Building a Relay Switch

[Joe Hart]

How easy/cost efficient is it to build a relay to switch mains voltages?  I see schematics and realize that I don't have some of the parts (and I'm not sure what some of the parts are, but that's what Google is for!).  But I can get a kit for cheap.  Here's one that's $7.00 US.

http://www.electronics-lab.com/projects/misc/022/index.html

There may even ben cheaper ones.  I just did a quick search to see what I could find.

Is it something that I should just buy a kit for?  I have no urgent need to build one from scratch, I just need one to power up a Leslie clone and another one to switch between the fast and slow motors.  Any thoughts?
-Joe Hart

[Ripthorn]

What do you mean by switching mains voltage?  Do you mean turning it on or off?  If so, then how are you powering the relay?  If it is to switch the value of the voltage, then I see why not as long as the relay is rated appropriately.  I would just go with a kit, seeing as how a good quality relay can be $4-5 or more depending on number of poles and such.

[R.G.]

How easy/cost efficient is it to build a relay to switch mains voltages?  I see schematics and realize that I don't have some of the parts (and I'm not sure what some of the parts are, but that's what Google is for!).  But I can get a kit for cheap.  Here's one that's $7.00 US.

http://www.electronics-lab.com/projects/misc/022/index.html

There may even ben cheaper ones.  I just did a quick search to see what I could find.

Is it something that I should just buy a kit for?  I have no urgent need to build one from scratch, I just need one to power up a Leslie clone and another one to switch between the fast and slow motors.  Any thoughts?
-Joe Hart

The relays and circuits for doing this are trivial, truly simple on a complexity basis, compared to something like wiring up an effects pedal with a bypass switch and three control pots.

What is NOT simple, and NOT trivial is the danger to yourself and others from messing with AC power line wiring. Unless you already know how to wire AC power line stuff in a safe way, you owe it to your self and your potential heirs to get some instruction and on-site help in doing that part of it. Electrocution is no laughing matter.

[Joe Hart]

R.G., I have wired light switches, wired outlets, installed GFI outlets, fixed table lamps, etc.  Not an expert, but I have some experience.  And I know enough to always double check that the power is shut off, and STILL I'm very careful not to just touch wires "willy nilly."  Then I always double check everything before turning the power back on, and even then I'm careful testing out the work I just did.  So I have little concern there.  I'm smart enough to know if I don't know enough to do something safely.

It's just that the schematics I've seen use things that look like coils.  I guess my question is this: some schematics show a coil and what looks like photoresistors or something and others show just a big square with poles -- are these just relays that you buy or do you actually get all of the stuff and build it "discreetly" (like you can build an op-amp)?  If you just buy a relay like you would buy an op-amp, then I can easily understand the schematic (and build it).  If that's the case, then it looks like you get a relay, add on some stuff to "support" it (like biasing an op-amp and filtering the power and creating the negative feedback loop for a distortion pedal), and you're good to go.  Is that overview correct?

I'm just confused with some of the symbols on some of the schematics -- they all seem to look very different!


Thanks.
-Joe Hart

[Joe Hart]

Ripthorn, I was looking at powering the relay circuit with 9V or 12V and switching the mains voltage between the slow and fast motors of a Leslie clone.  That way I could switch them with a footpedal and not have dangerous voltages running through wires all over the stage.
-Joe Hart

[Nasse]

http://www.elektor.com/magazine/construction-electrical-safety.83362.lynkx

There used to be info like this and pictures in every magazine, dont know if they still have it, like mechanical contruction and making a box for diy thing and how the safety things should be done. I think some info is for 240 volts mains so its a little safe side if you have less

[PRR]

> How easy/cost efficient is it to build a relay

Wind an electromagnet around a U-shape of iron. Hinge and spring an iron flapper so it moves when you energize the electromagnet. Attach a low-force switch. Done.


You can buy mechanical relays for a few bucks; it makes no sense to DIY one. I think I got 12V-coil relays rated for fan motor load (much larger than your Leslie motors) for $5 each.



The solid-state relay you pointed to is a bit quieter, and less prone to mechanical trouble (but more prone to line-surge trouble). You don't need a kit, you can buy this function in a sealed module and get an actual current rating. And RU/CE tested. (That kit does not give a spec. It looks beefy enough to turn my whole house off, but who knows? And if it is that beefy, do you need all that for your half-Amp motor?)

[R.G.]

R.G., I have wired light switches, wired outlets, installed GFI outlets, fixed table lamps, etc.  Not an expert, but I have some experience.  And I know enough to always double check that the power is shut off, and STILL I'm very careful not to just touch wires "willy nilly."  Then I always double check everything before turning the power back on, and even then I'm careful testing out the work I just did.  So I have little concern there.  I'm smart enough to know if I don't know enough to do something safely.

OK. I had to issue the warning in case you were an enthusiastic 14 year old.  If you're aware you can die doing this and have some modicum of experience, you know that you are taking your life in your hands.

It's just that the schematics I've seen use things that look like coils.  I guess my question is this: some schematics show a coil and what looks like photoresistors or something and others show just a big square with poles -- are these just relays that you buy or do you actually get all of the stuff and build it "discreetly" (like you can build an op-amp)?  If you just buy a relay like you would buy an op-amp, then I can easily understand the schematic (and build it).  If that's the case, then it looks like you get a relay, add on some stuff to "support" it (like biasing an op-amp and filtering the power and creating the negative feedback loop for a distortion pedal), and you're good to go.  Is that overview correct?

I'm just confused with some of the symbols on some of the schematics -- they all seem to look very different!

I actually did an article on relays at GEOFEX, "Relays for Audio Switching" or something similar. You might want to read that. I spend a surprising amount of time telling people that their answers are in the articles at GEOFEX. 

A classical relay is an electromagnet that operates a mechanical switch. The coil is really a coil, albeit with a significant resistance. When you put current through the coil, it attracts the metal part that moves the switch. There are some variations in how that functions.

Relays designed to be driven by DC across their coils are different from relays designed for AC on their coils because of the pulsating nature of AC. Finally there are relays which can be pulsed to make them change the switch position, and don't need to have coil current all the time to keep the switch in the activated position. These are called latching relays, and are more complicated to drive, but don't need coil power all the time.

The schematics are different sometimes. But the basics for classical relays are always that a current through a coil causes a magnetic push/pull to move a mechanical switch. The mechanical switch can be almost anything.

There are other kinds of "relays". The essence of relays is switching isolated from the activating current. Once we (humans) developed reliable light-activated devices, the next step was to use a light source isolated from the switching device to activate it. There are light-activated "relays" made from every kind of light-activated electronic device. The earliest was probably the neon-bulb and Light-Dependent-Resistor (LDR). These were popular on amps, and Fender still uses them. Modern optoelectronic relays tend to use LEDs for light sources, and then transistors, MOSFETs, or thyristors to conduct the current on the output. The action is similar: a current on one side causes a switch to activate on the other side, and there is no conductive path between them.

For what you want, you will need a classical relay: coil voltage to match your supply voltage, and switches with configuration and current rating to match the load.

Read the article on relays at GEOFEX and then ask for clarification where I've muddied the water instead of clearing it.