General Question

heydrew's avatar

How to wire a relay into a circuit?

Asked by heydrew (38 points ) March 29th, 2010

I am setting up a home automation setup to allow me to turn on my hot tub over the phone. I am going to use a telephone controller from Honeywell (http://www.aubetech.com/products/produitsDetails.php?noProduit=46&noLangue=2) and a relay I bought from McMaster. The relay says the control voltage is 20–60 volts. Does this mean when I connect it to the Honeywell box that it will ONLY send the 20 volts the relay needs? The wires wont be in conduit and will be running through my walls. I want to make sure I wont start a fire!

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11 Answers

Tropical_Willie's avatar

@heydrew

Get an electrician. Sounds like you’re on your way to burning your house down.

And your homeowner’s insurance won’t cover the house because of the DIY wiring that you did!

cockswain's avatar

The relay is rated to be operated by an input between 20–60V. Less than 20V wouldn’t be enough to activate the relay and provide sufficient hold-in current to keep the relay closed. Greater than 60V means the relay will get hot, and may fail in the stuck closed position, meaning the current to your hot tub won’t stop by activating the switch any longer.

Your Honeywell controller should say what its output voltage is. You also need to know if it is outputting AC or DC, and also need to know if your relay input needs to be AC or DC.

For safety, I’d calculate the amp draw of your hot tub and install a proper fuse or breaker inline.

In summary, if you don’t understand what I’m talking about, you shouldn’t mess with this. Your hot tub needs lots of juice, so you’re working with enough voltage to cause a serious problem if you don’t know what you’re doing.

DarkScribe's avatar

Why are you using an extra relay? The device appears to function as a switch or relay in itself at up to nominal domestic current ratings. It is rated for 10amps at 120/240 volts. The only requirement is that at voltages higher than thirty it needs to be electrically isolated in junction box.

As others have suggested – you need to have this installed by an electrician.

heydrew's avatar

Darkscribe: yes, this device technically is a relay but I will be wiring another relay in line with the mechanical rocker switch the turns the hot tub on, and then put that switch in the “on” position. The new relay will keep the hot tub from being on until the Honeywell controller opens it.

The Honeywell controller outputs 30–120 volts, and relay I bought has a control voltage of 20–120 volts (AC). So I am assuming that the relay will only “ask” for 30 volts—low enough voltage to not cause problems. This is my basic question. If a relay has a range it will accept, will it only draw the minimum it needs?

DarkScribe's avatar

@heydrew This is my basic question. If a relay has a range it will accept, will it only draw the minimum it needs?

No. Get an electrician. Really – it will be both safer and probably more cost effective in the long term. One device does not select a voltage from a supply, the supply has to regulate that, (but it can determine a current flow).

cockswain's avatar

That’s not quite how it works. The relay has no “asking” mechanism. Basically a circuit has x amount of resistance. The more voltage you apply, the more current that resistance will allow to pass. I don’t know how you control the output voltage of that controller (and don’t feel like reading the link). It may well be that if you hook up the controller to your household current of 120V, if there isn’t an internal transformer or a way to select the output voltage yourself, then 120V will exit the controller and fry your relay.

Tropical_Willie's avatar

@heydrew
They removed a hot tub at my house before we moved in. The circuit was 40 AMPS and 240 VOLTS. You don’t want to play with 240 VOLTS. It does not shock you, IT kills.

DarkScribe's avatar

@Tropical_Willie _You don’t want to play with 240 VOLTS. It does not shock you, IT kills. _

It is current that kills, not voltage. A colour TV HT supply can give a jolt that exceeds twenty thousand volts – I have had many of them – but they don’t kill. Well, not so that you would notice – I don’t seem to able to walk through walls. Not enough current. A car with a Kettering ignition system can also give thousand of volts, but they don’t kill.

gasman's avatar

” So I am assuming that the relay will only “ask” for 30 volts—low enough voltage to not cause problems. This is my basic question. If a relay has a range it will accept, will it only draw the minimum it needs?”

A relays is not ‘smart’ enough to ‘ask’ for anything. It presents a fixed coil resistance to the controller. Some controllers might sense this resistance & adjust output appropriately, but I surely wouldn’t assume that.

If the relay’s input coil is rated for 20–60 V but the Honeywell outputs 120 V, then coil current will be too high & it will overheat—power dissipation is proportional to square of voltage.

malevolentbutticklish's avatar

@Tropical_Willie @DarkScribe: If this 240V is in the USA and you receive a shock it will only be 120V AC. To receive a 240V shock you would have to be electrically isolated and grab both hot wires at once (highly unlikely). It is also highly unlikely that you would be killed by a 120V shock. Lots of people are killed by 120V but for every person who is killed MANY more are shocked and walk away just fine. In particular the hot tub should be on a GFCI breaker and it is very hard to get killed by 120V in the 15–30 ms it takes the GFCI to trip. I don’t have stats but the number of deaths are extremely low on GFCI-breakered circuits.

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