Relay causing noise - need suggestions - Electronic Circuits Projects Diagrams Free

tsasala · 8th March 2009, 01:33 AM

Hello everyone. I have a small 5V relay connected to a PIC via a ULN2803 driver. I have the diode on the 2803 tied to VCC to serve as a flyback diode, and it appears to be working properly. I'm switching 16VAC with the relay's contacts, which are connected to a small motor. The motor pulls about 60mA. The relay's contacts are not tied to the board's VCC or GND in any way - only to the motor and the AC source.

When the relay switches I get a significant amount of noise on VCC and throughout the circuit board. So much noise at times that the PIC actually resets. I'm really at a lost where all the noise is coming from - I would not expect so much induced noise on VCC just by turning a small motor on and off. If I remove the load, the problem goes away.

In the interest of full disclosure, the contact traces on the PCB do not cross VCC in anyway. I do have a ground plane on the bottom side of the board and the trace for the AC source lines is on the back side.

Any suggestions? Do I need a snubber on the relay contacts to kill the noise? If so, any suggestions on R/C values? Any help or constructive suggestions would be greatly appreciated. Thanks.

-Tom

be80be · 8th March 2009, 01:56 AM

Put a capacitor on the relay a 104 should do it. And make sure you have 1 on the pic from vdd to ground I have had to add a 10uf to my pic because of the motor

kchriste · 8th March 2009, 03:00 AM

Put the RC snubber across the motor, right at the motor. Make R the same value as, or slightly smaller than, the resistance of the motor windings. Try a 1uF non-polarized cap for C. A scope would help you select the right value for C.

rahulcet · 8th March 2009, 09:05 AM

The cap of value about 100 uF at the MCLR pin is very importnat. Micro chip themselves reccomends this which I have wondered Y. May be this is Y. Good LUCk.

tsasala · 8th March 2009, 04:23 PM

Thanks for the suggestions everyone. Using parts on hand (94Ω/.1µF) I put in a snubber circuit and it did limit some of the noise, but it is not perfect yet. Once I get more accurate parts, I'll see if it gets better. Thanks again.

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8th March 2009, 01:33 AM	#1
tsasala	Relay causing noise - need suggestions Hello everyone. I have a small 5V relay connected to a PIC via a ULN2803 driver. I have the diode on the 2803 tied to VCC to serve as a flyback diode, and it appears to be working properly. I'm switching 16VAC with the relay's contacts, which are connected to a small motor. The motor pulls about 60mA. The relay's contacts are not tied to the board's VCC or GND in any way - only to the motor and the AC source. When the relay switches I get a significant amount of noise on VCC and throughout the circuit board. So much noise at times that the PIC actually resets. I'm really at a lost where all the noise is coming from - I would not expect so much induced noise on VCC just by turning a small motor on and off. If I remove the load, the problem goes away. In the interest of full disclosure, the contact traces on the PCB do not cross VCC in anyway. I do have a ground plane on the bottom side of the board and the trace for the AC source lines is on the back side. Any suggestions? Do I need a snubber on the relay contacts to kill the noise? If so, any suggestions on R/C values? Any help or constructive suggestions would be greatly appreciated. Thanks. -Tom

8th March 2009, 01:56 AM	#2
be80be	Put a capacitor on the relay a 104 should do it. And make sure you have 1 on the pic from vdd to ground I have had to add a 10uf to my pic because of the motor __________________ Burt http://mybe80be.com http://stores.shop.ebay.com/mybe80be__W0QQ_armrsZ1 Last edited by be80be; 8th March 2009 at 03:01 AM.

8th March 2009, 03:00 AM	#3
kchriste	Put the RC snubber across the motor, right at the motor. Make R the same value as, or slightly smaller than, the resistance of the motor windings. Try a 1uF non-polarized cap for C. A scope would help you select the right value for C. __________________ Inside every little problem, is a big problem trying to get out.

8th March 2009, 09:05 AM	#4
rahulcet	The cap of value about 100 uF at the MCLR pin is very importnat. Micro chip themselves reccomends this which I have wondered Y. May be this is Y. Good LUCk.

8th March 2009, 04:23 PM	#5
tsasala	Thanks for the suggestions everyone. Using parts on hand (94Ω/.1µF) I put in a snubber circuit and it did limit some of the noise, but it is not perfect yet. Once I get more accurate parts, I'll see if it gets better. Thanks again.