I have a situation where I see spikes (with ringing) on the output of DC power supplies whenever something AC powered is switched in the room. The spikes change in magnitude; they can be as bad as 3 volts peak-to-peak; I suspect they change "randomly" because they are related to when the device is switched (relative to the 60Hz cycle) but I don't want to put words in anyone's mouth. The ringing after the spike can be several cycles and seems to be on the order of 10 to 20 MHz. Currently this spike is interfering with a differential to single-ended driver I use to feed an encoder into an acquisition card on a PC, causing the card to count false pulses thus making the encoder useless. I have a 1uF metal film capacitor as a bypass capacitor on the single-ended driver chip. I don't want to go into the details here because I can reproduce the problem with a very simple test. Power supply A is a 12VDC switching power supply with a 5VDC regulator to create a 5/12 output power supply (not my idea). This one is plugged directly into AC. Power supply B is a linear power supply outputting 5VDC (a Power-One HN5-9/OVP-AG). This one is plugged into AC via a Lambda MAW-1205-22 EMI filter. Both of these are properly enclosed and grounded. Power supply A has a vented box with maybe a 1/2" by 1/4" diamond mesh on one side. Power supply B is in a solid Hoffman box with no holes. My "test" circuit is a 100 Ohm resistor connected to 5VDC (either power supply gives the same results). Looking at the voltage across the resistor on a scope, i can see the spike and ringing clearly. Adding a 0.01 uF metal film capacitor across the resistor seems to make the ringing last longer. No combination of capacitors I have (all metal film, up to 1 uF) seem to improve the situation, though I can clearly see the bypassing effect because they will really clean up the voltage from the 5VDC regulator in power supply A. I cause the spike by switching anything AC, even if it's not on the same electrical circuit. I was doing it with one of those AC-powered breadboarding kits, but I also did it with one of those under-cabinet fluorescent lights and, most importantly, a 120VAC relay which has to be switched while the encoder is monitored (thus the problem). Having bought some Red Lion RC Snubbers (SNUB0000) along with the EMI filters, I tried connecting them across the switches in the relay in question. It's a double pole relay so I added snubbers between each common and the three of the four switching contacts that are connected to something. This made no difference on the spike in my test circuit generated by my switching the relay. Someone suggested using zero-crossing triac switches, but clearly if every AC device causes the problem then my only choice is somehow impeding the effect from going into the DC outputs. For some background: I have never solved a noise problem in my life. I've seen people wrap things in foil and do other such crazy things, but I've never met anyone that could explain the sources of noise and how you filter it. People seem to always wave their hands about but never have I been able to find some practical guidelines on what to try and why. Besides this one I have three other serious noise problems that I don't want to discuss yet so as to not dilute this. So, that being said, 1. Where is this noise coming from? How can it go "through" the power supplies (both switching and linear) and end up on the DC side? Does this necessarily mean it's going through the air? 2. How do I fix it? 3. Can anyone recommend a good "field" book that is not necessarily full of complex integrals but isn't just a flow chart with part numbers either?