I'm using the sainsmart 16 channel relay board to control some florescent ballasts. I connected two shift registers daisy chained together to control the relays from an Arduino. Without the line voltage connected into the relays, I can shift out a test pattern that activates the relays correctly. However, when I turn on the line voltage, the relays begin to flicker rapidly. I thought maybe it was an issue with some sort of EMF pulse from the ballasts causing the relays to toggle, but when I take out the shift registers, and ground out a few of the control pins (to activate the relays) the issue doesn't happen. Why would connecting a load into the relays cause the shift registers to rapidly reset?
Because you don’t have decoupling perhaps?
For information about decoupling see:- http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
That sounds very much like what is going on. Thanks to everyone for the replies.