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Relay question


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Warning: I know just enough about relays to be dangerous, so I want to bounce this off those more experienced than I. It may be possible a relay isn't what I want; I might want a transistor of some type. I just don't know.

I have an application where I need to push a button and complete an electrical circuit. I have plenty of spare channels on my LOR boxes so I thought I might be able to find a relay which will be open when AC power is off, and will close when AC power is on. When closed, the circuit I want to control will be complete and the effect will be as if I'm standing there pushing the button.

Sounds reasonable, right? So my big question is, what the heck kind of relay do I need? It won't be switching any current or voltage to speak of (though I haven't hit the circuit with my meter, I'm guessing 5v). It's just to close what's normally controlled by a momentary button.

Would something like this work or am I way off the mark in my thinking? http://www.mouser.com/ProductDetail/Tyco-Electronics-Schrack/V23061A1005A402/?qs=sGAEpiMZZMtSzCF3XBhmWzK2k7XZ%2fB23PQgBGJ3gaw4%3d

Or would I be better served by something like these (and is there a less expensive alternative?)

http://www.mouser.com/ProductDetail/Crouzet/SM-IAC5/?qs=sGAEpiMZZMuZ9PFRUZK2DP6wVca%252bFARDxWmtnyIigeM%3d

http://www.mouser.com/ProductDetail/Crouzet/IAC5/?qs=sGAEpiMZZMuZ9PFRUZK2DOsqNt3yPRZcIg1XHKTl%252bp0%3d

Thanks for any advice!

Jacob

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Sounds like a relay will work fine for what you described. You first want to be sure that the coil is rated for what will trigger the relay, in this case 125 volts AC from an LOR channel. Then you need to be sure that the contacts on the relay are rated for at least what you are switching, and that is still unknown. It is fine for the relay contacts to be rated way more than what you are switching. For instance you could use a relay with 10 amp contacts to switch something that is only 1 amp.

I would use a simple mechanical relay like this one at Radio Shack http://www.radioshack.com/product/index.jsp?productId=2049721 and it looks like it's stocked at most stores. With the ones in the links that you posted you would have to be sure that what you are switching is low enough voltage and current for them to work.

Edited by Scott4864
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I was getting tripped up by the rating on the load side of the relay. I was thinking I needed to get something rated for the type of circuit it would be closing, but - duh - it's just making or breaking a set of terminals isn't it?

Thanks for the info; I think this will get me down the path to testing!

Jacob

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Jcazz,

Yes and no. You can always have a relay that the contacts are way over rated and no problem. But I would not like to use a relay the size of those small ice cube that hotel ice makers make for say a 220V 15A. circuit. Cause within seconds after closing if it didnt start arcing the minute you turn power on, it burns up and smokes. So, yes you do need to be mindful of the load you are going to be putting on those contact. So, make sure the rating of the contacts are greater than the load requirements.

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Jcazz,

Yes and no. You can always have a relay that the contacts are way over rated and no problem. But I would not like to use a relay the size of those small ice cube that hotel ice makers make for say a 220V 15A. circuit. Cause within seconds after closing if it didnt start arcing the minute you turn power on, it burns up and smokes. So, yes you do need to be mindful of the load you are going to be putting on those contact. So, make sure the rating of the contacts are greater than the load requirements.

Hi,

I think you are reading some of this out of context. We were talking about the contacts being rated for at least what it is switching but that it could be rated for way more. Jcazz's comment was that it doesn't have to be rated for exactly (no more no less) what it is switching.

Scott

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First of all you need to describe what it is that you are trying to control with the relay.

Is the relay going to operate a light? In which case you have a resistive load.

Is the relay going to operate a motor? In that case you have an inductive load.

Is the relay going to operate something that is DC or AC?

Contacts of a relay are rated by load.

The heavier the load the larger the relay.

Also, DC loads can be tricky in that they are prone to burning contacts.

I am assuming that you do want a 120 V. AC coil on the relay.

Please, give us a little more information so that we can help better.

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A relay isn't just a switch, it really does matter what you're switching. There's a reason why there are so many different relays at Mouser.

Yes the contacts do have to be rated for more than the load but also all switch contacts have a minimum load too. You really should measure the current. If the current is only a couple of milliamperes, I wouldn't necessarily recommend a 10 amp relay to control it.

It also matters whether it's AC or DC too.

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As a followup; I want to use a LOR channel & a relay to control a fogger. So I need a relay I can control with 110 volts, but it will only be closing a dc circuit of a few volts with a few miliamps.

I think I've got my head on straight about what and how I should go about this from reading these replies and other forum threads.

Thanks, guys!

Jacob

PS - Whoo hoo, this is/was my 300th post to PC! It's rather anti-climactic actually... ;)

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Actually here's the way I did my fogger. I have the main of the fogger plugged in all of the time, so when the main to my display kicks on, it heats the fogger up. My fogger has a remote control that is triggered by (either 4v dc or 6v dc, can't remember which). I cut off the remote and wired a wall wart into the switch, after I tested to see which one controlled the actual on/off of the fog. Then I just plug it into one of my LOR channels, and turn the wall wart on/off with it, works great.

Depending on what kind of fogger you have, a simpler way would be using x-10 with LOR.

Bill

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Bill - I'm actually going to be using LSP this year, not 100% sure if it's x10 compatible off the top of my head.

The fogger is a Chauvet Hurricane 1300. The remote does more than on/off, but I haven't tackled it yet to see if it has a microprocessor or just wires I need to bridge. I'm thinking I'll have to solder onto the existing momentary button leads with the relay to make it work.

Thanks for the ideas though!

Jacob

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