# relay AC using DC?



## magnethead (Mar 18, 2006)

Ok, this is going to sound whacky, but my roomates and I are, quite whacky. 

I have 3 strobes for a car plus a 10 foot rope light. 

I'm going to power them with a converted ATX/bench power supply. And we want them to power on when the dorm door opens...so i got a set of magnetic switches, and put the sensor between the green and ground. 

Everything works as we will want it too. 

But now..new deal. I have a blacklight, ball of light, and he has a light display, that we want to power on when door opens as well. 

Can I put a relay (what type/rating?) across the green wire and use it to turn on an AC line ?

I know ground and neutral carry over, I put the relay across the black wires. But is it possible to conduct the black/hot AC wires using the green PS_on DC wire?


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## Done_Fishin (Oct 10, 2006)

1st question .. what voltage is on the green .. since you will need a DCrelay that will operate at that voltage 

2nd question what is the power rating or what current will need to go through the relay contacts .. make sure that the relay contact rating is higher than your requirements.

make sure that the relay and all wiring is thoroughly isolated in a plastic box and that teh wiring from the relay coil CANNOT come in contact with the wiring from the relay contacts

usually relays will have a coil rating and a contact rating
the contact rating may be something like 12Vdc 1A or 230Vac 5A, possibly 115Vac 10A,

It's normal for the DC rating to be much lower in amperes than the AC rating 

the dc coil will require a diode in reverse polarity to the dc voltage across the coil if you are going to use transistor to turn it on.
a magnetic switch driving the relay will most likely burn out quite quickly if you don't use a transistor to drive the relay. Use the magnetic switch to bias the base of a transistor to ON when it closes the circuit.


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## magnethead (Mar 18, 2006)

PS_ON is 5 volts. 

I know to use a plastic box and heatshrink/insulate cabling

I know DC arcs a switch when AC doesnt, hence lower DC rating

Here's what i want to do

The magnetic switch closes the circuit when the magnet is near it (taken from a security system window install) which turns the PSU on, hence lighting the 10 foot rope light and 3 strobes.

But im wanting to put a single duplex on the line (18 AWG feed wire) that we can plug the ball of light and whatever else we want to into it to activate when PSU does.

Should a 5 amp glass inline fuse be put on the neutral wire or the hot wire? If hot, which side of the relay?

edit: suppose the relay could also be put on the 12 volt rail with the lights/strobes to make selection easier?


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## magnethead (Mar 18, 2006)

Here's on 12V switched line..










FYI the PSU is rated 8 amps @ 12VDC..and it's 10 years old. 

It's working on the bench.


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## Done_Fishin (Oct 10, 2006)

Fuses ALWAYS on LIVE wire, ALWAYS as close to SOURCE as possible (we Brits have them in our power plugs regardless of whether there is also one inside the unit being powered)

5A @ 120Vac = 600W 

there are two types of fuses 

Fast & Time delay

fast means that an initial surge on the fuse might blow the fuse so you have to use a fuse than can cope with initial surge due to the sudden flow of current. These fuses have either an F or NO letter in front of the fuse rating. ie F1A or 1A 

Time delay fuses have a retarding mechanism in the form of a coil like winding, perhaps a ball of solder or even a zigzag effect along the conductor. All designed to allow that initial current surge to pass without blowing.
They are usually marked with a T to indicate that the blow slower than FAST fuses.
ie T1A.
I remember many years ago playing with some units in production and trying different types of fuse on the input to a transformer. I found that, in THAT PARTICULAR UNIT, a T1A was the minimum value "slow blow fuse" whilst a F4A was the smallest value "fast blow fuse" that I could use on the same unit and have it running at the point that I threw the mains switch.

Also note that fuses DO NOT blow at rated value .. normally you can expect the fuse to blow at approx one and a half times the rated value. However the fuse is also liable to fail due to aging if the current flowing through it is approximately the rated value. This may happen though after a few weeks or even a few years


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## magnethead (Mar 18, 2006)

so look for a 5 amp 120VAC fast blow fuse and a relay with a 12 VAC coil and 5 Amp 120VAC terminals?


