VAC/VDC Control input voltage in Solid state relay

Question

I have been trying to understand the datasheet of solid state relays from different manufacturers and I was not much familiar with solid state relays. A confusion arose when I came to see the control voltage or input voltage mentioned as VAC/VDC.

You can see in the snap taken from a datasheet of Sensata-crydom attached above, control voltage given as "208-265 VAC", "18-30 VDC", "36-35 VAC/VDC". To my knowledge I understand if given "VAC" it is an ac voltage input and if given "VDC" then it is dc input voltage. But what in the case of VAC/VDC?

I have also came across some relays with both ac and dc control but they mentioned different VAC and VDC input voltage

here in this snap under CWU the value is "20-48 VDC/ 20-280 VAC"

So what is the difference between this case and that VAC/VDC case?

If both are same , can we consider both ac and dc voltage range are same in VAC/VDC case?

Or does VAC, VDC have any other meaning than I understood?

Answer 1

With a standard DC-triggered SSR the turn-on can be instantaneous (at any point in the mains cycle) or can be zero-cross (within a couple of degrees of the mains zero-cross point). See what I've written on Opto-triacs, solid-state relays (SSR), zero-cross and how they work to learn more about this.

For an AC triggered triac you have to consider that the trigger may be on a different AC phase than the load side of the SSR and that even if on the same phase various problems may arise. e.g. You would not be able to turn on the SSR until the AC input has risen above a certain minimum voltage.

To solve this it is most likely that the AC-triggered SSR input has a bridge rectifier, smoothing capacitor and some sort of Schmitt trigger circuit to switch cleanly on and off again. This will require a delay for the circuit to charge up and turn on and a further delay on turn off to carry it through the zero-crosses.

The datasheet shows that the DC-triggered SSRs are quick turn-on / turn-off (1/2 cycles) whereas the AC-triggered are 20 ms turn-on (one mains cycles at 50 Hz) and 30 to turn off (1.5 mains cycles at 50 Hz).