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Use Triac as SCR?

Discussion in 'Electronic Basics' started by Bill Bowden, Dec 15, 2005.

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  1. Bill Bowden

    Bill Bowden Guest

    I'm thinking about reworking a 120VAC lamp dimmer so I don't have to
    turn the brightness to max to turn it off. I would rather turn the
    control to minimum and have the light go completely out and eliminate
    the switch.

    The dimmer installed turns on at full brightness and doesn't quite go
    all the off at the minimum setting. There is an adjustment to set the
    low level, but it's not very stable.

    This circuit from an old 1960s book uses a full wave bridge and SCR and
    can be adjusted to minimum so the light goes completely out. But I
    would like to use the existing triac and just add the bridge and other

    Is there any problem substituting the triac for the SCR?

  2. Andrew Holme

    Andrew Holme Guest

    You probably could substitute a triac for the SCR, but that circuit is not
    very green - it constantly wastes 7W in the 2K resistor, and "needs some

    Can't you just reverse the outside connections on your on/off potentiometer
    so it starts from minimum brightness?
  3. Chris

    Chris Guest

    Good morning, Bill. I'm not sure what you've got going with the dimmer
    (I'm assuming it's a store-bought item, and most of them work the other
    way, "off" being on the dim side of the dimmer pot).

    I'm also not too sure you'll be happy with the circuit shown in the
    link. There are a couple of problems here, which might suggest a
    better circuit.

    First, according to the text accompanying the schematic, the SCR will
    trigger when the cap reaches about 8V. This will mean the SCR gate is
    triggered by about 4 or 5mA of current (through the 1.5K resistor).
    This is enough for some sensitive gate SCRs, and there are sensitive
    gate triacs which will do this job (I would think something like the
    Teccor/Littelfuse L6004L3 would work, 4A(RMS)@600V, I(GT) = 3mA max in
    Quadrant 1), but if you want something beefier than that, I'm not sure.
    I'm sure you would also like a circuit that uses the triac you already
    have, which certainly isn't a sensitive gate triac.

    Second, the triggering voltage itself is not going to be very
    consistent. You can visualize it working well on the first half-cycle,
    but the question is, what happens after that. There's no mechanism to
    discharge the cap every cycle or every half-cycle to ensure consistent

    Third, the circuit itself is somewhat uneconomical, especially at
    higher currents. You have to pay for power diodes that will conduct
    the current of the load, just in order to get the benefit of control at
    low voltages without "snap-on hysteresis" (that's the effect you're
    seeing that's so frustrating when you turn the lamp dimmer down).
    Power diodes aren't a big problem with a 60 watt lamp (you can just use
    1N4003+), but at 20 amps, it kind of whispers in your ear that there
    must be a better way.

    There is a better way. I first saw this trick in the GE SCR Manual,
    4th edition (another "old 60's book"), and it's just as good today.
    Not only that, but you can probably scrounge the triac, diac and the
    cap in your lamp dimmer, reducing your cost. Look at page 4 of Teccor
    AN-1003, "Phase Control Using Thyristors":

    and you'll see two little trick circuits on the lower right corner of
    the page that'll do the job quite nicely. The key is that they
    discharge the cap, leading to more precise control of load voltage at
    the low end. Circuit a) in figure 12 is just the ticket for lamp
    loads, because the two diodes only discharge the cap on the positive
    half-cycle. It only costs two diodes and a 1/2 watt resistor. This
    puts a major DC component into the lamp load, but there's no problem
    with that. If you're thinking about running something that will not
    like a DC component, try circuit b). It only costs two more diodes and
    another 1/2 watt resistor. And all these diodes are only responsible
    for discharging the cap -- they don't have to carry the load current.

    Just for grins and giggles, I'd modify the app note circuit by adding a
    Quencharc snubber across the lamp load or triac, and also put a high
    current torroidal choke (you can scrounge these from PC power supplies)
    in series with the works, to minimize the possibility of RFI problems.
    You might also have the choke in the lamp dimmer -- look there, first.

    You may want to consider having your switch in the circuit, just for
    safety purposes, if nothing else. When you turn the pot up to maximum
    resistance, the load is "off", but it's still "hot". This could make
    someone very unhappy if they're, say, replacing the bulb. Be sure to
    follow local code, and be careful about installing home brew line
    voltage stuff inside the walls without consulting someone who knows
    what they're doing. And of course, use appropriate safety precautions
    during construction and troubleshooting.

    I hope this has been of use to you.

    Good luck
  4. Bill Bowden

    Bill Bowden Guest

    You probably could substitute a triac for the SCR, but that circuit is not
    You may be looking at the wrong diagram. I don't see any 2K resistor.
    It's the drawing titled "120VAC Lamp Dimmer"
    Yes, thought about doing that, but I want to eliminate the switch and
    have the light go completely out at the extreme dim setting. And it
    won't do that, because the phase needs to be exactly 180 degrees, and
    it isn't stable enough to maintain 180.

  5. Bill Bowden

    Bill Bowden Guest

    I'm also not too sure you'll be happy with the circuit shown in the
    Actually, the 1.5k resistor just establishes the trigger level. The two
    transistors will conduct at the trigger level and then discharge the
    cap through the gate at more than 20mA. The cap will discharge to well
    below the trigger level before the transistors turn off.
    The triggering voltage should be consistent since the mechanism to
    discharge the cap on every cycle is the dual transistor arrangement
    that acts like a diac and becomes a very low resistance when triggered.
    I think it's called a SCS (silicon controlled switch).

    It works well, I tried it with a 25 amp SCR with good results.

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