1

I am tinkering with BT136 & MOC3021 optocoupler to control AC loads, and below is the schematic i have put together. I have a few doubts here.

  1. Is the value of 100 ohm 1 watt correct on pin 6 of MOC3021
  2. Is 50 ohm, 1 watt sufficient in snubber circuit.
  3. Is 10 nF, 250 V sufficient or should I use 400 V.
  4. Any other design issues / sugesstions for improvement.

enter image description here

enter image description here

Suresh Kumar
  • 163
  • 3
  • 9

2 Answers2

4
  1. Is the value of 100 ohm 1 watt correct on pin 6 of MOC3021

enter image description here

Figure 1. The datasheet suggests this circuit.

Remember that when the MOC triac is off that there is no current through your R16 so no power will be dissipated. When the MOC turns on there will be a very brief current through R16 but Q4 will turn on almost instantly and the voltage across the circuit will drop close to zero. The power dissipated in R16 will be very low.

According to the datasheet you can increase your R16 value. The datasheet circuit includes a low-pass filter (the two resistors and capacitor) to improve noise immunity. You'll probably be OK with your simpler circuit.

  1. Is 50 ohm, 1 watt sufficient in snubber circuit.

The approach here is to work out the worst-case current through C7.

\$ Z_C = \frac 1 {2 \pi f C} = 318\ \text k \Omega \$ at 50 Hz.

\$ I_C = \frac V Z = \frac {250}{318k} = 0.8 \ \text {mA}\$.

\$ P_{R15} = I^2 R = (0.8 \times 10^{-3})^2 \times 50 = 32 \ \mu \text W\$. Do you think 1 W might be a little over-cautious?

  1. Is 10 nF, 250 V sufficient or should I use 400 V.

250 VRMS peaks at 250√2 = 353 V. You need 400 V rating.

  1. Any other design issues / sugesstions for improvement.

Just be aware that the opto-isolator you have chosen is a "random-phase" type as opposed to a zero-cross type. I have written about them on Opto-triacs, solid-state relays (SSR), zero-cross and how they work.

Links

Transistor
  • 168,990
  • 12
  • 186
  • 385
  • Thank you for the details. I was now thinking if i need snubber circuit at all ?. Can i get away without R15 & C7 – Suresh Kumar Mar 28 '20 at 16:55
  • Read the links I've added for more on the importance of snubbers. – Transistor Mar 28 '20 at 17:14
  • thank you for the details, i have updated the schematic to match your suggestion. – Suresh Kumar Mar 30 '20 at 14:44
  • what is waveform of load ? when Data/IO/Pin2 is high? pure sine or trimmed sine? – kdm6389 May 25 '22 at 20:47
  • Re-read the paragraph before the Links section. – Transistor May 25 '22 at 20:55
  • I can add that resistor in series with TRIAC gate is responsible for smallest turn-on voltage. If it is too big, TRIAC will not be able to turn on at low voltages due to its finite gate trigger current. This limits smallest phase angle and increases EMI in general. Smallest acceptable value is limited by maximum gate current and maximum optocoupler current, as Marko Buršič wrote. Another factor, resistors have limited rated voltage. Consult datasheet of yours. With leaded resistors and 220 V, at least 0,25 W is recommended, 0,5 W is best of all. – Vladimir May 25 '22 at 21:24
1

Peak Repetitive Surge Current = \$\dfrac{220V\cdot\sqrt{2}}{100\Omega}=3.1 > 1A (MOC3021)\$

So the 100 ohm is to low.

Marko Buršič
  • 23,562
  • 2
  • 20
  • 33
  • How much should that be? 1k/10k/100k etc. my hits and tries works with 560ohm 1/4w but I am not sure if that will fry it later.. – Harsh Shankar Oct 14 '20 at 20:23