A garden light at 230 V and a gate motor at 24 V, from the same module in the consumer unit. A Shelly Pro 3 will do it, and the reason is what its relays are: dry contacts, also called potential-free.
What a dry contact is
Picture an ordinary wall switch: it opens or closes a circuit, and that is all it does. A dry-contact relay does exactly that, and the important part is that it is not internally connected to the Shelly’s own supply.
Two consequences follow, and they are the ones that matter:
- It is independent. The current and voltage you put through the relay can be entirely different from the ones powering the Pro 3 — which is always AC, 110 to 240 V.
- It isolates circuits. One device can drive loads running from different supplies.
One channel for DC, another for AC
With a single Pro 3 you can have, at the same time:
- A DC line on output O1 — a 24 V DC garage motor, say.
- An AC line on O2 — the garden lighting at 230 V.
- And O3 free for a third circuit, AC or DC, even at a different voltage again.
Each relay behaves as an independent switch. They share nothing but the body of the device.
The two limits that are not negotiable
The flexibility ends where the specifications begin, and going past them damages the device or the installation:
- On DC: 30 V maximum, 16 A per channel.
- On AC: 240 V maximum, 16 A per channel.
The 30 V DC ceiling is the one that catches people out. If your motor runs at 48 V DC this is not your relay, however comfortably the current fits.
What this is actually for
- Fewer interposing relays. What used to need a contactor or a pilot relay just to match voltages now connects directly.
- Less room in the board. One Pro 3 does the work of several separate modules.
- More freedom when planning. You can put devices with different electrical requirements into one system without redesigning the board around them.
A ceiling fan on AC and the irrigation on DC, driven by the same module and the same app. That is what the dry contact buys you.