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Thread: SSR TRIAC Gate Resistor values

  1. #1
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    Default SSR TRIAC Gate Resistor values

    This topic often comes up, so I'll put down all the math for the various combinations of supply volage and make the assumption you are using the 'standard' Sean Bowf SSR, which is the same as my SSROZ PCB.


    Gate Resistor value

    With reference to SimpleIO.com and some additional math and editing by me
    The value of the series gate resistor is a balancing act between limiting the peak current through the opto, and allowing enough gate current to turn on the triac.

    From the Fairchild Application Note AN-3003:
    The max surge current rating of the optoisolator, ITSM, is 1 A for the MOC series of optos.

    120VAC. The peak voltage for a 120 VAC line is 120 x 1.414 = 170 V, so R = 170 V / 1 A = 170 ohms minimum. At 120VAC, Round up to 180 ohms for a standard value.

    240VAC.
    The peak voltage for a 240 VAC line is 240 x 1.414 = 338 V, so R = 338 V / 1 A = 338 ohms minimum. At 240VAC, I rounded to 330 ohms for a standard value.

    24VAC. The peak voltage for a 24 VAC line is 24 x 1.414 = 33 V, so R = 34 V / 1 A = 33 ohms minimum. At 24VAC, I rounded to 33 ohms for a standard value.

    12VAC. The peak voltage for a 12 VAC line is 12 x 1.414 = 17 V, so R = 17 V / 1 A = 17 ohms minimum. At 12VAC, I rounded to 22 ohms for a standard value.

    The balance comes in here for the gate current and the line voltage to drive it. The minimum voltage needed to turn on the triac is determined by adding up the gate current through the resistor IGT, the triac gate voltage VGT, and the opto on-state output voltage VTM.

    120 VAC operation. R x IGT + VGT + VTM = 180 ohms x 50 mA + 1.3 V + 3 V = 13 V.

    Therefore, the supply voltate for a 120VAC circuit must reach 13VAC before the TRIAC will trigger. This is well below the minimum voltate required to get the lights to illuminate.

    12 VAC operation. R x IGT + VGT + VTM = 22 ohms x 50 mA + 1.3 V + 3 V = 5.4 V.

    Therefore, the supply voltate for a 12 VAC circuit must reach 5.4 VAC before the TRIAC will trigger. This is close to the minimum voltate required to get the lights to illuminate. You would just need to take not of the setting in Vixen for dimming range @ 12VAC is likely to be different to 120VAC operation.

    The next question is "I have used 180 ohm and have 24 Volt lights and they work, why bother changing the gate resistor value?" Well, the answer is - you can, and it will work. The only impact is that the lights wont turn on until the AC waveform gets to 13VAC, on its way to 33VAC peak (24Volt RMS). This also explains why using 180ohms @ 12VAC will be marginal at best, not functional at worst.

    The last question is what size (rating) should the resistor be. For all cases quoted above, the power disappation of the gate resistor will be less than 10mW, so a 1/4 watt (250mW) resistor will be fine.

    John Wilson,
    (B Elec Engr, M Telecom Engr)

  2. #2
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    Default Re: SSR TRIAC Gate Resistor values

    Quote Originally Posted by wjohn
    This topic often comes up, so I'll put down all the math for the various combinations of supply volage and make the assumption you are using the 'standard' Sean Bowf SSR, which is the same as my SSROZ PCB.
    Here is some good information on calculating resistor values. It comes from Dan Baldwin and was originally posted in the Computer Christmas forums.

    ----------------------------------------------------
    Beginning of original quote from Dan Baldwin:
    ----------------------------------------------------

    Calculating resistor values for control side of an opto:
    ----------------------------------------------------
    First from the spec for the opto determine the current flow necessary to guarantee it will trigger. This is rated in ma (1/000s of an amp). optos generally call for 5ma, 10ma or 15ma. I then add a few ma to it to make sure...

    I use 5ma optos so I design for 8ma to be supplied to the opto.

    R = V/I (Resistance = Voltage / Current ) (Ohms =Volts/Amps)

    Lets compute it for 8ma with a 5 volt supply and here is where some people make a mistake.

    R = 5/0.008 = 625ohms .... (THIS IS WRONG because it really isn’t 5v)

    Why is it wrong? Well the opto uses a LED and LEDs have an inherent property that causes a forward voltage drop this is listed in the specs as Vf. A Vf of 1.3 is pretty typical for an opto. Different color/types of LEDs have different Vf values.

    So the actual calculation is:

    R = (Control Voltage-Vf) / I

    For control 5v - 5ma opto I do:
    R = (5-1.3)/0.008 = 462 ( I use a 470 ohm resistor )

    For control 5v - 10ma opto I do:
    R = (5-1.3)/0.013 = 284 ( I use a 330 ohm resistor )

    For control 5v - 15ma opto I do:
    R = (5-1.3)/0.018 = 202 ( I use a 220 ohm resistor )
    ----------------------------------------------------
    End of quote from Dan.
    ----------------------------------------------------

    Note that the MOC3023 opto that is frequently used is 5ma. Also note that for a 10ma opto a 300 ohm resistor could also be used. I hope this information was helpful.

    TED

  3. #3
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    Something you need to remember when calculating out the resistor value for the input resistor to the opto (Ted's post....John's post is for the gate resistor for the TRIAC, which is on the other side of the opto) is the capability of your 5V power supply. It sounds great to add a little extra into certain values, but remember that a lot of folks are using 595 controllers. With the 595 we send a constant 5V discrete signals to our SSRs (unlike the renard that uses short on pulses twice per AC cycle).

    The IFT of an opto is the minimum that an opto is GUARANTEED to turn on at...so it will probably turn on at less. Most of us are using moc3023 optos which has an IFT of 5ma. Calculating at the 5ma leaves us with...

    5 - 1.3 / .005 = 740 ohms (nearest value being 750 ohms).

    I had a box of CAT3 cable that had about 850 ft of cable left in it. I put an RJ45 connnecter on both ends of the cable and hooked it to my 595 controller, and it worked fine. My point being, is that I think the SSRs will work fine at 5ma, and you will be using less current on your controller side of your display.

    The reason I use the higher value resistor (750 ohms), is due to the total current draw of the controller and SSRs. If you are running 128 channels through an Olsen 595...then you will have 5ma for each channel at the SSR, plus you have a similar current draw at each of the LEDs on the controller board.

    .005 x 128 = .64 amps.. times 2 for the controller and you have a minimum of 1.28 amps that will be drawn from your 5V power supply any time you turn everything on in your display. The common thought was that we overbuild things to make sure we dont have falures...so you I would personal use a 5v power supply capable of handling 1.5 amps or more. Anyhow...

    If you look at using 8ma for both the SSRs and the controller...you now have. .008 x 128 = 1.024 amps just for the SSRs, and over 2 amps if you use a similar value resistor for the controller's LEDs. Now you need a power supply that is capable of over 2.5 amps or more.

    Truthfully, you can apply anything from 5ma to 50ma to each channel of your SSRs to make them come on...you just need to keep the big picture in mind...and think about your total current draw. If your 5V power supply can handle it, you are probably better off building a little overhead into the resistor value. Personally, I have had no ill affects using 5ma when calculating my opto input resistor value.

    Happy Holidays,
    Sean

  4. #4
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    Sean,

    good discussion,

    Thanks,


    John.
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