Electrical – protection device on power line and io lines


I have seen many design to protect nput circuitry from surge , EFT and ESD
Mov's for surge protection, Choke for Conducted emission TVS for ESD. still i am confused on below point

Q1) If surge is applied to both input power line and all I/O lines then why most of design only had mov on Power lines but not on IO lines.

Q2) TVS diode on IO line will protect from ESD or EFT or both.

Q3) Mov is for surge protection but what device protect circuit from EFT on power lines.

Q4) what protection device used for power lines to protect from ESD.

Best Answer

Your being slightly confused by the choices made for a given application, and your questions overlap a bit, but I will try to clear them up. To simplify descriptions, surge suppression and ESD and EFT devices are considered the same. This does not include high-impedance grounds for 3-phase Delta power or basic wrist-strap ESD grounds.

1) MOV's are best for power line surge suppression, both line to line and line to ground. Large and small MOV's have a high capacitive value, so they cannot be used on data lines as they would choke off the signal. Because of the low frequency ac power (50/60/400HZ), inductors and capacitors are sometimes combined with MOV's to provide superior noise and surge filtering.

2) TVS diodes have much less surge absorption in terms of peak current, but have much less capacitance and are often used on low to medium speed data lines. Any inductive filters are seldom more than a few uH. If capacitors are used, they are less than 1nF, and normally used for DC blocking and as a basic HPF.

3) Gas tubes and tiny smd type TVS surge suppressors have only a few pF of capacitance and are used to protect high speed data lines and cable TV, so they may have to 'work' at frequencies up to 2GHZ. Any inductive filters are seldom more than a few nH at most. If capacitors are used, they are just a few pF to 1nF at most and normally used for DC blocking and as a basic HPF.

NOTES: For most defined data links such as USB/RS-485/RS-232 and Ethernet only the surge suppression devices are 'add-ons' to a basic design at the PCB level. Depending on the manufactures quality and durability requirements surge suppression and noise filtering maybe built into the product, or not included at all, forcing the end user to decide if such devices are needed. However, surge suppression for power lines are fairly common in many countries, based on my last employers sales of TVSS units to many countries, even those with poor quality power grids.

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