http://www.thesunrain.com/Amazon/4%20relay%20board/4-ch%20relay%20module.pdf
In the Schematic there are 4 Items Labeled U1,U3,U5,U7
They look like small black boxes on the actual boards. I can't really tell what they are, it looks like some sort of LED icon and then a transistor? If so whats their purpose? These are Mechanical Relays btw.
Answer
These components** are opto-couplers. Their purpose is to convey the digital signal while providing galvanic isolation. If galvanic isolation is required, VCC and JD-VCC should come from separate power supplies, which are in turn isolated from each-other.
** U1, U3, U5, U7 in the schematic.
Galvanic isolation is used to:
- stop high voltage events from getting into vulnerable areas
- block conducted EMI on the signal lines
- allow the isolated parts of the circuit to float with respect to each-other
For what it's worth, the schematic could have been drawn better.
- Isolated grounds should have different symbols to make it more obvious that there are isolate grounds in the circuit. Isolation barrier should be drawn as a line.
- There are 2 connectors on the board with "J?" designators.
- It's better to have a separate power connector for each of the isolated sides. Where's the ground pin for the right side power supply?
- No revision number. No part number.
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