its the incorrect part. Lulled into a false sense of security ;)

    Personally I wouldnt take the risk, I would buy a MOV or 2 one for each leg of the mains (phase and neutral). Then I would know I had a correct part in the circuit :)

    Dave
     
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Here's a good description of how a Metal Oxide Varistor (MOV) works. Regardless of their particular ratings they all have one thing in common... They present a high resistance below their threshold voltage and a very low resistance above it. I've never seen an MOV used in series with the PS input as you want to do.

    http://en.wikipedia.org/wiki/Varistor

    Chris
     
  6. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    Neither have I Chris,
    The circuit board has it labeled R1.The outside coating was cooked but it looked like whats left of an MOV. Like a big disc capacitor. It is definitely in series with L1. The 110VAC is applied thru R1 (the MOV) to the primary of a 1;1 Torroid xfmr. The secondary feeds a 600V 25A Bridge rectifier. It makes sense it's a MOV by your definition. The DC is used to control VFD circuitry, a 3 phase 230V, 2.4A 3/4 HP motor. I think the 25A bridge is overkill because the torroid feeding it has only 16 ga wire in its windings. When I put a clamp amp meter on the line and ran the motor the in-rush was 13A.
     
  7. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    I noticed bigger VFD's (5 Hp and up) use large inductors to filter incoming line spikes. I guess the MOV is an alternative. I always seen MOV from L1 to L2, and or to ground. Never in series with the line, but why not?
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    It should be obvious. Did you read the wiki link? When you read it don't skip over the degradation that can occur each time an MOV fires from an over voltage spike.

    Chris
     
  9. Y2KEDDIE

    Y2KEDDIE

    252
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    Sep 23, 2012
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
  11. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
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    May 8, 2012
    If anyone interested in PSpice models for LittleFuse MOVs I found a bunch of them here...

    http://www.littelfuse.com/technical-resources/spice-models/varistor-spice-models.aspx

    It should be noted that some of the files on this page are mislabeled. Some are marked PSpice files but when you click it for download you will see that it does not have a lib extension. I just went through them all and download only the .lib files.

    Chris
     
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