Relays are suitable for switching different voltages and currents and are very universal. Controlling and controlled circuits are isolated from each-other and this makes them quite safe to use – if you burn down relay because of switching too high voltage or current, most likely your controller stays intact. Wide-spread relays could be divided to mechanical and solid-state relays. Both of them can switch electrical circuits on and off. Nothing more. Mechanical are cheap but they cant be used to switch high currents and/or high frequencies. Their contacts will just burn. Solid-state are much better in handling high loads and higher frequencies but they are expensive and its unlikely that something in a regular household needs switching so fast. In addition to max allowed switching current one should pay attention also to max allowed voltage as higher-than-allowed voltages can cause relay and or contacts isolation to fail. You can read more about relays in here
As I had need for switching voltages 5V DC, 12V DC and 220V AC, I chose relays as a way to go if I need to switch something just on and off. I counted my needs and it turned out that I need at least 8 relays. Preferred interface is USB as it can be used both with Raspberry Pi and regular PC-s. So the search began for relay-controllers which have at least 8 relays and have USB interface.
Results – there are probably hundreds of solutions around but few were actually good in my mind. I will list few cons what some products had:
- Big power consumption – especially if all relays are switched on.
- Need for external power to switch relays, most often 12V was also needed additionally to the 5V what USB provides.
- Latching relays – relays keep their position when power is lost – just not needed for me.
- Small amount of relays – some products were really good candidates but they only had 4 relays. I want to expand the “system” but not now, sorry.
One product cought my eye which did not have any of those cons:
Really. It can run only on power provided by USB. It has bunch of other interfaces additionally to USB. Even IR (InfraRed) – you could teach some “tricks” to your old TV remote. It has very good documentation in English and its available through manufacturer website and Ebay from inside EU (no customs bs).
Forgot to say that this BV4627 supports switching with timer – you can delay switching on or off up to about 4700 seconds (~79 minutes) using internal timer.
All the relays on that board are SPDT – Single Pole Double Throw and have following contacts:
- Common (CO, Change-Over but I like Common more)
- NO – Normally Open
- NC – Normally Closed
These contacts can be used in any way you like, just need to remember that “normally” relay is powered OFF:
- If relay is OFF, Common and NC contacts are closed.
- If relay is ON, Common and NO contacts are closed
Common contact can be used both for input and output in a world of flows. So you could either switch one input between two outputs or you can switch two inputs between one output. That last sentence was really scientific ….
If you want to switch just two wires together you can choose when they are connected – when relays are on or off. I would say that its best to decide based on the duration – when these two wires should be connected more time than they should be apart, its better to use NC contact and send command to relay to turn on when you actually want to disconnect these wires. Or decide based on priority or based on damage if you happen to lose control or something else or who really cares?
Lots of BS above. Now its time for actions.
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