Relay Board - Components and Schematic

Other Hardware General Electronics
1

I need to create a PCB with 8 relays, an Arduino to control the relays, 120VAC to 5VDC converter for power to the relays, and a bunch of other things. There are a bunch of 8 relay boards with optocoupler, status LEDs, etc on the market. Are there any schematics and component lists (resistors values, optocoupler parts, etc) for such boards as the one shown in the image below? If there are any layouts available, even better. I asked google but all I find is some breadboard approaches that just use transistors and not optocouplers.

8RelayBoard

2

This version allows for the Input to be either Hi or Lo.

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This 4 channel version needs a Lo to energize the relay.

board

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3

Is a transistor really required? Depending on the relay specs an optocoupler alone may be sufficient.

4

  • As long as the Opto CTR is large enough, that would work, however, most companies use cheap Optos and a transistor makes up for LOW CTR.

  • A 4N32 has a CTR of 500% and can handle 100mA :slight_smile:

1 Like
5

Thank you for this information. I have the 1N4148 and I have SS8550 (which should be better than the S8550 shown in the screenshot). So I think I am set on those components.

I don't have any PS2705 (plus I need through-hole anyways). I do have LTV-817s, though. They have a better CTR but the current - Output / Channel is only 50mA for the LTV-817 vs 80mA for the PS2705. The relays (SRD-05VDC-SL-C) have a coil current of 70-90mA so I guess the LTV-817s won't do? Or is this where the SS8550 comes in, i.e. the current output of the optocoupler is just driving the SS8550 and then the SS8550 (from VCC) is providing the required coil current? So the LTV-817 should still work?

Looking at the VCCs, currents and resistor values, I believe 1/8 Ohm resistors will do. No need to go to 1/4" resistors which are quite a bit bigger.

6

Yes, something like this;

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Tom.. :+1: :+1::coffee: :australia:

7

I am so confused
:woozy_face:

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8

Flip the OPTO and everything falls into place :slight_smile:

9

Hah
:sunglasses:

10

Sorry, to much blood in the caffeine steam..
Revised;

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At least someone is awake.

Tom.. :grinning: :+1: :coffee: :australia:

11

I feel much better now !

:+1:

  • R1 could be removed.
1 Like
12

In @LarryD's post #2:
I would keep logic Vcc and coil power Vcc totally separate (call them LVcc and PVcc) as well as grounds (LGND and PGND) for complete isolation. Also 12V relays would need only 30mA per coil vs 72mA for 5V. Also also, I would think about a power shift register like TPIC6B595, 8 transistor switches in one package controlled by 3 MCU pins.
tpic6b595.pdf (1.5 MB)

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