I need help with an electrical Relay

[digitalmonkeyman]

I'm putting together a "one Button Power Strip" and have an electrical question.  I have the Build Your Own Arcade Book from another member here, and he recommends this;
http://www.radioshack.com/product/index.jsp?productId=2049722&cp

Not being an electrical guy, I was wondering if I could safely use this;
https://www.alliedelec.com/Search/ProductDetail.asp?DESC=W78RCSX%2D7&tab=specs&MPN=W78RCSX%2D7&sid=459309002DFE17F&R=850%2D0410&SKU=850%2D0410

more info on the alternative;
https://www.relayspec.com/specs/026940/78.pdf

Any help someone can provide would be greatly appreciated!

Thanks

[BobA]

Looks like a good alternative.     Basically the same relay.

[crashwg]

I have not read the book yet so I'm not familiar with the circuit but I'm assuming that you're supposed to get the 12v from the PC power supply right?

I'd suggest getting a 5v relay and using a USB cable for the signal power.  Personally, I think it just looks cleaner especially if you have the PC in a case.

[whammoed]

I have a product you may be interested in:
http://www.nicemite.com/PowermiteDD/PowermiteDD.htm

[digitalmonkeyman]

Here is a pic of what I am trying to accomplish.  When you plug something into the first plug and it goes on (computer) the down line pugs get power, monitor, speakers. marquee, etc.
Thats what I'm going for, and I was hoping to not burn down the house!
 
I was hoping a technical electrical guy could assure me these are the same relay...
Thanks

[extremepong]

I'm not an expert - just a hobbyist. But here is my thinking.
I think I get it. I had thought that you merely wanted to swap out the switch on the power strip with a relay that would get its  low voltage power from another source like your computer. But you are presenting a slightly different problem in that the computer must be plugged into an always- on source not controlled by any relay and it looks like you want that to be on the strip (which is do-able if you rewire for  that ).   Anyway, it  looks like your relay can handle 10 amps or more at 120 volts. I think it looks okay if you are drawing fewer amps than the rating but then we don't really know what you are going to draw do we? So you could consider a fuse that would blow before the relay  coils burn.  But if you don't plug in any waffle irons or coffee pots I would guess you would be okay.  You might feel better if you measure with a VOM/ammeter  the current being drawn by all your loads so you could know that you are within bounds.  You could wire it up and let run for a few hours open - then check it closed too and you would know if it getting hot.
But couldn't the whole  problem be flipped . Suppose you were to turn on your "system" by pushing the red power strip button. All the plugs would be energized. But on one plug you have placed a small relay that  *momentarily*  closes the circuit to your computer power switch - which has a safe low voltage. That way the relay that you fear burning out will never be carrying a heavy load since it's only job is to briefly close the circuit that switches on your computer's power supply.  You won't need to lose your computer's  switch - just bridge it so you have two switching methods.  Lastly, you could move that red switch to wherever you want it on your cab. So, now,  if there are any experts out there. Would that imperil the computer in any way?

[BobA]

Looks like a good alternative.     Basically the same relay.

That's what I said the RS and the alternative are relays with the same specs.

[Druxar]

THe relay coil wouldnt burn out if the load exceeded the 10amp rating, that is for the switch contacts. The coil is just going to see the 12volt energizing source always.

I didn't look TOO deep into the specs but another consideration, this 10 amp rating, is it PEAK or continuous RMS? If it's 10amps peak, then your load average is really going to be less than 10 amps. I would assume it's average current rating and I think overall it is sufficient to run a few downstream things in a cab.  Keep in mind a typical house circiut is rated 15amps and you can run a lot on that.  A cab with this relay isn't even going to power the whole cab, the computer isn't powered by the relay (if I understand). 

But even if we have issues with 10amp contacts not being hefty enough, there is always the option to run TWO relays if the 12 volt coil power source can handle enough current to energize two relays (should be no problem).  Check the coil current to see how much it requires and find out how much current the 12volt supply can drive and how much of that is availble to drive the relay.  Then connect the two relay coils basically in parallel so they both come on at the same time when 12 is applied, then take the AC source being switched and branch that to each relay so you get to drive two 10 amp circuit branches.  Starts to sound messy without drawing it out but you're taking your incoming main AC and connecting to the first relay wherever it was originally planned for switching it,   and then duplicate that on a second relay so you're tapping into the same source AC and feeding the two branches equally set up.

Of course don't forget basics of driving the relay coil, use a rectifier diode (1n4001 thru 1n4007) connected across the coil with the diode's anode to the ground and cathode (stripe side) to the +12 side of the coil to help suppress the inductive voltage switching spikes.  DOn't connect the diode backwards with cathode to ground or the diode will "come on" and short the 12volt supply.

[extremepong]

Ahh - yes the contacts not the coil would burn. I wouldn't have thought of the diode either. Would it make any sense to solder an automotive type fuse (of say,  8 amps)  to protect the contacts and provide more peace of mind?

[AwesomeAlbert]

has anyone tried a smartstrip or Mini Power Minder? I was thinking of doing the relay thing but i read about  the mini power minder around here, for around 15 bucks i think that is a much easier route to take for me.

[Druxar]

I would probably add a fuse somewhere along the AC path if I were building this setup, even if there is a general fault in the wiring where something eventually falls on it and shorts it, it's good to fuse it in addition to protecting the relay rating.   Whatever type of fuse is used, it has to be rated for the 120volt AC (or 250) along with the proper current handling. I am not too familiar with automotive fuses, being a 12volt system, are those fuses rated for household AC voltages?  If not, have to maybe buy an inline fuse holder (anywhere, radio shack even) that can accept a "normal" AC fuse and has wires you would solder in line with the AC hot side.

So with that application of using the fuse, I'd wire it physically close to the source AC lines before they go to feed the relay contacts rather than fusing the downstream load network after the relay contacts switch the AC.  That way if there is a short or overload on the main AC source wiring, the fuse will do a better job in protecting both the relay/load network as well as any general AC wiring faults that show up on the path towards the relay circuit.  If you put the fuse somewhere after the relay and before the load, and somehow a short develops on the AC source before it gets to the relay, there is no fuse there to protect the short (the relay and its load and the fuse would be all shorted out of the equation).


As for what fuse current rating, I think this application is light enough that just guessing on a value like 8A would be fine, because we dont expect the load to come close to the relay rating and having a fuse with more than the load but less than the contacts would allow the load to have extra start up current spikes without nuissance fuse blowing and still stay within the contact ratings.  So as close to 10A as possible I'd use.

[digitalmonkeyman]

Thanks a million!!!