switching 120V load with relay causing noise in circuit

I have a simple mcu-based circuit board that turns on and off 12V mechanical relays. The board requires 12V DC to operate which goes through a 5V regulator to the mcu and the rest of the circuit. There are 10uF tantalum caps on both the 12V signal coming into regulator and the 5V DC going out. The relays are turned on and off using a ULN2003 driver which has a diode built in to prevent back emf when the relay is turned on and off. When there is no load connected to the relays I don't have any problems. However, when I have an AC or DC load of some kind routed through the relay, I see significant spikes on the 5V DC signal on my board. I am confused as to why this is happening - my assumption was that the load on the relay was essentially isolated from the circuit that is driving the relay and it that it didn't really make any difference what the load on the relay was. Can anyone enlighten me as to what is going on?

Hello ,
it seems to me like the switching spikes are going back through the 12VDC line in some way ( maybe through mains ), however if you 5v regulator is 7805 series , can you try using 0.33uf capacitor on the 12 V side and 0.1uf on the 5V side of the regulator (as per the data sheet ) and see what happens ?
Regards

It's just normal operation. Contact arcing generates wideband "noise" that's propagated through cables, capacitively and inductively coupled to other circuits, radiated and received again.

Thinks you can do:
- reduce contact arcing by RC snubbers, varistors or freewheeling diodes (the latter with DC switching only)
- reduce succeptibility of your mcu circuit to switching interferences by good ground layout, power supply bypassing, filtering of external connected signal and supply lines by series resistors or ferrite beads and parallel capacitors or voltage limiting diodes