# Are MOVs and Inductors good enough for power supply protection?

I want to know if MOVs (Metal Oxide Varistors) and Inductors are good enough for power supply protection from surge.

I'm trying to design a power supply but our power distribution network is plagued by frequent power outages, surges, harmonics, which claims the life of many power supplies.

What protective components will provide best protection?

I would also like to know if an SMPS (Switched-Mode Power Supply) will be better than a standard transformer based power supply in such an area as this.

Thanks

Location: Lagos, Nigeria.

Nothing works perfectly.
Some things work better than others.

If you are serious you want a UPS or equivalent.

A transformer supply is less likely to die than a SMPS

If you want load "hold up" you want energy storage. This is usually in capacitors in a SMPS. Storing energy in capacitors for more than a brief blip requires vast capacitance - not realistically done.

Using an isolation transformer as a surge and spike suppressor has some merit.

Spike removal can use MOVs, gas discharge tubes, zeners and similar. Gas discharge tubes are the "gold standard".

Batteries are used to get holdup from tenths of a second through minutes + with a smps.

If budget is low a pretending "Ward Leonard machine" is a possibility. WL is an AC motor driving a DC generator but you could drive an AC alternator with an AC motor. Hold up time depends on flywheel size.

You MAY be able to get an OK result with an unloaded induction motor running on the circuit storing energy mechanically and acting as an energy store. . More information is require in on power level required

may prove useful - he is using a petrol motor to power an alternator which is based on an induction motor but the principle is the same.

Mechanical storage in a flywheel or similar has the advantage of possibly being able to use surplus parts. How well that applies in Lagos I don't know.

• how about using thermistors? – TiOLUWA Feb 1 '12 at 16:42
• @TiOLUWA Negative Temperature Coefficient (NTC) thermistors are useful as simple inrush current limiter (another common / basic solution is to use a fixed low-$\Omega$ resistor that is switched in / out via relay with a RC delay) Ref: circuits on Pg 2 & 3 of EPOCS App Note – mctylr Jun 19 '12 at 8:25

You must define what kind of surge and how strong surge you want to be protected from. If a lightning strikes directly to your house, then any kind of varistors and/or inductors are usually pretty useless. But this kind of disaster is very unlikely. More likely it hits nearby, not directly at your house, and then these little helpers can save your devices, because the surge spread hundred of meters around but is less and less strong. Even more likely there will be no real lightning, but just reglar smaller overvoltage events. Those can be also "solved" by varistors quite well.

You can visit for example www.dehn.de - a famous german company specializing in protection equipment. As you can see there, there are many much more sophisticated solutions you can buy, but it is often quite expensive. It depends on the value of electric equipment you want to protect. If you have for example 300 computers in one small building then it is wise to invest into surge protection more money. But if you have just one computer at home, it is much cheaper to let it burn and buy a new one whan this disaster happens.

So a short answer is: MOVs cannot protect from all possible surges, but they are so cheap you should always use them when designing a power supply. Their benefit is much higher than the cost.

Update: UPS is a necessity in the case of problematic electric lines as described in the question.

• This answer would be good for an IT or an advanced home computer user, but I don't think it does a great job with helping with someone who is designing the electronics to provide protection. – Kellenjb Feb 1 '12 at 16:10
• I am guessing you added that last paragraph... at least I swear it wasn't there when I first read your answer. Even with that paragraph, I don't know if your answer would be much help to the poster, for example, you haven't answered his question of if SMPS would be better protected or not. And saying that you should just go ahead and use MOVs doesn't answer the question of if they are "good enough" or if there is something better to use. – Kellenjb Feb 1 '12 at 16:17
• You are right. But he asked too many questions together (a "tell me everything" question), and I can only help with those parts what I am really familiar with. Btw. I think UPS is better in those "bad electric lines" cases than a special power supplies. – Al Kepp Feb 1 '12 at 16:21
• But I don't think he's going to put an UPS in a DIY power supply... – clabacchio Feb 1 '12 at 16:24
• Really the power supply is for an electronic device, specifically an energy meter, we get alot of them burnt up so i would like to know what design will be most tolerant. – TiOLUWA Feb 1 '12 at 16:31