magnethead

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?

Done_Fishin

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.

magnethead

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?

magnethead

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.

Done_Fishin

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

magnethead

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

Done_Fishin

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 ..

magnethead

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?

Done_Fishin

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

magnethead

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?

Done_Fishin

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/DkSea...v%20dc%20relay

magnethead

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.

Done_Fishin

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

magnethead

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....

Done_Fishin

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?

magnethead

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!

Done_Fishin

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

wrench97

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

http://www.grainger.com/Grainger/wwg...exId=1&typeOf=

Done_Fishin

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

wrench97

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