Triac 101

[NivagSwerdna]

I've never done this before.... I have a door bell solenoid... ok... four of them that I wish to turn on and off using a ESP32

Literally years ago I came up with a little circuit...



But now it's come to the time to populate the board and I'm no longer convinced!

This is a one off and I've never used a Triac before so I am a bit worried.

Does this look reasonable?  AC voltage is 12V and control voltage is 3V3

[Benta]

LED current is too low.
MOC3021: 15 mA
MOC3022: 10 mA
MOC3023: 5 mA

What's the purpose of R4?

R3 looks low (depends on the triac).

[NivagSwerdna]

TBH I'm not sure where I got this from but it is currently baked into the PCB.

I think R4 is to keep it stable off when the opto isn't active?

For the LED side... 15mA is max for a MOC3021... I think typical is 8mA is typical but maybe R1 is a bit high.

Does it matter which side the LOAD is applied?

[Benta]

R4 is unnecessary and will probably impair performance.

I don't do designs using typical values, that's looking for trouble. But right now the LED is getting 6.5 mA.
Load can be applied both sides.

[PCB.Wiz]

R4 is unnecessary and will probably impair performance.

Not quite.

Wiki says this
Because turn-ons are caused by internal capacitive currents flowing into the gate as a consequence of a high dv/dt, (i.e., rapid voltage change) a gate resistor or capacitor (or both in parallel) may be connected between the gate and MT1 to provide a low-impedance path to MT1 and further prevent false triggering. This, however, increases the required trigger current or adds latency due to capacitor charging. On the other hand, a resistor between the gate and MT1 helps draw leakage currents out of the device, thus improving the performance of the TRIAC at high temperature, where the maximum allowed dv/dt is lower. Values of resistors less than 1kΩ and capacitors of 100nF are generally suitable for this purpose, although the fine-tuning should be done on the particular device model.[5]

Renesas says this
For high-sensitivity and low-current thyristors, it is generally recommended to insert a resistor of 1 kΩ between the gate and the cathode.

[RoV]

I would put a small snubber in parallel with the load, to limit dV/dt. 100 nF class X2 in series with 100 ohm are sufficient, because low energy is expected (the triac opens close to zero current).

[Benta]

R4 is unnecessary and will probably impair performance.

Not quite.

Wiki says this
Because turn-ons are caused by internal capacitive currents flowing into the gate as a consequence of a high dv/dt, (i.e., rapid voltage change) a gate resistor or capacitor (or both in parallel) may be connected between the gate and MT1 to provide a low-impedance path to MT1 and further prevent false triggering. This, however, increases the required trigger current or adds latency due to capacitor charging. On the other hand, a resistor between the gate and MT1 helps draw leakage currents out of the device, thus improving the performance of the TRIAC at high temperature, where the maximum allowed dv/dt is lower. Values of resistors less than 1kΩ and capacitors of 100nF are generally suitable for this purpose, although the fine-tuning should be done on the particular device model.[5]

Renesas says this
For high-sensitivity and low-current thyristors, it is generally recommended to insert a resistor of 1 kΩ between the gate and the cathode.

Well, the Z0103 is hardly high-sensitivity. And there's a certain difference between 300 and 1k.

[PCB.Wiz]

Well, the Z0103 is hardly high-sensitivity. And there's a certain difference between 300 and 1k.

The supplier claims differently

Z0103MN0
Planar passivated very sensitive gate four quadrant triac
• Very sensitive gate in four quadrants

Gate trigger is 200uA~3mA

[Benta]

Well, you're (as always) right. Feel better?

[PCB.Wiz]

Well, you're (as always) right. Feel better?

Many people read these forums, over a long time, so it helps to have accurate information. 

[bostonman]

FYI, this month (or issue) of Nuts and Volts has a multi part series on TRIACs.