Mcu immunity of relay noise

[mah]

Hi, i want to design relay circuit to switch ac motor without causing noise to the MCU. I put diode on the relay coil but what about the contactor on ac side?

 

[MaxHeadRoom78]

A RC snubber is typically used or a MOV, I prefer the former, you can buy them as one unit.
Max.

 

[mah]

Is this enough for relay switching noise?
How to calculate its value?

 

[MaxHeadRoom78]

I have used them many times and it has cured intermittent noise problems.
Is this on the contactor coil or the contacts?
Max.

 

[mah]

The problem is the mcu and connected lcd are disturbed when i connect the ac power so i think the problem is with contacts. In general i want to protect mcu from coil and contact

 

[dknguyen]

Is this enough for relay switching noise?
How to calculate its value?

The value isn't too critical unless you're optimizing. If you have an oscilloscope you can use trial and error until the switching overshoot/undershoot is criticall damped. If R is too big and C is too small, then there will be little effect. If R is too small and C is too large then the RC snubber might heat up too much and you'll have a little bit less efficiency.

0.1uF is pretty big for a snubber capacitor so you could just try that and try a bunch of different resistors from 1ohm to 5ohm until you find one where the components don't heat up too much. If the cap heats up then use a higher resistance. If the resistor heats up too much, use a higher wattage resistor (or you could also increase the resistance as well but this reduces the snubbing).

The main goal of an RC snubber is to slow down the rise time of the spike rather than to actually clamp or suppress the spike (although it does that too to a limited extent).

 

[MaxHeadRoom78]

What I Would do in future is try to use L.V. DC for contactor coils, they often have the option of AC or DC versions.
Some also have a RC snubber module or diode that fits across the coil in a custom manner.
Max.

 

[HarryC]

What mah is describing won't be cured by a snubber, most likely. It sounds like the AC is coupling through the coil of the relay and back to the MCU. We'll probably need to see the schematic and board layout of that area to understand what's going on.

I use SSRs (Solid-State Relays) for cases like this 'cos the trigger is usually optocoupled to the triac, giving added separation between AC and your electronics. The Fotek brand that you can get cheap out of China gets iffy with 3.3V logic, and really needs a 5V drive signal even though the specs say it'll run down to 2.4V input threshold. Due to the high inductive load of a motor, you might still need a snubber on the AC side of an SSR, so keep those PDFs (posted above by MaxHeadroom78) handy.

 

[mah]

this is my pcb

 

[MaxHeadRoom78]

What mah is describing won't be cured by a snubber, most likely.

Has always done it for me, but I have stopped using AC contactor coils for some time now in L.V. circuits.
You could also reference your L.V. common to Earth ground, which often takes care of the problem.
Max.

 

[mah]

Has always done it for me, but I have stopped using AC contactor coils for some time now in L.V. circuits.
You could also reference your L.V. common to Earth ground, which often takes care of the problem.
Max.

you put snubber on coil or contact?

 

[MaxHeadRoom78]

I have only had to put it on the coil.
Max.

 

[mah]

in parallel to free wheeling diode ?
Could be my problem because the ac suppy is close to mcu?

 

[MaxHeadRoom78]

Some confusion here, I took it you had a DC relay picking up the AC contactor coil?
To be clear, BEMF or free-wheeling diodes go across DC inductive devices or coils , snubbers, across the AC fed versions.
If you have both, then you need the appropriate device across the coil.
Max.

 

[mah]

No i use just relay connected to ac motor. Do you mean i don't need snubber in my case? So how to avoid noise relay?

 

[dknguyen]

What mah is describing won't be cured by a snubber, most likely. It sounds like the AC is coupling through the coil of the relay and back to the MCU. We'll probably need to see the schematic and board layout of that area to understand what's going on.

I use SSRs (Solid-State Relays) for cases like this 'cos the trigger is usually optocoupled to the triac, giving added separation between AC and your electronics. The Fotek brand that you can get cheap out of China gets iffy with 3.3V logic, and really needs a 5V drive signal even though the specs say it'll run down to 2.4V input threshold. Due to the high inductive load of a motor, you might still need a snubber on the AC side of an SSR, so keep those PDFs (posted above by MaxHeadroom78) handy.

Well he says it only happens during switching.

OP: When does the noise occur? While the motor has power? Only at the moment the motor is switched off? Only when the motor is switched on?

If only when it's switched off, then an RC snubber will fix your issue.

 

[mah]

noise happens when i connect ac power ,whether there is load or not. But it increased with load even with light pulp

 

[MaxHeadRoom78]

noise happens when i connect ac power ,whether there is load or not. But it increased with load even with light pulp

What are you feeding the MCU with power wise?
Max.

 

[mah]

What are you feeding the MCU with power wise?
Max.

i feeding it with 12 v adapter ( before regulator)

 

[dknguyen]

noise happens when i connect ac power ,whether there is load or not. But it increased with load even with light pulp

This is a little bit unclear.

What do you mean by "when you connect AC power"? Do you mean when you plug AC power into the circuit even when the relay is open and no current is flowing? Or do you mean when you connect AC power to the load by closing the relay?

And you did not make it clear whether it occurs only when it switches or if it occurs while it conducts.