Purpose of 100 ohm resistors on 74HC595 input pins?

[addmoss]

What is the purpose of the 100ohm series resistors on Data, Clock, Latch & OE pins of 74HC595?
I haven’t seen any resistors in this configuration in other schematics.

I’m referring to Pi OpenSprinkler v1.43 schematic

[Ray]

The resistors act as current limiters: in the event of damage to 74HC595, they reduce the chance of the damage to RPi.

[addmoss]

I’m not aware of how 74HC595 typically fails but I’m thinking that, in a worst case scenario, if 24V gets from the triac gate to that pin, the resistor is not much of a help.
Maybe you have more experience with these circuits.

Have you considered using SSRs on the driver side?
AQY282SX from Panasonic looks a good pick and are quite cheap.
Would this be feasible?

[Ray]

It’s not for protecting the circuit from 24V, rather, in earlier versions of the circuit design, I used 5V on 74HC595, and so the resistor is there to be a current limiter between 5V and 3.3V. It didn’t turn out to be necessary, but I kept the resistors anyways.

Yes, I’ve looked at SSRs for driver. They used to be quite expensive. Now they are cheaper, so it’s certainly feasible. One thing you may want to look at is the maximum current they can drive. Some of them are quite low, like below 500mA, which would be an issue if someone wants to plug in two solenoids into one port.

[addmoss]

Makes sense now. Thanks for the info.
So far, I have found this 30VAC, 1A SSR from Panasonic as the best fit.
(1A should be enough for 2 solenoids; my RainBird DV-100 have an in-rush current of 0.3A then stabilize to 0.23A)

But SSR prices explode even for small voltage/current increments. Maybe it’s not yet the time for SSRs…

My other option is using MOC3063 + the same BT1308-600 triacs. More components but way better specs and price than the SSR.

[Author]

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