I want to make a control panel in my hobby room. Something similar to the control panel to a homebrew setup. All I will be doing is controlling various 120v items (exhaust fan, lights, desk fan, stereo, computer, etc). My aim was to use nice small electronic switches to make it look cool. I cannot figure out how to make the circuit so that I can take 120Vac into a relay that brings is down to 12Vdc and sends the signal to a 12v rocker switch, which sends the signal to another relay that uses the 12v impulse to fire up 120V and close the circuit to the appliance. Basically, I want to turn stuff on and off from a central location using 12v switches using a control panel similar to the one in this picture.

I am new to electronics, so even some basic items may seem confusing to me at this point yet.

 

Simply use a 12V "wall wart" or similar power supply inside your control cabinet for use with all the switches (coil side of the relay)..
All the 120V is only switched/connected to the "contact" side of the relay..

This keeps total isolation between the "Switches" and the "Switched" loads..

A relay has 2 "sides".. the coil and the contacts..
You apply voltage to the coil and the contacts switch states

 

Personally, I wouldn't use 12V switching to do this. I'd just build a box and use AC switches. But the size would be an issue and I suppose that's why you want 12V.

Use a single AC adapter 12V supply to power all your relays. You'll want a power supply rated to an amperage that is about double the combined current required by all your relays. This means the power supply will be perfectly happy even if/when it is powering all of the relay coils at once. It's Murphy's law that this worst case scenario WILL happen. So if your preferred relay requires 60mA to activate, and you have ten of them, you'd want a power supply rated to 1A or more. You might want to consider using 19V for your system. There are plenty of 12V adapters out there but all those laptop power bricks you can find at a recycling center typically have a 19V output and lots of capacity. In other words you could get a very nice power adapter for free.

The relays you need will depend on the the loads you want to switch. Find the biggest load (maybe your lights?, or stereo?) and then double it. Maybe that would be 300 watts. Every 120W requires a 1A current, so you'd need a relay rated to 120V AC or more and 3A or more. For ultimate safety, you might consider 15A relays. Your home circuit breaker likely trips at 15A and there's no reason to go over that. Using the high capacity would allow you to plug in anything you would ever plug into the wall, without concern. Hair dryer, vacuum cleaner, anything. Otherwise you'll need to label your receptacles and try to remember that they have a limited capacity.

Note that your entire panel can only switch a total of 15A, since any more than that will trip the circuit breaker. I'd consider including a 10-15A fuse in your panel on the AC side. This would get smaller if you go with smaller relays. The fuse should cover the maximum combined load.

All of your control switches will be on the 12V DC side, directing power to your relay coils, which in turn will switch the AC loads.

 

Thank you both for the excellent guidance. I truly understand what I am to do know, and I have not been able to figure that out on my own for over a month. Extra thanks for the diagram/schematic. So, my desk fan says "Power 1w". I understand this to be the load...is that correct? that would mean my cap would be 1800 watts per 15 amp circuit. I think that is a lofty number, and I am certain I will not come close to that. Now, about the relay...I should get a 120V relay with a 12V coil...and - depending on load for that circuit - double the calculated amp draw...Is that correct? So, I could end up with various types of relays in the control box depending on what each circuit is running. Do I get the ice cube relays, or can I use automotive relays?

 

I found a relay on an electronics site - 12vdc coil, max switch current of 1A, DPDT (2 form c). Am I right to think a nice pile of these would do just fine? Each circuit would be limited to 120W load because of the 1A limit, right. The only thing I do not understand on this relay is the 2 form c used to describe the switch. I have never heard of 2 form c.

 

2 form C to me means 2 sets of change over contacts (N.O.-N.C.).
Max.

 

You will definitely want better than 1A contacts on a relay. A 100W light bulb will pull .8 amps and that is just a little too close. Look for 5A at least, 10 is better. They won't be that expensive. Overkill is good in this case.

 

While I have no idea what you may have for a budget something I like is Octal Base relays in the DPDT variety. Here is an example. I have seen them much cheaper than the link and a nice feature is the base sockets can be easily mounted or simply snapped on a piece of DIN RAIL making for neat and easy layout. Relays like this are very standard and come in a wide range of coil voltages. While 12 VDC control is nice I like using 24 VDC as then each coil only requires half the current of a 12 VDC coil. A Google of octal base relays" should bring up dozens of hits from assorted manufacturers. Anyway it is all something for thought depending on your budget.

