Thyristor circuit help

Discussion in 'The Projects Forum' started by CVCTFMaint, Sep 26, 2012.

  1. CVCTFMaint

    Thread Starter New Member

    Sep 26, 2012
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    I need assistance in identifying a semiconductor component in a High / Low speed control circuit for a floor buffer machine. Heres what I know as fact since I have another "working" machine and am trying to reverse engineer the circuit.
    The motor ia 1.5 HP. it is fed from a bridge rectifier.The bridge rectifier is fed directly from the 120 VAC supply in the high speed mode (150 VDC output). In low speed mode (75 VDC output) a thyristor circuit is introduced in series between the 120 VAC supply and the bridge rectifier. The thyristor circuit consists of the following (4) components: (1) 56K RESISTOR, (1) .1uF "104K" CAPACITOR, (1) LINE THYRISTOR "Semikon # SKT40/16E", and (1) unknown semiconductor that feeds the gate of the thyristor. Three of the components (thyristor, resistor, capacitor) are installed in the circuit and are working with the old semiconductor that feeds the gate of the thyristor. I am unable to identify either the type or properties of the "gate" semiconductor. The component in question is .150" long with a .070 diameter and two leads. It appears to be made of a glass type material and looks similar to a DIAC or a SIDAC. I have tried a SIDAC (NTE6415) and a DIAC (NTE6412) without success.
    The working thyristor circuit and voltage readings are are follows:
    A 56K resistor (120 VAC before resistor, 81VAC after resistor) is in series with a .1uF capacitor and inserted in parallel across the line "in/out" of the thyristor. The gate of the thyristor is supplied (72 VAC) through the unknown semiconductor which intersects the series circuit between the resistor and the capacitor (81 VAC). These voltage readings are from the working thyristor circuit. 120 VAC before the resistor, 81 VAC after the resistor but before the capacitor. Out of the unknown semiconductor and into the gate of the thyristor is 72 VAC. 72 VAC is also the reading exiting the thyristor and feeding the bridge rectifier.
    I would appreciate help in identifying the "unknown semiconductor" and possibly it's required properties for the above mentioned circuit.

    Thank you in advance.....:)
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    can you draw the circuit and post it, or take a photo of the pcb both sides (components side and track side) and post?
     
  3. CVCTFMaint

    Thread Starter New Member

    Sep 26, 2012
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    Attached is a PDF file of the circuit I need assistance with. Thank You in advance!:)
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    looks like diac, can you measure the voltage across the device,it will prob be 30v.
     
  5. vrainom

    Member

    Sep 8, 2011
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    19
    Hello there, most definitely a simple dimmer circuit, in which case the diac most commonly used is the db3, take a look at the schematic shown in the triac reference page here: http://www.allaboutcircuits.com/vol_3/chpt_7/6.html

    I read that you've tried replacing the diac and the circuit still doesn't work, in which case, having tested (or replaced as they're very cheap) both the resistance and the capacitor, the solution should be replacing the triac.
     
  6. CVCTFMaint

    Thread Starter New Member

    Sep 26, 2012
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    The voltage measurements across the semiconductors in the forementioned attached circuit are as follows:

    1. DIAC (assuming it's a DIAC) - 18 Volts
    2. Resistor - 51 Volts
    3. Capacitor - 18 Volts
    4. Thyristor - 60 Volts
    :cool:
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The capacitor will have the same voltage as the diac rating, because the diac wont conduct until the capacitor gets to its strike voltage, so looks like the diac is faulty as it should be 30V, so its conducting at an early voltage , which will make the motor run faster.
    Replace the diac, you can put a couple of zener diodes instead, Anode to Anode 30v each will do the same job.
     
    Last edited: Oct 1, 2012
  8. CVCTFMaint

    Thread Starter New Member

    Sep 26, 2012
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    0
    Perhaps I need to clarify. The "DIAC" is not faulty. The circuit diagram I drew is of a physically existing, properly working circuit. I'm trying to duplicate this circuit. The voltage readings I've previousely stated were taken with the circuit energized in low speed mode with the motor operating at the proper low speed RPM.
    I just need to identify the one semiconductor and it's properties. It "seems" to be a DIAC. I need to verify this and determine the electrical characteristics so I can build it.

    Thank You.....
     
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