Replacing Transformers in Doorbell System

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  #1  
Old 05-31-16, 03:10 PM
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Replacing Transformers in Doorbell System

New member, with minimal electrical expertise, living in Florida.

We have a 4-year-old house with a lighted doorbell button at the front door and two transformers, each serving a chime. One chime is in the living room and the other chime is in the rear patio. That setup permits us to hear the doorbell even when we're out back.

Both transformers are in a junction box behind the chime in the living room. The wiring from the doorbell button to the two transformers and from there to the two chimes is Cat 5.

I just noticed the doorbell button light was out, and discovered both chimes were inoperative. Total silence.

I've removed the button and found the wires have no power. I removed the chime assembly in the living room and checked the two transformers. Both are getting good house current, but I get no reading on the other sides, where the low voltage is connected to the chimes via the Cat 5 wire. It seems that both transformers have gone bad, after only four years of service, and both at the same time, which I understand is a very unlikely occurrence.

Two questions: 1) What could have caused BOTH transformers to go bad? 2) Is there something I should do before replacing the two transformers, so this doesn't happen again.

Any assistance will be appreciated.
 
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Old 05-31-16, 03:16 PM
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I've removed the button and found the wires have no power.
If only two wires there you really can't measure power there. Were you using a multimeter to test for power? A non contact tester won't work. If that is what you used at the transformers to check voltage then they may not actually have power.
 
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Old 05-31-16, 03:36 PM
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On one side of the transformer are the three wires to connect to the 110V house current. On the other side of the transformer are two screws for attaching the Cat 5 wires. From the doorbell button to the transformers and from the transformers to the chimes, only two wires of the Cat 5 are used.

I used a multimeter and got 110V probing the wire nuts connecting the transformers to the house wiring. Switching the sensitivity higher, I then tested the two screws connecting the transformers to the chimes - that's where the multimeter showed no current.

At the doorbell button, I touched the two wires together to complete the circuit, but the chimes didn't work. Since the light in the button is also off, I'm certain that there's no power to the button.
 
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Old 05-31-16, 03:54 PM
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You usually cannot connect the low voltage output of two transformers. You need to use one large transformer.
 
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Old 05-31-16, 04:03 PM
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So in order for the front doorbell button to ring chimes in two locations there should be only one transformer rather than two?

Why are two not OK? Is that the reason these two failed? And how do I get the proper size transformer to replace them?

(You may notice a large amount of electrical ignorance on my part. I presumed the electrician who installed this system knew how to configure it.)
 
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Old 05-31-16, 05:27 PM
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CAT5 is a bit on the light side for doorbells. I would double up on the copper, that is, use one pair twisted together as a single conductor. I don't know if that has anything to do with your problem but it wouldn't hurt.
 
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Old 05-31-16, 05:30 PM
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AC current changes polarity sixty times a second. For parallel Leg A of transformer "1" must always be the same polarity as the leg of transformer "2" in real ife that is very hard to achieve. Second each transformer should put out exactly the same voltage. Not likely with a very cheaply made transformer. I suspect one failed early on and both chimes have been running on one transformer.

What is the VA rating of the failed transformers?
 
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Old 05-31-16, 05:42 PM
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Great idea! Yes, the wires are pretty thin. Luckily the installer left all the other strands in place, so doubling up should be do-able.
 
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Old 05-31-16, 05:46 PM
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The stampings on the case say 120V 60 Hz. There are other notations for classes in wet and dry conditions.

Is that helpful?
 
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Old 05-31-16, 05:53 PM
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Your transformers were probably 10 VA* and need to be replaced with a single 20 or 30 VA* transformer.

*A volt-ampere (VA) is the unit used for the apparent power in an electrical circuit, equal to the product of root-mean-square (RMS) voltage and RMS current. Read more at https://www.google.com/url?sa=t&rct=...8kHJ1gLRtt1hUA
 
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Old 06-01-16, 04:57 AM
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I would suspect a short circuit between two wires possibly within one of the CAT5 cables.

Given that each transformer has two low voltage output terminals, it is okay for one terminal of each to be tied together and the wire then goes to the button. From the button the wiring splits off (either at the button or a little further on) to go to the two chimes, respectively.

From each chime a wire goes back to the transformer for that chime respectively. These last two wires must not be tied together anywhere otherwise you can burn out the transformers. Now I still cannot explain why both transformers burned out at or about the same time.
 
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Old 06-01-16, 08:26 AM
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Wiring Diagram for the Broken Doorbell

Name:  Doorbell Wiring Diagram.jpg
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Here's a crude sketch of the low voltage wires as I trace them (the three circles represent wire nuts). It seems that Allan and Ray both suggest the wiring was done incorrectly and that might have caused one transformer to fail, then the second at a later time. Do I have that right? Does this sketch show the problem with connecting the two transformers together that Ray described?

So the solution would appear to be removing both existing transformers and replacing them with a single, larger transformer. Is that the consensus?

I really appreciate the time and trouble you have taken to help me.
 
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Old 06-01-16, 08:41 AM
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Another question: The chime units have a caution stating "Must use 16VAC and 10VA transformer." Does that mean a larger transformer (the 20 or 30 VA that Ray recommends) can't be used?
 
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Old 06-01-16, 08:46 AM
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You must use a 16 volt transformer. The VA rating needs to be increased for multiple chimes. Since they state 10VA is required then the minimum size for the new transformer would be 20 VA. It does not hurt to increase the VA size over the minimum requirements and I personally would use the 30VA transformer or if that were not available then go to a 40VA model.

Also, definitely "double up" on that cat.5 wiring as was previously mentioned.
 
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Old 06-01-16, 10:17 AM
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If both existing transformers are bad then you might as well substitute one larger transformer for the pair.

Even if you get a much oversized transformer, the chimes will not draw more VA than they want.

If the wires are too thin for the distance involved and also you failed to double up (parallel, vt) two wires in the cable for each wire run, then a larger transformer will not pump more VA into the line to make up for the voltage drop that could cause the chime(s) to not sound.

Do not arbitrarily or blithely jack up the voltage (say, select different taps or terminals on a multi voltage transformer) to make up for voltage drop. You would burn out the light in the button and possibly burn out a chime if that is electronic.

It is still possible to use two small transformers if, the diagram above, you break up the junction represented by the left little circle.
Then connect the loose wire end from the top transformer only to the loose wire end for the top chime.
Connect the loose end from the bottom transformer only to the loose end for the bottom chime.
 
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Old 06-01-16, 10:28 AM
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Davids diagram shows both transformers in parallel and both chimes in parallel.
The ONLY proper way to power that circuit is with one transformer.... 20va - 40va in size.
 
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Old 06-01-16, 10:49 AM
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PJ, Allan, Furd, and Ray -

Thank you all, sincerely, for helping me through this issue. You have given me the information and confidence to fix my doorbell despite my first inclination to be overwhelmed and call an electrician.

I just ordered a NuTone C907 16VAC 30VA transformer through Amazon. While waiting for that to arrive I'll double-up the wires for the entire low-voltage wiring diagram.

Once I get the transformer installed and the chimes to operate, I'll sit back with a beer and feel smug and superior to the electrician who installed the system.

Again, thank you all for your help.
 
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