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## Done_Fishin (Oct 10, 2006)

Give it a try like that .. you may find that if you look at the rated watts of the device then divide by 120 to get the current, a smaller T fuse will suffice. If the watts show 100W then you'll be drawing ~1A so a T1,5A or T2A may suffice .. 5A @ 120V is ~600W and that's quite a load ..


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## magnethead (Mar 18, 2006)

Done_Fishin said:


> Give it a try like that .. you may find that if you look at the rated watts of the device then divide by 120 to get the current, a smaller T fuse will suffice. If the watts show 100W then you'll be drawing ~1A so a T1,5A or T2A may suffice .. 5A @ 120V is ~600W and that's quite a load ..


Up to but not exceeding 5 amps. I figure an incandescent lamp is half an amp, and i cant see lighting devices taking more than 3 or 4 lamps. So maybe even a 2A @ 250V standard glass fuse. 

What do i need to look for on a relay? Are they listed by coil or switch rating? I'm seeing 12VDC 20 amp relays and 120VAC 5 amp relays...which am i looking for?


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## Done_Fishin (Oct 10, 2006)

relays you will find are rated with both coil rating and contact rating 

look for your coil rating first then the contact ratings to select what is suitable


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## magnethead (Mar 18, 2006)

Done_Fishin said:


> relays you will find are rated with both coil rating and contact rating
> 
> look for your coil rating first then the contact ratings to select what is suitable


I just checked radioshack, none of them are clearly marked. closest i could find was a PCB/solder relay that said 12V coil but didnt have a contact rating. another said 135VAC coil rating/250VAC contact rating.

But you cant run a DC current through an AC coil, right?


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## Done_Fishin (Oct 10, 2006)

not usually .. also you have to make sure that you get the coil polarity correct .. some require correct polarity 

look here at DigiKey for examples

http://www.digikey.com/scripts/DkSearch/dksus.dll?Cat=1049447&keywords=12v dc relay


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## magnethead (Mar 18, 2006)

okay. I was hoping i could get in store, since our goal was to have this going by tomorrow when we move in to the dorms for spring. oh well.


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## Done_Fishin (Oct 10, 2006)

That was only an indication .. go to your local store or phone them and tell them you want a 12V relay with 250Vac 5A (or similar) contacts. Are them what they have and see what is most suitable for your needs


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## magnethead (Mar 18, 2006)

Done_Fishin said:


> That was only an indication .. go to your local store or phone them and tell them you want a 12V relay with 250Vac 5A (or similar) contacts. Are them what they have and see what is most suitable for your needs


the radioshack at home, only has 1 expert..3 days in a row i missed him. 

The radioshack down here by school....I can't describe their inteligence. They didn't know what F or BNC connectors were....


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## Done_Fishin (Oct 10, 2006)

you have no other stores to purchase from ?? 

If they have no idea about the stuff they are selling and you have no idea about what you want to buy .. how on earth are you doing all this stuff you have done with the robot? :laugh:


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## magnethead (Mar 18, 2006)

Done_Fishin said:


> you have no other stores to purchase from ??
> 
> If they have no idea about the stuff they are selling and you have no idea about what you want to buy .. how on earth are you doing all this stuff you have done with the robot? :laugh:


LOL

because the first robot was a home project. The second one, which will be getting crated between home and school, i am somewhat stumped on, still not sure what the electrical is going to look like on it, being a 12 volt system.

I have radioshack, fry's, and best buy......and im in the city!


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## Done_Fishin (Oct 10, 2006)

well good luck with that ..

I suggest you head for the shops in order of your own preference, ask what 12V relays they have in stock, and then look at the covers to see what contact ratings are available .. they normally show coil rating and contact rating on the outside .. tell teh assistant what you are looking for and pray that he is someone with an interest in electronics rather than just an interest in his paycheck at the end of each month .. the latter tend to know nothing and are oblivious to the damage that they can cause the end user :grin:


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## Wrench97 (May 10, 2008)

Hey Mag try www.Grainger.com you will probably find a store near you.

http://www.grainger.com/Grainger/ww...=Relays,+Socket-Mount&searchIndexId=1&typeOf=


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## Done_Fishin (Oct 10, 2006)

at first glance this one looks like it will do you 

http://www.grainger.com/Grainger/items/2W937

Contact Form SPDT 
Pins 5 
Contact Amp Rating (Resistive) 5 
Contact Amp Rating (Inductive) 5 
HP @ 120V 1/10 
Coil Volts 12VDC


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## Wrench97 (May 10, 2008)

I have a feeling that is a solder on pcb relay though.


