Offset issues when Relay is on

[kanurys]

Hi folks, I have another temperature control project which is exhibiting an offset issue. I'm powering 2 5v regulators with a 12v switching power supply. 1 regulator powers an LCD screen, back-light (the real power hog) and the arduino uno, the other powers the 2x5v relay board for switching AC.

The sensor input op amps and conditioning runs off of the 12v supply so I can use the full 5v range with a low noise floor going into the Uno. Everything has a common ground and there are a few decoupling caps on the regulators and in the arduino. My 12v bench supply can deliver 22A, so plenty of headroom, there.

The Problem:
When the relays click on, the input voltage to the Uno analog pin (from the sensor circuitry) drops slightly, giving a false reading. The offset changes depending how close I place the relay ground and power to the 12v supply. Is there an easy way to better-isolate the relay power from the rest of the system?

My other thought was to separate the analog sensor circuitry from the rest of the system using a 24v supply - by the use of a regulator to isolate the sensor circuitry at 12v and see if the relays effect the 12v regulated side. I worry that a bad ground would introduce a ground offset and effect the Uno reference. In the end I want a single power supply for the entire system so it is compact. Thoughts? Thanks for reading

[kanurys]

I got the ground voltage offset down to about 8mV with the 24v supply and a separate 12v reg for the sensor circuit. Man those 5v regulators are cookin' with a 24v input!

As I move the ground for the relays all the way back to the 24v power supply terminals I get near 0 offset. I guess I could chalk this up to poor connections on the breadboard or something. It would probably be better all soldered up. Any other ideas on how to eliminate this problem?

[kanurys]

Here is a post from the arduino forum by dc42 which I will execute:

"You need to separate the grounds. Dedicate one of the Arduino ground pins as analog ground, and use it as the ground reference for the op amps, sensors and associated circuitry. Use a different ground pin to connect Arduino ground to the power supply grounds, relay board grounds etc. Also connect a resistor (perhaps 100 ohms, lower if necessary) between the 12V supply and the op amps, with decoupling caps local to the op amps. The purpose of the resistor is to prevent noise on the 12V supply getting through the decoupling caps to analog ground."

[Neilm]

Is this on a breadboard or a PCB? If it is a breadboard then you could just have issues with the return current paths. If it is on a PCB, then is there a 0V plane?

In either event, pay attention to how the current flows. Remember it goes back to your power supplies, not magically "flow through" an IC.

I have done quite a lot of work on EMC / SI situations, and I keep finding that a single 0V plane with carefully laid out and partitioned components works 9 times out of 10. (the tenth just needed a small ferrite)

Neil

[kanurys]

Thank you kindly for your reply. This is on a generic white breadboard - two actually since I'm trying to keep the analog stuff away from the regulators. I think I'll set up a choke resistor on the op amp input and a cap, then reference it's ground to the Arduino analog side, then ground the Arduino to the other part of the system making sure the relay ground is the closest one to the supply.

The finished product will have a ground plane on it. If I can eliminate this problem on a bread board then it will work even better on the printed circuit board. Thanks for the input.

[kanurys]

Confirmed: grounding the op amps through the Arduino eliminated the offset almost completely even without the choke. I'll throw the choke resistor and cap on there to see how the noise looks with that. Thanks again.