I would like to make a good surge protector for a device that will have a hardwired mains connection. The protected electronics will consist of a ~10W SMPS and a low-power AC load (<100W) controlled with a triac.
I plan to use a mains filter (nothing fancy) behind some combination of GDTs (Gas Discharge Tubes), MOVs (Metal-Oxide Varistors) and/or TVSs (Transient Voltage Suppressors). This is meant to work at 110V (US) but the SMPS is universal so it would be nice if it worked with 230V as well (mostly because this is the standard voltage in the place where I live).
I understand that I want the faster device (MOV, TVS) to have higher voltage rating than the GDT so both have a chance to work when really needed. Unfortunately both GDTs and MOVs have rather lousily specified I-V characteristics and their "breakdown voltage" is more of a "breakdown region".
A GDT with typical 230V breakdown won't work with 230V AC since it may start conducting at as low as 180V. If OTOH I go with a 350V typical (280V minimal) my protection is guaranteed only at 420V since this is the maximum ignition voltage, right? This leaves little room for the choice of the MOV breakdown voltage (which is a voltage range as well) and still places heavy burden on the downstream elements. Is this a lost case and the only solution is to have all the downstream components rated >900 V?
I welcome any articles, suggestions or rules of thumb for designing such (simple) surge protectors.
Best Answer
A MOV is an essential part of your mains-side safety circuitry (despite what user886922 says). 300V MOVs are commonly used to protect universal input (85-264VAC) power supplies. The MOV is sized such that the energy it can crowbar safely is more than enough to blow the mains fuse, which will provide the necessary disconnect. It should be located 'inside' any EMI filter components as surges tend to be amplified by EMI inductors (think boost converters).
EMI filter capacitors (X- and Y- rated parts) have dielectric strength far beyond their rated voltage, so there's little concern there. Sometimes I see GDTs in parallel with the Y-caps (line-to-earth, neutral-to-earth) but often they're omitted because of the toughness of the safety capacitors.
I wouldn't be surprised if your existing 10W SMPS doesn't already have some combination of GDTs and MOVs.