Transformer open circuit output voltage and current limiting resistor at gate of Triac

Question

Im kinda new to electronics and have some questions before i solder my projekt together. I want to make an Arduino controlled soldering iron and designed the circuit below. The optocoupler i used is a LTV-817 and by iron i mean the heating element at around 3Ω.

1. My problem is that the transformer features an open circuit voltage of around 50V AC peak to peak and I am not sure how the optocouler IC1 is able to handle this. Do I have to choose a different resister value?
2. The second Problem is the current limiting resistor at the gate of the triac. How is the value calculated?

Answer 1

One thing that jumps out is that you are using a transistor opto-isolator to turn on the triac. This won't end well because

1. It could, at best, only turn on when the collector is positive so it would only switch on positive half-cycles of the transformer output.

2. The LTV-8X7 datasheet, section 4 gives absolute maximum ratings as

   • Collector - Emitter Voltage, V_CEO: 35 V
   • Emitter - Collector Voltage, V_ECO: 6 V

The first parameter means it will be destroyed by your 50 V supply on the first positive half-cycle it sees.

The second means that it will be destroyed on the first negative half-cycle it sees.

Either way you're doomed.

---

What you need is an opto-triac.

Figure 1. A sample opto-triac controlled circuit (for mains voltage). Source: Triac Switching circuit with Optocoupler.

You may need to modify the circuit a little to work with your lower transformer voltages.

For more reading on the matter have a look at my article Opto-triacs, solid-state relays (SSR), zero-cross and how they work which may be of help. You may decide that a zero-cross opto-triac means that you don't need to monitor the zero-cross of the transformer.