24VAC irrigation solenoid controller

[ilium007]

R21 prevents any leakage through the opo-coupler from triggering the TRIAC. Frustratingly, the minimum trigger current is often not specified on the TRIAC data sheet, but the minimum voltage is, which is 0.2V. Looking a the graphs on the opto-coupler data sheet gives a maximum leakage of 4µA, so a <50k resistor would be required to ensure a voltage drop of under 0.2V. Intuitively, I think it will be fine without R21, but resistors are cheap.

Thanks! This is the last piece of the schematic puzzle for me. I am going to try and incorporate a PCB spark gap as well, that's going to be another rabbit warren to go down with KiCAD.

[thinkfat]

Custom pad shapes are how you do spark gaps in KiCAD. Don't forget to remove the solder stop mask.

[ilium007]

Custom pad shapes are how you do spark gaps in KiCAD. Don't forget to remove the solder stop mask.

Yeah, I played around with them a while back. Hoping to find someones footprint library in GitHub.

[ilium007]

R21 prevents any leakage through the opo-coupler from triggering the TRIAC. Frustratingly, the minimum trigger current is often not specified on the TRIAC data sheet, but the minimum voltage is, which is 0.2V. Looking a the graphs on the opto-coupler data sheet gives a maximum leakage of 4µA, so a <50k resistor would be required to ensure a voltage drop of under 0.2V. Intuitively, I think it will be fine without R21, but resistors are cheap.

In the datasheet for the Z0109MN0 it says min Gate Trigger Current is 0.4mA = 400µA https://www.ween-semi.com/sites/default/files/2018-10/z0109mn0.pdf

So the optocoupler leakage current at 4µA isn't enough to trigger the Z0109MN0 TRIAC? What triggers the TRIAC? Voltage or current?

[Ian.M]

What triggers the TRIAC? Voltage or current?

Now *THAT* is a philosophical question!  The gate is effectively part of a BJT so a semiconductor physicist or an engineer performing quantitative circuit analysis would say Voltage and trot out an exponential equation for the resulting gate current, but a technician or hobbyist would say Current . . .

[ilium007]

What triggers the TRIAC? Voltage or current?

Now *THAT* is a philosophical question!  The gate is effectively part of a BJT so a semiconductor physicist or an engineer performing quantitative circuit analysis would say Voltage and trot out an exponential equation for the resulting gate current, but a technician or hobbyist would say Current . . .

hahahah I thought the answer would be along those lines 

[Zero999]

R21 prevents any leakage through the opo-coupler from triggering the TRIAC. Frustratingly, the minimum trigger current is often not specified on the TRIAC data sheet, but the minimum voltage is, which is 0.2V. Looking a the graphs on the opto-coupler data sheet gives a maximum leakage of 4µA, so a <50k resistor would be required to ensure a voltage drop of under 0.2V. Intuitively, I think it will be fine without R21, but resistors are cheap.

In the datasheet for the Z0109MN0 it says min Gate Trigger Current is 0.4mA = 400µA https://www.ween-semi.com/sites/default/files/2018-10/z0109mn0.pdf

So the optocoupler leakage current at 4µA isn't enough to trigger the Z0109MN0 TRIAC?

That's a better data sheet, than the one I found. Good, then you don't have to worry about the resistor.

What triggers the TRIAC? Voltage or current?

Unless it's a superconductor, there can't be one, without the other!

[markitus]

I’m also trying to learn about the opensprinkler design and looking at the schematic I can’t understand why the mcu pin does not get fried.

For example P0 when it’s HIGH (5v / 3.3v) by the MCU or the shift register it turns ON the triac, but it’s aparently connected with a pulldown to the ACRET which I understand it closes the AC part of the circuit to energize the solenoid. How this 24VAC don’t destroy the MCU pin /multiplexer/shiftregister connected to it?

[SeanB]

Resistor is also there to reduce false triggering due to noise on the mains, which can capacitively couple either through the PCB traces, or through the TRIAC itself.  Easiest to keep it there, as while the TRIAC is likely not to trigger with 4uA of current flowing into the gate, with the gate at 0.15V, it is a lot less immune to noise then, and only a tiny bit more can start the triac conducting, and with it then slowly turning on from limited gate current, the voltage across it will be high while the current is rising, leading to very high instantaneous power dissipation. This can long term damage it, as the die area closest to the gate is stressed by heating, and this degrades that area of the junction. good reason why you have a gate pull down that handles any leakage, and you drive with more than the minimum gate current.

[thinkfat]

I’m also trying to learn about the opensprinkler design and looking at the schematic I can’t understand why the mcu pin does not get fried.

For example P0 when it’s HIGH (5v / 3.3v) by the MCU or the shift register it turns ON the triac, but it’s aparently connected with a pulldown to the ACRET which I understand it closes the AC part of the circuit to energize the solenoid. How this 24VAC don’t destroy the MCU pin /multiplexer/shiftregister connected to it?

(Attachment Link)

If I'm not mistaken the whole circuit GND "rides" on one of the AC lines. On the controller side, ACRET is likely connected to GND. I've made my own version of the OpenSprinkler circuit as a cape for the BeagleBone Black Wireless a few years ago. Looking back at the schematic, this is how I did it.