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Mains Switches - How do they work??

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cljunior

New Member
Hey everyone

I've been looking into mains switches for a project and i can't seem to find any circuits or data on how they actually work? I've been told that, for example, a normal light switch, that its not just an open/close switch, because with such high voltage, its just going to leap over the open connection? Not sure if this is correct, but i can't seem to find any info as to how they actually work. Could anyone help me out?

I also need to find a switch to use in a design project, which needs to be triggered by a signal received from an IR receiver. What kind of switch would i need to use in this case?

Thanx in advance
Colin

PS. I accidentally posted this in the wrong section before, feel free to delete the other one.
 

Phasor

Member
I've been told that, for example, a normal light switch, that its not just an open/close switch, because with such high voltage, its just going to leap over the open connection?
Not true. It is a simple open/close. However, they are designed such that there is enough clearance between the contacts, that it should not arc.
 

mxitman

New Member
Hi, Something I actually know a bit about. A mains switch is just an open/close switch that has usually a small gap across it, and it will produce an arc... thats why they tell you not to turn a light switch on if you smell gas in your house! Overtime this arcing will produce pitting on the contacts in switches and could lead to them becoming welded together. :idea: Depending on your application.. How many amps you plan to switch and how many poles, you can use anything from a pilot relay to a contactor or even mag-starter. That way you can use anything from a small dc volt or ac volt to controll something else, i.e. Air compressor from a wireless transmitter, hope that helps.

Mxitman :wink:
 

mxitman

New Member
A relay uses a magnetic field to pull/push contacts together, which are seperated from the mains supply. I deal mostly with 24vac relays and contactors, the seperate poles are typically rated from 5-50A and 600v max. You would simply supply the relay(coil) voltage when you want to switch the mains supply on. What are you looking for?

Mxitman
 

cljunior

New Member
We have to convert a signal which we get from a receiver, which is a voltage pulse, and this drives a toggle switch, which switches our device on/off. sort of like pressing the on/off button on your remote for your hifi or vcr. I was wondering how i would have to implement this, i assume a relay is necessary?
 

stevez

Active Member
In very heavy duty (high voltage, high current) applications the resulting arc as the contacts open is a serious issue and many solutions to the problem have been developed over the years. While probably not an issue (beyond what's already been said here) within the scope of your immediate application your intuition or reasoning that the resulting arc could be a problem is right on target.
 

Phasor

Member
In very heavy duty (high voltage, high current) applications the resulting arc as the contacts open is a serious issue and many solutions to the problem have been developed over the years.
This is particularly a problem in high voltage circuit breakers (11kV and higher). It is desirable to have a method of arc extinction, and probably the most common method is by an oil-chute - In an oil-circuit-breaker (OCB), there is a partial vacuum created when the contacts open. This vacuum draws in oil from a reservoir, and the oil extinguishes the arc as it passes between the contacts.

As stevez says, there are many other solutions - I have only explained one.
 

daviddoria

New Member
there are also SCR (sillicon controlled relays) that i learned about when making my coil gun :) i dont really know much about them ,but i was told they are great for switching high voltages.

good luck
david
 

cljunior

New Member
So could anyone suggest a way in which to implement a switch, which is toggled via a voltage pulse.

I have come up with a vague idea of using a D flip flop and a BJT arrangement, but this is still vague at best.

Regards
Colin
 

Phasor

Member
The flip/flop-BJT arrangement is fine - just use your BJT to drive a relay. This removes any "vagueness" :idea:
 

bmcculla

New Member
Hello,

If you're just switching mains voltage at <15A you should look into a SCR (Silicon Controller Rectifier) or Triac. The SCR is like a diode with an on switch. When you put Current into the SCR's gate it switches on and stays on until current stops flowing i.e. when the AC cycle ends and current reverses. A triac just two SCR's (one for current flowing each way) Fabicated into the same package (there are some differences in the polarity of pulses to turn on a triac vs a SCR but they shouldn't be a problem if you use a driver chip). There are Opto-isolated Triac drivers you can buy at Digikey.com that should make turning on your triac a snap.

-Brent
 

cljunior

New Member
Thanx, that's a good idea. But we have cost constraints and a triac would seem to be a lot more expensive in this case.
 

Phasor

Member
Would this kind of arrangement be reliable enough?
I don't see why not, so long as you don't throw it around the room :p

And how about current drain?
That depends on the relay you choose. Most small relays have a current consumption of around 50-200mA for the coil, though you may be able to get a smaller one, if it suits you better.
 
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