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Wiring an "Ice Cube" Relay to Latch

01/16/2012 9:28 AM

So i have a basic understanding of relays but I'm kinda stumped here.

I'm wiring up an alarm for a metal detector. The signal coming from the detector is 120AC. The relay is a 2pdt and has a 120 coil. I need the relay to latch until it is broken manually by a push button on the alarm box. I have a separate 120 line coming into the box.

Any help with a diagram would be greatly appreciated!

Thanks for your time,

Kris

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#1

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 9:37 AM

The thing that turns the relay on needs a normally-open contact, driven by the relay, wired across it to sustain it. Then wire a normally-closed contact from the push button in series with the relay contact.

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#2

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 9:40 AM

Assuming you are American, the symbols should be familiar. This is not for a relay per se, but will work all the same. Please ask if you are not clear about something here....

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#3
In reply to #2

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 9:56 AM

Yep. That's the baby. Of course, if L2 were N then the fuse adjacent to it would not be fitted under BS7671.

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#4

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 12:22 PM

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#5
In reply to #4

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 3:37 PM

Thanks for all of the quick responses. I should have been more specific with my question. I know how to loop the contact to get it to latch. What I'm confused with is how to tie in the signal wires to avoid ending up with 240 at the relay. The signal voltage and the alarm box voltage are separate due to the signal being momentary. Its a metal detector on a conveyor line and when it fires, it only does so for a split second.

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#6
In reply to #5

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 3:39 PM

What I'm trying to avoid is sending 240 across the coil.

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#7
In reply to #6

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 3:41 PM

Even if it is just for a split second....

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#8
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Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 3:48 PM

If wired as shown above, then the alarm signal only needs to be active for a fraction of a second to power the relay coil and latch the circuit.

If one side of you relay's coil is to neutral, and there is only one 120v source, then you will never have 240v across the coil.

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#17
In reply to #6

Re: Wiring an "Ice Cube" Relay to Latch

01/17/2012 7:21 AM

So why not use a 240V coil?

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#9

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 4:35 PM

You need to get a 240V lamp or a [240V] Neon lamp test screwdriver or a 250V voltmeter to find out the voltage between the wires of the two supplies you have. Test readings or brightness on known 120 and 240V supplies before using and also test voltage to earth/ground of each wire to find which are neutral.

It is far the best thing to find out what supplies each of your supplies are from back at the distribution breaker or fuses (by opening or removing said items and your lamp going out) than to hope and pray something is NOT connected you do not know about, such as a supply from UPS or supply changeover switch which can change phase suddenly. You may have to disconnect all wires at the supply source, follow them visually via conduits or ducts and "ring" them out to your alarm box to be sure what is connected together. Always make sure wires you test between have zero volts to each other before using a ringer or ohmmeter. Label everything when you have identified it and put a diagram in your alarm box to help yourself and anyone else!

If your not confident about sorting this out, get an expert. And complain in the necessary places about lack of diagrams and labeling if they are missing.

I can understand your problem if you have wires from unknown sources - which may be connected to different phases or have opposite polarity, as could happen with 120/240 V USA type supplies.

Unless you can clarify this, it would be dangerous to make a suggestion.

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#10
In reply to #9

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 5:07 PM

I'm not unclear about where the supplies are coming from. I have 120 coming into the box to power it and i have 120 coming in as a signal to fire the box. I'm just confused as to how I should wire the relay. If I tie 120 to a NO contact on the relay and then loop it back to the coil... and then tie the signal 120 to the coil, wouldn't there be 240 at the coil when the device signals?

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#11
In reply to #10

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 8:17 PM

Get a qualified electricians help - PLEASE!

If you didn't understand what 67model said about "different phases", then you are not qualified to do this work. You could kill yourself or someone else later.

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#12
In reply to #11

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 10:14 PM

teach me something. not threaten. don't be a forum troll. I'm not a dummy. "qualified" or not i'm going to figure it out. i know the neutral is shared between the device and the "power" source for the box via ohm meter.

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#13
In reply to #12

Re: Wiring an "Ice Cube" Relay to Latch

01/16/2012 11:59 PM

Since you say the Neutral is common, all you have to do is put an AC voltmeter between the two 120V lines, when they are both active. If you measure 240V, then they are of opposite phases and your concern about 240V is valid.

