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    Resistor Burning Issue.

    I have made the following circuit to control my water pump with Arduino(UNO R3). I have tried many resistors (calculated myself) to limit current in the optocoupler and triac, but failed to carry the load for a long time. At one time either resistor get burned or triac gets too hot. Kindly suggest what resistance I should use in the resistors with '?'. Any other suggestions are also welcome.
    Click image for larger version. 

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    MOC 3021 datasheet - https://www.alldatasheet.com/datashe...D/MOC3021.html

    BT136 datasheet - http://www.alldatasheet.com/datashee...IPS/BT136.html

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    Re: Resistor Burning Issue.

    I have tried many resistors (calculated myself) to limit current in the optocoupler and triac, but failed to carry the load for a long time.
    To classify the problem, we must surely know which resistor values you have actually tried.

    You should have noticed that MOC30xx application notes suggest a RC circuit when using inductive loads.


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    Re: Resistor Burning Issue.

    Thanks for your reply.
    Initially, I have tried only 1K ohm between A1 and pin 6 and 1K ohm at 5V dc anode. That went well with a 5watt bulb and a fan (100 watts approx). But when I tried same with the pump(373watt) triac get too much heated. Which prevents it from switching off. MOC also failed. Now I have added 200 ohm across dc (as shown in fig) and it is working fine. Then I tried some high resistance 2K ohm (bw pin 6 and A1) and 4700 ohm(bw pin 4 and A2) with the bulb. It worked but the resistor immediately burned. I only wanted to lower the current passing through MOC and triac to prevent them from overheating.
    I also considerd adding capacitor but the elecric motor is alreday provied with a capacitor within. However I will reconsider it.
    I have to turn on my pump for maximum 15 minutes at a time.I would be glad if you could tell what resistor should I use. Thank You again.



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    Re: Resistor Burning Issue.

    Hi,

    did you use - or at least calculate whether you need a heatsink?

    Klaus
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    Re: Resistor Burning Issue.

    Resistor burning can be well explained by using inappropriate resistor values. One point is to assure the specified LED current for MOC3021 (15 mA according to datasheet). 200 ohm sounds O.K. for 5V VCC.

    I only wanted to lower the current passing through MOC and triac to prevent them from overheating.
    Completely wrong consideration. You want to trigger the triac quick and reliably, after triggering the MOC output current stops automatically. A popular circuit is using a capacitor to reduce triac dV/dt, preventing false triggering with inductive load and also provides a short strong trigger pulse. R2 may be varies, I have often used 220 ohms for R1 and R2. C1 must be rated for 250 VAC.

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    I agree with KlausST about checking heatsink need. Not turning off can however happen due to inductive load and missing RC filter for MOC3021 (dV/dt self trigger).


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    Re: Resistor Burning Issue.

    I added 1.3K ohm(three 3.9K ohm resistors in parallel) between A1 and pin 6. It is working fine. I illuminated 100W bulb for 45 minutes. My triac, mounted with 2cm long aluminium heat sink, just get a little warm. When I connected the 373W electric motor for 2 minutes, the triac(with heat sink) was just unberable to touch(about 60-70 celsius) and working perfectly. I need to run the motor for maximum 15-20 minutes. What heat sink should I use.
    Thank you guys for all your support.



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    Re: Resistor Burning Issue.

    Hi,

    What heat sink should I use.
    Try this:
    https://lmgtfy.com/?q=online+heatsink+calculator

    Klaus
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    Re: Resistor Burning Issue.

    resistor burning can also be caused by using too low a power rating.

    one 1000 ohm burned up, but 1300 ohm, when made from 3 parallel 3900 ohm resistors did not
    because you tripled the power rating

    it is likely that three 1000 ohm in parallel would also not burn up

    note: i have assumed all four resistors were the same power rating - i.e the same case size



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  9. #9
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    Re: Resistor Burning Issue.

    Hey wwfeldman, your assumption is correct. Also I have overcome the heating by using a cooling fan. My problem is solved. Thanks all of you guys.



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