Ron

 

Also for 12v relays, if there is an autowrecker near you, you can usually get a hand full for a couple of $'s, especially if it is a strip-your-own.
If you only need one contact for each relay.
The insulation material used is the same for 240vac relays.
Max.

 

I would recommend using DPDT relays and switch both legs of AC. While not strictly necessary, it's a safety thing and not that expensive. With a single pole relay, you would switch the hot (black) wire but it's easy to get get miswired outlets and power cords. Or, with a non-polarized plug. Switching neutral means that hot is always connected through your wiring.

 

my biggest struggle appears to be choosing the correct relay. I know I have to stick with a 12VDC system because I am using automotive switches. If I send 12VDC to a relay with a 24VDC coil, will the relay still switch? Also, am I correct to assume that all my relays could be 15 amps even though no single one will approach 15 amps of current/load/draw? Assuming so, I could use:

12VDC - 24VDC coil even if only providing 12VDC - the relay will still switch
from 1 amp to 15 amp Contact Current Rating depending on specific circuit load


the only question left would be if the relay should be latching or not. Does latching refer to the contact side or the coil side? If I want the switch to operate like a normal light switch, should the relay be latching or non-latching?

 

While I have no idea what you may have for a budget something I like is Octal Base relays in the DPDT variety. Here is an example. I have seen them much cheaper than the link and a nice feature is the base sockets can be easily mounted or simply snapped on a piece of DIN RAIL making for neat and easy layout. Relays like this are very standard and come in a wide range of coil voltages. While 12 VDC control is nice I like using 24 VDC as then each coil only requires half the current of a 12 VDC coil. A Google of octal base relays" should bring up dozens of hits from assorted manufacturers. Anyway it is all something for thought depending on your budget.

Ron

Once I understand how to choose the correct specs on a relay, this is the route I would like to take. I could hook up multiple circuits to one relay this way, right?

 

The supply voltage decides the coil voltage, 12v, use a 12vdc coil relay.
A latching circuit or relay is a whole different device.
You can get 4 contact relays such as the Omron MY4ZN series.
Max.

 

I would recommend using DPDT relays and switch both legs of AC. While not strictly necessary, it's a safety thing and not that expensive. With a single pole relay, you would switch the hot (black) wire but it's easy to get get miswired outlets and power cords. Or, with a non-polarized plug. Switching neutral means that hot is always connected through your wiring.

I am 100% on board with this, and thank you for sharing it. It makes sense. Once I figure out what kind of relay to get, I will design a layout like this.

 

The supply voltage decides the coil voltage, 12v, use a 12vdc coil relay.
A latching circuit or relay is a whole different device.
You can get 4 contact relays such as the Omron MY4ZN series.
Max.

Is there no way to use a relay that does not cost 20-30 bucks? I will do that if there is no other way, but the goal was to have as many circuits as possible - to make it a cool nerdy control panel.

 

No, you can get relays for a lot less than that. Here's a DPST 12V, 8A contact relay that costs $2.28. Doesn't look that big, either.

 

No, you can get relays for a lot less than that. Here's a DPST 12V, 8A contact relay that costs $2.28. Doesn't look that big, either.

Awesome!! Thank you! I really appreciate that help.

 

I would recommend using DPDT relays and switch both legs of AC. While not strictly necessary, it's a safety thing and not that expensive. With a single pole relay, you would switch the hot (black) wire but it's easy to get get miswired outlets and power cords. Or, with a non-polarized plug. Switching neutral means that hot is always connected through your wiring.

I think I have that wiring figured out, but because electricity is such an unforgiving wraith, would you mind sketching out the appropriate way to wire for this?

 

hmmm, my drawing packages don't have much help in that area. The way I make sure I get it right is:

• google for outlet polarity to determine which slot is hot. (can never remember which is which)
• buzz out the power cord to determine which plug pin connects to the hot slot
• use black wire for hot
• use white wire for neutral
• make sure I connect black to the hot side of the new outlet, white to the neutral. Packaging should tell you which is which.
• Afterwards, use a polarity tester (cheap from the HW store) to verify my new outlets are correct.

 

On your N.A. socket, the wider pin should be the neutral, not all cords are fitted with a polarized plug.
Max.