Safely using a relay

I am new to using relays and am getting mixed answers when I search on YouTube on online tutorials. I have an Arduino UNO and I bought this:

I want to switch off a power strip when the temperature gets too high in an enclosure.

I have seen people power the relay directly from the Arduino by using the 5V output pin on the Arduino and the GND pin.

Then I see that you should NEVER power a relay through the Arduino and that you should always use a separate 5V supply for the relay.

And yet another option I have seen is to use some type of diode in the circuit, but then you can power the relay off the Arduino...

All this has done is make me nervous. I am learning as fast as I can and reading as many books as I can, but this has really confused me. It seemed like such as easy idea but the contrasting thoughts on the subject has me lost.

What is the best solution for safely using a relay with my Arduino? Any help would be greatly appreciated.


If you are only driving this one relay and nothing else, you probably are okay using the Arduino 5 volts to power this board.
You will pull ~90ma from the arduino.

Something like this could work to pick the relay.

Relay Circuit.png


It seems like that board has a transistor driver.

Ignore the connections on the right side of the image going to the terminal block.

Potential problem.
I only see one resistor on the board where there should be two.
One to set the transistor base current, one to set the LED current.


How much current will the relay contacts be switching? 5 Amps or less I hope. :slight_smile:

Thank you very much for the help. Time to see what happens. And in response to edgemoron, It will be less than 3 amps.

Perhaps something from this company might be useful?

Please power the Arduino from somewhere other than your power strip!


Merely curious, what are you using to sense temperature?

Something like this could work to pick the relay.

Relay Circuit.png

Hi guys,

I´m working on something similar..
What does the "+ 0.8" on the diagram mean?


Vbe = .8 volts.
Voltage measured, base to emitter.

Probably closer to .6 to .7 volts

Thank you!

Hello all.
I dont know to answer but because i have a question about relays and relays modules
i choosed to post here to be easier for someone in future to solve his issues.

First of all ,i am very beginner in arduino and microchip programming
and …i have almost no idea about electronics !!!

Lets try my 1st and main question.

1.Ok…i ve managed to finish my 1st arduino project that is
A load scale (10 kg) sending sms 1 per day …from gms in some phone number.

And i bought a relay MODULE that already has transistor,resitors and diode…(and 2 leds)

You can see on attached image

Now the problem is that i DONT want to use the hole MODULE
a)i need to shrink the things
b)I dont want the led of the module
c)i want to learn

As so,The question is.

-Can i use a SINGLE relay (not module) with
ONLY one DIODE (just before the wire coming from the pin that i am controlling
with digitalWrite(Pin, LOW or HIGH) the relay ?
…avoiding the TRANSISTOR and RESISTORs ???

If yes,is there (on the market) RELAYS (not modules) that can be triggered with
the few amperes 20mA of arduino?

2nd question …maybe stupid but forgive me.
In the above module…that i installed fine and working fine

-WHY i must use 3 (Vcc,Grnd, Controlling Pin)
and not 2 ( Grnd ,Controlling Pin) [avoiding Vcc]
wirings from arduino ?

I just want to give HIGH (if digital) or 255 (if analogue) to the controlling PIn
so that when has voltage will give it to the RELAY
in order for the COIL to function and have the results on the OTHER side
( on the other non relative CIRCUIT… Normally open or normally close depends etc)
PS…I ve tried also with analogueWrite (2 wires)but not succesfully)

…I dont know…if this question is understandable…

Also the strange thing is that i must give LOW to trigger the coil …
but i was expecting …that i am suppose to give High to trigger and low to rest
…in order …also …to avoid baterry-energy consumption…

Thank you in advance anyway …


You need to start your own thread. Anyone trying to help now has to wade through an old conversation before they even see your post. They may try and assist the OP who is long since forgotten this problem. Try reading the forum guidelines