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## Done_Fishin (Oct 10, 2006)

he can always solder the wires direct to the correct pins or use a bit of prototype board to mount it, solder the wires then mount in a small plastic box for safety. But apart from that it will give hima an idea of what is available and what he should be looking for.

Most relays require either an electrical cabinet to mount them in, a base fitting (increases costs) that in turn require mounting in or on something. The worst part is usually making sure that soldered wires don't yank off the pins that they are soldered to. One way of getting around that is to solder short flexible wires to the pins then extend them using those plastic blocks which have two screws in them for input output. Sorry so long since i used teh stuff I no longer remember the name. poke wire in one side screw down, then poke wire in other side and screw down .. then you have extended your wire


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## magnethead (Mar 18, 2006)

Done_Fishin said:


> he can always solder the wires direct to the correct pins or use a bit of prototype board to mount it, solder the wires then mount in a small plastic box for safety. But apart from that it will give hima an idea of what is available and what he should be looking for.
> 
> Most relays require either an electrical cabinet to mount them in, a base fitting (increases costs) that in turn require mounting in or on something. The worst part is usually making sure that soldered wires don't yank off the pins that they are soldered to. One way of getting around that is to solder short flexible wires to the pins then extend them using those plastic blocks which have two screws in them for input output. Sorry so long since i used teh stuff I no longer remember the name. poke wire in one side screw down, then poke wire in other side and screw down .. then you have extended your wire


barrier strips- i use them extensively.

I dun forgot bout granger. I bet there is a nearby store.


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## magnethead (Mar 18, 2006)

http://www.grainger.com/Grainger/items/6C873

yea i know way overpriced.

http://www.radioshack.com/product/index.jsp?productId=2049722

50 cents cheaper..am i seeing double?

I may have to go see if the local radioshack has that one and compare the numbers. Otherwise, I'll have to peck the minds of the grainger employees...not a bad idea anyways.

grainger is about 8 miles away, on the other side of the new cowboys stadium from here.


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## Done_Fishin (Oct 10, 2006)

I make it $1 cheaper .. $9.46 - $8.49 .. bar a few cents.

of course that is without taking into consideration time & travel costs 

however they look very similar and eitrher would probably do you

(I wasn't aware that Radio Shack was an economical store :grin: must be a first~!)


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## magnethead (Mar 18, 2006)

Done_Fishin said:


> I make it $1 cheaper .. $9.46 - $8.49 .. bar a few cents.
> 
> of course that is without taking into consideration time & travel costs
> 
> ...


radioshack has one that says 12VDC and 120VAC. but if the clear poly has writing on it, i'l have to look closer and read.


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## Done_Fishin (Oct 10, 2006)

Granger specs


> Square Base General Purpose Relay, Number of Pins 8, Contact Form DPDT, Contact Current Rating Resistive 10 Amps, Contact Current Rating Inductive 7.5 Amps, Coil Voltage Rating 12 VDC, Coil Current Rating 75 Milliamps, HP Rating @ 120VAC 1/2 HP


 Contact Current Rating Resistive 10 Amps, 
this is most likely a DC rating 

Contact Current Rating Inductive 7.5 Amps

this is most likely an AC rating 

Radio Shack



> Features
> 
> * Contacts and Ratings: DPDT 10A at 125VDC
> 
> ...


As to be expected Radio Shack aren't telling you enough .. although with a double throw you could parallel up the contacts, if you are uncertain 
looks like a heavy duty contact anyway from the specs


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## magnethead (Mar 18, 2006)

Done_Fishin said:


> Granger specs
> 
> 
> Contact Current Rating Resistive 10 Amps,
> ...


thats why im not sure. But it looks like the clear piece has the specs lasered on it. I'll have to read it to choose.


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