SPDT Relay coil wattage?

[XERTYX]

Sometimes I get ideas for circuits. I had an idea earlier for an ... almost automatic transfer switch. 

Actually it would be to turn on a DC circuit when AC isn't available. The easiest way I can think of is to to energize an SPDT relay with AC to close the NO pole. When AC is removed the relay would return to NC and switch on the DC circuit.

I'm not worried about stuck relays. And the how many watts question has a LOT of factors. I know this.

But from exercising my GoogleFu about the best answer I can find is to calculate the resistance and impedance and multiply them by the molecular weight of the sun and divide by your body weight or some such ridiculousness. 

The most common answer is to multiply the AC voltage by the amperage. But that gives the wattage of the CONTACTS. Not the electromagnetic core.

I want a ballpark. A couple of watts or 120?! The DC circuit being switched on would be under 1 amp 12v DC so the cure cant be worse than the disease.

 

[bullfrog]

I guess I am wondering why you would be concerned about the 120 Volt coil power usage as it would be minimal I would think and normally coming from the grid or generator. They sell rail mounted switching relays that have plastic bodies so heat build up from power usage would be small. I seem to remember a discussion about turning on an inverter with a converted 12 volt DC temp sensor just before a 120 volt AC refrigerator compressor was needed in an attempt to keep from having the inverter waste battery power when it wasn't needed while using a cheap 120 volt AC refrigerator in a van with solar and an inverter. I believe there was a lock up circuit so the compressor could keep running until it had lowered the temp in the refrigerator. All this to say I would assume there is a relay that will work for you as the 120 volt source would be generator or most likely grid power instead of a battery powered inverter. I guess I would simply wire up the relay and check it as there is a lot of variance in electrical components with very small loads. You might find the old thread useful if you search it out.

 

[jasper]

[font=Tahoma, Verdana, Arial, sans-serif]I have done that for four years in a small camper trailer.  As bullfrog mentioned, I am using a 120vac dorm fridge with the thermostat bypassed to run continuously when connected to 120vac.[/font]

[font=Tahoma, Verdana, Arial, sans-serif]The fridge's now isolated thermostat interrupts 12vdc applied to a 12vdc relay's coil.  That relay's normally open contacts close with the thermostat and turns on a 750 watt inverter that then feeds 120vac to the fridge. The inverter turns off when the fridge cools down and the thermostat opens. 12vdc relay contacts wired across the inverter's on/off switch.[/font]

[font=Tahoma, Verdana, Arial, sans-serif]A relay with a 120vac coil is used to provide automatic transfer as you outlined. I am using an "ice cube" relay for automatic transfer.  As you mentioned, if the contacts weld, AC only.  If the coil fails, no transfer.  It's energizing current is less than 100 milliamps (.1 amp).  As bullfrog also mentioned, no concern with most 120vac sources.[/font]

[font=Tahoma, Verdana, Arial, sans-serif]The first 120vac transfer was a simple knife-blade type underneath a seat.  I got tired of using it. Good luck.[/font]

 

[tx2sturgis]

Most general purpose 120v relays will have a coil current below 50-100 milliamps or in that range. 

Generally speaking, a continuous duty relay coil will not pull a lot of amperage/wattage, otherwise it would get too hot and burn up.

There are exceptions, such as high power industrial intermittent duty contactors...which are like big relays on steroids...but you wont be using those anyway.

 

[Weight]

Every relay manufacturer I researched has a specification list with that information for each of their offerings.

 

[jonyjoe303]

If you only plan to switch 1 amp at 12 volts DC, you can use a 30 amp DC relay or even one of the smaller 10 amps relays. I built a dc to dc transfer switch with a 30 amp automotive relay to switch between 2 batterybanks. This dc to dc transfer switch can also be used as a UPS. With the automotive relay, the transfer is almost instantaneously, the only sign I get is the click of the relay and the lights either dimming or getting brighter as it switches between different voltage power banks.

I got the idea from something I saw on ebay "Backup power transfer switch Repeater Battery Backup DC Switching System PWRgate" , what you mention is practically the same as this backup power transfer, except this is all powered by dc. Its a very simple design that works.

I only tested these 2 relays to see their power use in amps when energized.

 

[highdesertranger]

jony tear the 200amp one open I want to see the insides. Highdesertranger

 

[bullfrog]

So now I have a question, could that 200 amp relay be used as a less power consuming isolation relay for a house battery charging circuit?