trying to build a stamp circuit that will control power switches without starti

Taylortronic

hello, parallaxers, i have a pretty specific issue here.· i'm working on a basic stamp project and i need it to switch a few power relays to control a couple appliances.· i am definitely planning to err on the side of caution and i want to use relays that can handle 10 amps @ 120 volts AC.
very simply, i need to set up a subprogram that will switch each of these whenever called.· i guess i'm just nervous about the proper way to wire this up and i want to feel safe when i'm away from the lab.· whenever i google the issue i get a number of very different ways of doing this same thing (including a couple small third party manufacturers that sell units that perform a similar function).· i prefer to build my own and i want to make it totally idiot proof (as in, almost anything that you plug into this power outlet is safe for switching by this circuit - i am really paranoid about fires, etc.)
right now i am looking at a 125VAC/10A SPDT Mini Relay from radio shack (#275-248) but i am unclear as to how to build them into my stamp circuit reliably.· is there a parallax-recommended component that is a better choice?· and is there·a document on the site that can provide a walk-through for·someone who's doing this for the first time?
thanks,
taylortronic

Chris Savage

Hello,

·· While this is an older article, it may help you out.

http://www.parallax.com/dl/docs/cols/nv/vol1/col/nv6.pdf

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Taylortronic

hey, chris, thanks for the help... it definitely does answer quite a few questions... but i wonder if the transistors described can handle the necessary loads. one of these relays says it can handle up to 500mA and i don't want to leave any room for mistakes. i mean, i won't be able to sleep unless i can be sure that i could hook up my washing machine to this thing. that would really set my mind at ease about any levels below that.

the transistor switches make sense, i'm also seeing a cheap relay at radio shack - http://www.radioshack.com/search/index.jsp?kwCatId=&kw=275-248&kw=#275-248 and it's rated @ 125VAC/10A. the website claims that it's perfect for a pc mount. surely i can't use my demo board to build this circuit if it requires a full 120VAC power supply, right?

the article elucidated the logic-controlled circuit switching with transistors, but i'm just nervous to send electricity out of the wall to a circuit.

thanks for the help,
taylortronic

ejdarling

In general, you make every effort to isolate the line volts from the low volts. Usually the electrician's instinct is to set a row of icecube relays in the middle of a box, bring the control wires in one side of the box and to the relay sockets in the neatest most tied-down way possible, and bring the power in the other side and land those wires on the relays also in the neatest safest way. It is a code violation to mix low and high voltage conductors in the same raceway, the sole exception being if they are for the same system and if you use conductors which are all insulated for the highest voltage in the system.
Little control boxes are also factory-made containing a 120 -> 24 volt transformer, and a DPDT? 24 volt relay, for the purpose of controlling line-voltage loads from low-voltage sources and devices; they are sold thru furnace suppliersand also thru hot-tub places as freeze, and at electrical supplies. I'm thinking of Honeywell brand....
Hope that helps. I've been a master elec. too long.....

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thanks for your time,· Eric

LoopyByteloose

Safety first.

The LINE should always be fused due to high amps. Especially if it is 120VAC. If you want to plug it into a wall outlet and be universally safe, it must have its own fuse on board. Outlets will vary from 15amps to 30amps, so there is no reliable guess about your device being shutdown by a house's circuit breaker.

While 10amp relays are good, they need to have 10amp wire [noparse][[/noparse]unless you use something like a 2amp fuse to de-rate them] and do not use a flimsy printed circuit board. Some people try to put a 10amp relay on a 2amp circuitboard and then expect to switch 10amps. It is much better to buy the relay and relay holder assembly. I know it takes more space, but it certainly has been certified for high AMPs and fire safety. Some have fuse holders and/or circuit breakers that go along side. 12 or 10 guage multistrand wire will usually err on the side of caution, but the unit will be rock solid. Don't use aluminum wire as it requires special anti-oxidation gel to protect the connections. Don't use solid wire if you have a choice as it is harder to work with and less tolerant to flex. Above all the fuse should be the lowest rated device in the design.

You can use solid state relays for AC, but if you want DPST you need two. If you want DPDT, you are probably wasting your money - mechanical relays can just do more logic in one package. You can now buy a 25amp solid state relays for 120VAC that can be directly switched from a BS-24. I suppose that if you use it in a 'de-rated' fashion for only 5amps, it will enhance its saftey and useful life. Still need the fuse or circuit breaker though and that should be 'de-rated' too.

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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan