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Topic: 120 volt control through relay (Read 2318 times) previous topic - next topic


I got a couple relays from sparkfun.  see http://www.sparkfun.com/products/10924 for relay.  The device I want to control says it is 200 watts at 120 volts.  Unless Watt's law was repealed while I was away this means that there is ruffly 2 amps of current.  I'm sorry if that's not close enough I only do digital computing ( I count on my fingers.)  So the relays I got from sparkfun are rated at 240v and 20 amps.  This should be more then enough, yes?

About 40 years ago I built a five tube radio but I have written a lot of code in those years ( I'm a comp sci guy not an eecs guy.)  To teach my son about coding and EE I have gotten into the arduino and love it.  Now I am getting my feet wet with a house current and voltage project.  I want to make my own pcb for this project.  Sparkfun has one already made for the relay but I want to add other stuff and would like it all on one board.  So here are some questions:

1:  The last time I made a board I used tape to protect the copper on the board from the etching solution ( Nixon was president at the time).  I am having a had time finding blank boards let alone any tape.  I did find some board material through Radio Shack but no tape and dots.  What are people using to prevent the etching solution from eating off the trace?  Is there a tutorial for making pcbs at home?  I have looked but haven't found any useful ones.  I would really like to not have to make a light box to do photo etching but if that's the only way I will.

2:  This pcb will have to handle a couple of amps at 120 volts.  I have strong doubts about the ability of a little trace to do that.  I think the board will just go up in smoke.  How should I deal with that?  The pcb from sparkfun doesn't seem to make any allowances for this but in the pictures they don't show the bottom of the board. 

3: Any other thoughts or suggestions will be greatly appreciated.



If you're looking for the PCB to etch do a google search for "copper clad board". You will also need an etching solution (Ferric Chloride / Sodium Persulfate, etc.). If you're looking for cheap and simple you can use a Sharpie marker to draw the traces, otherwise you can look into printing your design with a laser printer and ironing it onto the board (requires a bit of trial and error) -- this is referred to as "toner transfer".

A little trace has no problem handling 2A; you can find a calculator here. 2A comes out to .03" on a board with 1oz copper thickness which is much smaller than you'd want to make it on a board you're building yourself (i.e. thicker traces are more reliable).


Thanks for the info.  I bought some Ferric Chloride, same stuff I used when Nixon was in office, on line with a bunch of different size boards. 

I was thinking about making a an xyz cnc machine for making pcbs.  I guess this clinches that.  What a pain.  I used to walk a couple of blocks and get everything I needed.  How things have changed.  We have micro-controllers that are only a few bucks but it's a real PITA to make a pcb.  I wonder if wire wrap will make a comeback.  Probably already has I just missed it.


While we may no longer be able to buy tape and transfers for PCB design, we do at least, today, have great software for creating the design which will be turned into copper on substrate...


As for the craft of making the actual physical object...


BUT!... Household voltages are a bad place to go if you are not trained. Alternatives...


... but if you must, at least remember the snubbing diode across the relay coil...


=== Old fart rant....
Not only can we no longer get tape, by we can't get decent etch resist pens or a solvent to remove it! And while there is a "Sharpie" brand, the pens have been watered down, and don't do half the job they used to do, and the range of choice in the stores has evaporated, and good luck finding anything except the Sharpie brand. And yet the legislators allow leaf-blowers and patio heaters? (Not that I'd want them to ban anything else!)


First, veroboard-style breadboard is quite useful for smaller circuits.
If you want to do mains power, you might want to Dremel out a trace in between the traces of the relay, to reduce the risk of iron filings causing shorts.
This is what I do for one-offs, and it works fine so far.

Second, to make PCBs yourself, people recommend "toner transfer." However, when looking at all that, I've come to the conclusion that paying $60 for a PCB from an online PCB printer is worth it.
Not only do you get 3 PCBs for the minimum order, but you get a full digital production process -- out of your layout, and into your mailbox, in 3 days! http://expresspcb.com/ExpressPCBHtm/SpecsMiniboard.htm
(There are others -- http://pcbexpress.com/products/prices.php#1pricing , http://www.pcbunlimited.com/us-quickturn-pcbs-p-604.html , etc)

Also, Circuit Specialists have nice aluminium enclosures under $10, that come with through-plated hole-array PCBs. This means you have to actually solder wiring between components, and/or use wire wrap posts (remember wire wrapping?) but it's an option if you're going to get enclosures anyway.
(They also have the kinds of DIY etching parts you're asking for: http://www.circuitspecialists.com/printed-circuit-board-fabrication )

Finally, -- tried a desktop CNC milling machine? It can cut isolation from traces on copper clad FR4 just fine, and probably even drill the holes ;-)

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