Moving 40V AC down to 16 - 24V AC

Thread Starter

mrchao

Joined Jul 13, 2020
2
Hey, Folks-

Hoping to see in anyone has a suggestion on how to take a 40V AC line and convert it to somewhere in the range of 16 - 24V AC.

I’m trying to add a Nest Hello to my mother-in-law’s doorbell intercom system which is on a 40V AC line. She’d like to keep the intercom for now and the speaker/buttons are on different circuits—fed by the same transformer—so I’d like to knock the voltage down to the Nest Hello’s input requirement in the (fairly large) doorbell box.

I do have the option to install a separate transformer for the doorbell circuit but there’s not an elegant way to do that with the way it’s currently wired. Hence I’m trying to see what my options might be in the receptacle at the door.

Thanks!
 

MisterBill2

Joined Jan 23, 2018
7,273
There is a whole lot of information missing here. Where are the 40 volt line supply connections? are they by the door, by the intercom , or by the bell, or someplace else? And what is presently connecter to the outlet by the door.

In short, a much more detailed description of what there is and where it is located.
 

Thread Starter

mrchao

Joined Jul 13, 2020
2
There is a whole lot of information missing here. Where are the 40 volt line supply connections? are they by the door, by the intercom , or by the bell, or someplace else? And what is presently connecter to the outlet by the door.

In short, a much more detailed description of what there is and where it is located.
I didn’t really omit anything that I know.

I’m not in a position to redesign the wiring, or run anything new—the transformer is on the other side of the (large) house—I just have 40V AC coming in to the doorbell button and am trying to figure out what might be an option to bring that down to the device’s range.

So that I’m better informed, could you help me understand how your questions affect the solution?
 

Irving

Joined Jan 30, 2016
1,064
Do you know the VA rating of the 40v transformer and the required VA on the 20v side, I'd be surprised if the 40v AC circuit had that much VA to spare.

However, 40 -> 20 is the same ratio as 220 -> 110.
So a suitably VA rated transformer with split 220/110 primary windings could be pressed into service as an autotransformer. Probably won't be much smaller than sourcing a new 24v transformer though.
 

LesJones

Joined Jan 8, 2017
2,726
You have still not given enough information. You have not said where the door bell is located. If the bell is located near the transformer that supplies the 40 volts AC then (Assuming only twin cable.) then the 40 volts available at the push button is in series with the bell. (So there will be no power available at the push button location while the push button is pressed.

Les.
 

MisterBill2

Joined Jan 23, 2018
7,273
OK, There was also a mention of an intercom back in post#1. That confused the issue a bit.
There is a button at one location, a bell at another location. and a transformer someplace else. Each of those devices has a pair of wires connected and someplace those three pairs are connected in series, since most installations do not have three wires each running between the devices. If that point where all three pairs are tied in series is accessible, and in a location where another item can be added then there is a simple way to change the terminal voltage. If the pair from the transformer and the pair from the button both meet at the bell, then it would be less convenient. And is the intercom part of that setup? So there are the questions.
40 volts is very high for a doorbell system, most of them use 12 to 18 volts. So is there something unusual about this installation? I am wondering if it is in a location with 220 volt mains and somehow a 120 volt primary transformer has been installed. OR, is the 40 volts AC also used by some other system in the building?
 

AlbertHall

Joined Jun 4, 2014
10,550
Do you know the VA rating of the 40v transformer and the required VA on the 20v side, I'd be surprised if the 40v AC circuit had that much VA to spare.

However, 40 -> 20 is the same ratio as 220 -> 110.
So a suitably VA rated transformer with split 220/110 primary windings could be pressed into service as an autotransformer. Probably won't be much smaller than sourcing a new 24v transformer though.
Or a transformer with 20-0-20 secondaries (but be careful to insulate the primary connections).
 

crutschow

Joined Mar 14, 2008
25,645
It would seem you need a transformer, either 40Vac to ≈20Vac or mains voltage to ≈20Vac.
Other ways to reduce AC voltage are likely problematic for your requirements.
 

LesJones

Joined Jan 8, 2017
2,726
I had been assuming that the "Nest Hello" required a direct feed of 16 to 24 volts AC. After looking at the manual (https://cjsperformance.com/wp-conte...17_NestHello_Pro_Installer_Guide_20180416.pdf) (Assuming I have found the correct manual online.) I realise that it picks up power via the current in the loop. I think the two parts of the "Nest Hello" (The bell push/camera and the chime connector.) must have rechargeable batteries inside them that are kept charged via the loop current. The chime connector now switches the current to the chime so the chime will now only by supplied with the 16 to 24 volts so the existing chime may not work. I can't find any current or voltage rating for the switching capacity of the chime connector so I assume it will not be rated to switch 40 volts to the chime. (Even if it was rated for 40 volts I have not worked out a way to connect it with the "Nest Hello " powered from 16 t0 24 volts and the chime from 40 volts.) I think you will also have to replace your chime with one that will work with 16 to 24 volts.
EDIT. NOTE Clicking on the link does not seem to work. You have to copy the URL to your browser.
Les.
 
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MisterBill2

Joined Jan 23, 2018
7,273
Once we learn the interconnect wiring arrangement we will be able to make rational suggestions. Until then folks are just making random guesses. And once again, post #1 calls it a doorbell intercom system. So there is a whole lot that we are not informed about. If there are more than two wires toward the door then there may b options available. But we are not told that, one way or the other.
 
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