Driving a solid state relay from ZX-40

[audioguy]

Just based on specs, it looks like driving a Kyotto KD20C40AX solid-state relay directly from a ZX-40 I/O pin should be ok. Anyone have any experience with that either way?

The ATmega644 data sheet shows that with VCC=5V, the minimum voltage of a high pin is 4.2V . The SSR data sheet shows that with 4-5V on the control input the current should be around 2.5-3ma .

Since the SSR input is already optically isolated from the load and I would include MOV's and ferrite snap-on's for slow and fast-rise inductive spikes and RFI on the output side, any reasons why I shouldn't do it this way? The application is to switch a 240V single/split phase 2 1/2 HP pump motor around 5-10 times a day with high reliability.

Thanks,
--jim

[GTBecker]

> ... with 4-5V on the control input the current should be around 2.5-3ma.

Sure, you can easily drive most opto-coupled devices with a pin.

It is common to drive them by sinking the opto cathode to ground,
though, instead of sourcing current to the anode. If you sink the opto
current, the processor doesn't need to supply it.

To do this, connect the SSR opto anode (the + control input) to Vcc and
the cathode(-) to the processor pin. In code, set the pin low to turn
the relay on and set the pin to high-impedance to turn the relay off.
High-impedance is also the Reset pin state, so the relay will be off
when the processor is in Reset.

Tom

[audioguy]

>
... To do this, connect the SSR opto anode (the + control input) to Vcc and
the cathode(-) to the processor pin. In code, set the pin low to turn
the relay on and set the pin to high-impedance to turn the relay off.
High-impedance is also the Reset pin state, so the relay will be off
when the processor is in Reset.

Tom

Excellent point with the uP reset Tom, thanks!

--jim