Any idea about dry contact?

If a smart relay only supports AC input and its dry contact supports DC load, what is the use scene of this smart relay?

If it’s truly dry contacts, I’m intrigued by it caring about the type, or even polarity, of the switched load. Many care about having an AC load so they have a zero crossing to switch on, but something requiring it be a DC load is odd to me. If it’s “supports AC or DC loads”, then it’s useful for switching pretty much any electrical load it can handle within its voltage/amperage limits. Generally, you find a job that needs doing, then find the tools to perform that job, though.

Thanks for your reply. I also wonder if this smart relay with dry contact, but the relay only supports AD or DC input instead of both. What is the influence?

A lot of solid state relays (SSRs) have dry contacts but only do AC loads, because they require a zero crossing point to switch off properly (which they do primarily to avoid arcing, I believe). I can’t recall seeing any “DC Only” load relays with proper dry contacts, generally all the DC relays I’ve dealt with don’t much care about the load itself, as long as it’s within limits. They may not list AC load limits, and they may explicitly call out that they are not meant for standard AC loads (referring to standard wall power), which run rather high voltages compared to typical DC circuits, but I can’t think of a design reason a relay could handle DC and all the pain in the neck switching it can be but can’t handle a similar AC load (bearing in mind that AC peak voltages are well above the RMS value that’s typically used).

Why don’t you show us the device?

Actually, I am researching dry contact and would like to develop a smart relay with dry contact. I try to figure out whether the input and output of the device’s voltage is required AC or DC.