Simple Load Transfer Switch

[keyboardcowboy]

Is it possible to create a Load transfer switch using just a pair of relays,the relay should switch to secondary power when primary is turned off and should switch back to primary when its turned on.

[Krankshaft]

Re: Simple Load Transfer Switch

You would need a single pole double throw relay.

Here is an image displaying what a SPDT relay looks like electronically:



You also will need a small DC transformer to power the relay coil. Also be sure to place a IN4009 (or equivalent) diode across the coil to prevent it from dying.

When primary power is applied to the relay (through the transformer) it will connect the primary power to the device (through 87) when the primary power is removed the relay contacts will return by spring tension to the secondary power contacts (87A).

You could get alot more advanced with a bit of circuitry but this is all I can think of since you want to keep parts to a minimum.

I'm assuming you intend to switch line current so that when power goes out the device is connected to a battery backup or something similar?

If your circuit already has low voltage DC that isn't higher than the relays max rated coil voltage eliminate the transformer and just power the coil directly off the primary power but don't forget the diode.

[keyboardcowboy]

Re: Simple Load Transfer Switch

can these type of relays be used to control a 220v line since i would be using it with mains or would i need a bigger relay.

[gonzo0815]

Re: Simple Load Transfer Switch

IMHO this is a typical low voltage relay, which is is not suited for 230v AC
If you want some detailed answers, you have to share your intended application.
there are to much things you wich can go wrong with switching between independent AC supplies.

[keyboardcowboy]

Re: Simple Load Transfer Switch

iam just trying to create a switch that would switch over to the inverter when the mains go off.so it requires to switch between 220V source

[arneson]

Re: Simple Load Transfer Switch

When a relay is used for AC load switching I call them Contactors instead of relays.
Like the one that starts your car is called a Solenoid, similare but different application.
Look inside an airconditioner and youl see a contactor, in fact most large motorized appliances turn on and off with a contactor. A large double pole, double throw, heavily insulated relay.
http://en.wikipedia.org/wiki/Contactor
I install these inverters and that feature is built in with the 2500 models.
http://www.inverterdoctor.com/news.htm

[Krankshaft]

Re: Simple Load Transfer Switch

Improper isolation was a worry of mine when I wrote that.

That was a low voltage relay but it was just an example of a SPDT relay and not an actual picture of the one needed.

Response time is also an issue if you will be powering computer equipment all mechanical relays have a lag (in milliseconds) that will equal an interuption in power to your equipment not to mention the inverter delay before it begins to work.

It sounds like what you want is a homemade UPS?

[keyboardcowboy]

Re: Simple Load Transfer Switch

"It sounds like what you want is a homemade UPS?"

well sort off,timing is not an issue,issue is the auto switching