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### Topic: leds and relay board (Read 209 times)previous topic - next topic

#### kli

##### Aug 10, 2017, 02:59 pm
hello all,

i need help about a project which implies running 64m of leds. I already have some materials like some 12V 16A power supplies, an ebay 8 channel relayboard and an arduino mega. I would like to switch my leds strips, not need dimming. Maybe a relay board is not the best device to do it, I should go with mosfet instead. But on the other hand i would like to finish the project with what i already have.

First of all i have a question about the way i can hook up relay board to the arduino. It's an active low relay board. I plan to use separate power supplies, one for the relay and one from the mega regulated 5v output for driving the relay. According to the joined schematic of the relay board there is a resistor on the path of the optocoupler and the status led, value is not indicated. When i look directly on the board i can see two resistors labelled respectively 511 and 102 (closest to status led).

The optocoupler is a 817C. According to the datasheet the forward voltage is 1.4V.
http://www.keepjump.com.tw/DataSheet/Others/DPC-817C.pdf
If the right resistor value is 102, thus 1K
(5V-1.4V)/1000ohm ~= 4mA

So i should have 4mA on the path to the arduino pin. Is that right ?
Now if i don't need all relays on board, for example if i just need 5 of them, it will be 20mA for all 5. Because it will be equal to the max 20mA per pin, can i hook them to the same arduino pin ?

Then my second question is about the calculation of the max power i can expect from the relay coil. On the relay marking i can read that :
10A 250VAC 10A 125VAC
10A 30VDC 10A 28VDC
so it's the maximum rating. But what if i want use 12V instead ? how much current can i expect ? can i calculate it like the following :
30VDC*10A = 300W
then to know the max current under 12V :
300W/12VDC = 25A
is it the right way to calculate ?

#### terryking228

#1
##### Aug 10, 2017, 03:11 pm
Hi,

Is the relay board like THIS ONE??

See the schematic there.  the 102 is 1000 ohms  " 1  0  (2 zeros) " and that is the one in series with the optocoupler internal LED.   The 511 is  "5 1 (1 zero)" or 510 ohms which is the base resistor to the transistor activating the relay.

DISCLAIMER: Mentioned stuff from my own shop... (Where I have my Info!)

So: I calculate 2mA as the current from Arduino.

Make sure you DO use a separate 5V supply for the relay board and DO isolate it.  See:

http://arduino-info.wikispaces.com/ArduinoPower

and specfically:

http://arduino-info.wikispaces.com/ArduinoPower#OI

POWER You Can Control:

The relays are limited to 10A, right?  So POWER 'per relay' is 12V * 10A = 120W so your calculation is not right.

Let us know how this works out...

Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
HOW-TO: http://ArduinoInfo.Info

#### kli

#2
##### Aug 11, 2017, 02:24 pm
Quote
Is the relay board like THIS ONE??
yes it looks the same.

Quote
DISCLAIMER: Mentioned stuff from my own shop... (Where I have my Info!)

So: I calculate 2mA as the current from Arduino.
oups yes you're right i forget the status diode forward voltage...
that's a good thing you show the schematic on your shop ! it's help a lot !

2mA is far less than the maximum current allowed on a arduino pin, does that mean i can hook the eight relays input on a single arduino pin, is that correct ? that way i can switch ON/OFF all 8 relays with just one pin ?

Quote
POWER You Can Control:

The relays are limited to 10A, right?  So POWER 'per relay' is 12V * 10A = 120W so your calculation is not right."
that's interesting ! whatever voltage is (<30VDC) it's always 10A. ok !

#### kli

#3
##### Aug 13, 2017, 02:10 pm
sorry for bumping !
but anyone can confirm this ? can i safely hook up the eight relays input to a single arduino pin ?

#### terryking228

#4
##### Aug 13, 2017, 03:00 pm
Quote
can i safely hook up the eight relays input to a single arduino pin ?
Yes, 2 mA * 8 = 16 mA and that should be OK. Test it first with just the relay board without external connections.

The relay board must have it's own 5V power supply that is not connected to Arduino ground.
Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
HOW-TO: http://ArduinoInfo.Info

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