Hi, i'm new with arduino, im setting up a relay that turn on/off a led light, but when the relay turns on, it leaks current through the whole arduino, and the laptop when I'm powering the arduino with it
This is a test for another project but I'm afraid it can cause fire or something.
Sorry for my bad english 
(mod edit)
**Edit:
I'm using 127V for the LED light.
That is about the cheapest relay module there is.
You may have been better with the next one up which would have OPTO ISOLATION.
In theory there should be no leakage although powering a relay directly from an Arduino is discouraged because the Arduino cannot supply the current often required.
Could you also take a few moments to Learn How To Use The Forum.
It will help you get the best out of the forum in the future.
Other general help and troubleshooting advice can be found here.
Then you will be able to post pictures and I wont have to intervene for you.
abel2598:
**Edit:
I'm using 127V for the LED light.
I guess that must be DC. Be advised that in the real world there is ALWAYS leakage of current if it goes through traces on a printed circuit board, or even insulated wires that are too close together. It will likely be so small it can be ignored on projects we work with.
That is why all commercial equipment that is powered by the commercial AC power must pass tests using a piece of equipment known as a "megger". We had one and only used it for one project in 20 years. I was surprised, but my manager knew exactly how to use it. I sure didn't.
It will apply a high voltage to a device for a specific length of time an try to discover leakage to other parts of the device.
Paul
abel2598:
Hi, i'm new with arduino, im setting up a relay that turn on/off a led light, but when the relay turns on, it leaks current through the whole arduino, and the laptop when I'm powering the arduino with itThis is a test for another project but I'm afraid it can cause fire or something.
Sorry for my bad english
Yes, possibly a translation problem.
What do you mean by current leaking through to your laptop?
Do you get an electric shock?
Can you post some pictures of your project so we can see your component layout?
Thanks.. Tom... 
Do you have a multimeter?
Unplug everything from the relay module, and check the resistance between low-voltage side and the
contacts is 'OL' (off scale).
If it isn't your unit is dangerous to life and limb and should be crushed and discarded to prevent anyone else
using it. Or perhaps opened up and investigated for the failure mode.
