Relay Switching - generating Noise
Relay Switching - generating Noise
(OP)
Hey all,
I need some help in designing a MCU system. I have a set of 4 relays and one of them switches an inverter. I have a MCU monitoring the system and so everytime the relay switches the MCU resets itself (noise). I am working on different PCB layouts so as to keep the relay as far away from the MCU as possible. I have tried caps ... but have not been successful. I would greatly appreciate any suggestions.
Thank you
SD
I need some help in designing a MCU system. I have a set of 4 relays and one of them switches an inverter. I have a MCU monitoring the system and so everytime the relay switches the MCU resets itself (noise). I am working on different PCB layouts so as to keep the relay as far away from the MCU as possible. I have tried caps ... but have not been successful. I would greatly appreciate any suggestions.
Thank you
SD





RE: Relay Switching - generating Noise
RE: Relay Switching - generating Noise
RE: Relay Switching - generating Noise
The load (a lamp load) is primary ont he normally-closed contacts (no load on the normally open)
When the relay switches from normally open to normally closed, it does not reset the micro but when it switches from normally closed back to normally open it causes the mcu to reset.
i appreciate your help
SD
RE: Relay Switching - generating Noise
RE: Relay Switching - generating Noise
1. Diode across relay coil. (You've done that).
2. The surge current when relay closes, energising the load momentarly drives the power supply into overload and it briefly drops its output voltage. If the MCU is connected to the same supply it will reset. Solution is to use a seperate power source and seperate wiring for the load.
3. Ensure that there are no common current paths for the MCU power and ground, and the load power and ground. The ground paths are just as important as the power paths because when the load energises there may be significant momentary voltage spikes on the power AND ground leads.
RE: Relay Switching - generating Noise
I think I have figured out the problem. Let me begin my explaing what i have... I have 3 circuits.... A charging circuit (charges the battery), and inverter circuit (in a black box.. I cannot redesign that) and a daughter board (this is where the microcontroller is.. and once again i cannot redesign that... )
The only thing i have access to (to redesign ) is the charging circuit. A 14.4 V battery is used to power the lamps (in an emergency) and after droping down the voltage (with a zener)it is fed into the MCU circuit.
Now this is what I did.. I disconnected the 14.4 V battery to the to the MCU board and I used a seperate battery to power the board.. and now I don't have any resets ...
Here is the problem I need to use the same battery that powers the inverter to power the MCU board. There is a little connector and after that is closed will the MCU board and the inverter be connected. Hence I can't get the power to the MCU board as close to the battery as possible....
I have a 0.01uF cap at the pins between the charger board and the MCU board.. Do y'all have any ideas on cleaning that power supply
Thank you once again.. I appreciate it
SD
RE: Relay Switching - generating Noise
RE: Relay Switching - generating Noise
Good luck and keep us posted!
RE: Relay Switching - generating Noise
Hi guys, just wanted to check how the MCU drives the relay(s)? If it drives them directly (doesnt sound likely in this case?) or via a bipolar transistor it may use enough current from the probably weak zenervoltage for VCC to drop? In such a case you may consider:
0. If possible increase series R to the relay driver transistor(MUST be checked that transistor saturates correctly or fire may occur!)
1. More powerful regulator for MCU, like a 7805 or 78L05
2. Put some buffer stage between the MCU and the relay driving circuitry
3. Exchange the bipolar relay driver with a FET (with some 10k gate pull down in case of MCU fail)
best of luck
RE: Relay Switching - generating Noise
RE: Relay Switching - generating Noise
These dropouts are also a major cause of failure of 3 terminal voltage regulators which find themselved momentarily reverse biased when power input droops but the output is held high by a cap in the load. They do not like being reverse biased.
RE: Relay Switching - generating Noise
A 14.4 V battery is used to power the lamps (in an emergency) and after droping down the voltage (with a zener)it is fed into the MCU circuit.
If that is a series zener feeding the micro, I would consider that sub-standard design in a power ckt. Dump the zener and use a 78xxx as suggested, even if other measures cure the immediate problem. A series diode may be good to add as well, though it will use up a bit of voltage head room.
RE: Relay Switching - generating Noise
I stuck with the zener and used a decoupling circuit to decouple my power supply into the MCU circuitry. I moved the relay away from the MCU circuit and that helped too.
When I get my new PCB i'll keep y'all informed. Thanks a bunch
SD