How to protect Arduino from HF

I am making cnc plasma project. Using Arduino UNO with grbl to control it. Problem is that sometimes arduino craches on plasma ignition. I spend a lot of time reading on internet how to protect arduino from HF. My arduino is in metal case which is grounded, all cables which are outside box are shielded and grounded and almost all cables in the box are shielded and grounded. Also i found solution to use capacitors and diods and there comes another problem because i dont realy know how to pic right capacitor and not sure where in the scheme i should put it. I attached scheme which i am using maybe somebody can help me telling what capacitor (type, nominal, model) and where should i use?

You describe everything I might have suggested. Please can you post photos of your project clearly showing the various parts and how they are wired up.

Are your shielded wires only grounded on ONE end? If not, they are not shields, but parallel conductors/capacitors which will induce noise from the shield to the inner conductor.

As you can see in the scheme which i posted earlier. I added bluetooth module with 5v power supply for it. To control plasma via bluetooth and i have to say that this solution helped. The only problem left with relay which is turning on plasma torch. Arduino doesnt crash anymore but relay works not stable. After command M04 relay turns on but then it turns off immediately when ignition is starting. So ignition doesnt start arduino keeps reading program and moving. Command M04 is active but relay doesnt work. I decided to change relay module with opto and shalded wires inside box but after that i got even worse result my arduino started to crash and freeze so i decided to change back wires and to put capacitors with diode because i cant really find any other solution.

yes i grounded only one end. No loops.

They don't look it! By putting the relay in the box you are bringing the noise into the box right up to the Uno. Separate the Uno from the power switching, perhaps by putting it inside it's own metal box.

I took this photo before i changed wires into shielded. Yes i thought about it. I am planing to print box for arduino with 3d printer and then put it into aluminum foil :slight_smile:

The box needs to be metal and grounded, buy a suitable size metal box. Difficult to get a decent connection to foil.

Hi @corvuscorax4 .
I saw this tip on another site a long time ago but I haven't proven its effectiveness yet.
Arduino pins, by default, are inputs, so to avoid unwanted external noise inputs set all pins that you are not using in your project as output, including analog pins.

RV mineirin

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1000uF capacitor on 5V relay , directly on module

To make clear, when nothing is connected to relay on right side relay is working properly ?

Yes, when plasma cutter not connected to relay everything is working perfect. So your suggestion is to use capacitor only on relay module? How about fan, stepper outgoing wires?

So this is your source of problem. When you turn on plasma manually by separate switch, you have problem or not ?
It is metal box connected to powerline ground ?

We really need to know what it looks like now, not what it looked like in some previous version, which is irrelevant.

Let see the answers to post #13

I will try to turn plasma cutter manually tomorrow. And let you know how it will be. Metal box is grounded to power line ground.

In this pictures you can see how my first version looks like. As i mentioned i changed almost all wires with shield in second version but i got worse result than before with no shielded wires inside box so tomorrow i am planing to return to my first version which you can see in the pictures.

Do you have 2 phase power supply coming to your place, use extension cord and plug in your plasma to different outlet.

plasma cutter and my cnc project are connected to different phases. Also where do you suggest to put capacitor on relay? Like in the picture? I bought metal corrugated tube today and planing to put wire from plasma cutter to torch into it and ground it.

capacitor on. left side, 5V and GDN , 500uF and 0.1uf. (in red 2 and 3 )

in red 1 - you may need power line filter


How far is arduino from plasma ?