It is much more likely that you will measure zero between the two 120V lines. This will indicate that both lines have the same phase, in which case you have no worries about 240V.

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#14
In reply to #12

Re: Wiring an "Ice Cube" Relay to Latch

01/17/2012 12:08 AM

1. You don't know the difference between a warning and threatening.

2. I'm not sure what you mean by "forum troll", but maybe you can look in the mirror and explain it to me.

3. I will repeat: If you don't know what "different phases" means, then you should not be playing with this circuit, you should be getting help or an education that is beyond this forum.

4. Just because neutral is common between the device and the box, does not mean they are not on 2 different phases. That is the whole crux of the problem, you don't know whether there is 0 volts Ac (RMS) difference between the 2 120 VAC supplies, or 240 VAC.

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#16
In reply to #10

Re: Wiring an "Ice Cube" Relay to Latch

01/17/2012 7:20 AM

<...Any help with a diagram would be greatly appreciated!...>

Both ways....

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#18
In reply to #10

Re: Wiring an "Ice Cube" Relay to Latch

01/17/2012 7:22 AM

Why not use the one 120V source instead of two?

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#20
In reply to #10

Re: Wiring an "Ice Cube" Relay to Latch

01/17/2012 10:00 AM

The diagram that I provided above would work if both the 120v signal wire and the 120v L1 to the ice cube relay contact and the neutral wire were all derived from the same transformer or generator.

Are you are saying that you don't know where the "hot" for the alarm contact originates - ie what transformer produces it? And in order to get around this, you are trying to use 120vac from a separate source like a distribution panel?

You could try wiring your circuit like this, but I won't guarantee that it will work because A2 for the coil does not complete the circuit back to L2 of the unknown source:

Note that use of the N.C. relay contact ensures that the two sources (X1 and L1) will never "meet" at point P.

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#21
In reply to #10

Re: Wiring an "Ice Cube" Relay to Latch

01/17/2012 12:07 PM

If there is any chance this would ever happen, because of a wire being put in the wrong place during maintenance, or for whatever reason, the quick, cheap, easy thing to do is to put an isolation relay between the detector and the warning device. It's coil is wired from the signal wire to neutral. It closes whenever there is signal, opens as soon as the signal goes away. It's NO contact sends 120 V to the alarm Coil & NO contact, then to neutral. (The other side of the NO goes to 120V supply) A NC push button in series with the Coil & NO contact is the reset. 120V can never get back to the signal, so never a chance of a phase change that would put 240 or 208 on the signal wire. -- JHF

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#15

Re: Wiring an "Ice Cube" Relay to Latch

01/17/2012 12:32 AM

Could you please clarify the following do you have 2 x 120v hot wires coming in to the box sharing the same neutral or are they on seperate circuits and each one has its own neutral in the box and can you fit another relay into the box if needed?

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#19

Re: Wiring an "Ice Cube" Relay to Latch

01/17/2012 7:23 AM

See? Without the diagram, the forum is uncertain too.

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#22
In reply to #19

Re: Wiring an "Ice Cube" Relay to Latch

01/17/2012 12:11 PM

True! But without a diagram the only thing that really confuses me is what all this has to do with an "Ice Cube". -- JHF

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#23
In reply to #19

Re: Wiring an "Ice Cube" Relay to Latch

01/17/2012 5:42 PM

Sorry for the attitude. A couple of wobbly pops and someone telling me not to do something kinda set me off.

Thanks for all the help guys. I've got it figured out. It just took me a while to wrap my head around it. Its been a while since I had to be sparky and I forgot that voltage coming from the same phase doesn't double...

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#24

Re: Wiring an "Ice Cube" Relay to Latch

01/18/2012 8:21 AM

Use 2 relays. One triggered from the sensor and driving the latching relay. This will isolate the 2 supplies. If you have trouble with wiring this kind of simple circuit you should consult a technician knowledgeable in basic electricity for safety sake.

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#25

Re: Wiring an "Ice Cube" Relay to Latch

01/10/2014 5:05 AM

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