# Understanding the power to my door bell

Hi guys,

I would like to understand how my outside door bell light works.

Based on what I understand (the outside wire) it's one wire getting interrupted & when pressing the door bell it creates contact resulting in the inside door chime to move and make ding dung sound.

But how does the actual light outside on the bell light up since it's one wire (I.e. Positive wire)? As well as when I measure the voltage on the wire itself I'm getting 24V.

How is that possible I would like to understand that.

Same thing with my thermostat wire, when I measure the voltage I'm getting 24 volts but based on my understanding it's one wire making contact when requesting for heat. (It's heat only zone) so how come I'm getting 24 volts

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@abrahamyaris that should be an AC circuit so no positive wire. What voltage do you get when you measure AC? That is a really interesting question :-)

I'm getting 24vac when I'm putting my multimeter on each of the door bell wires.

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@abrahamyaris , Abraham, are you sure your door bell button only has one wire, I have not seen this before. Check link below as this is what I have always seen for low voltage door bells with a transformer. Normally a lighted button will always be on because the bulb circuit is complete until you press the button. When you press the button it breaks contact with the button/bulb(button light goes out) and this is where the bell contacts come into play, while button pushed the bell rings because the second contacts in button assembly are being used, but there is no light on the button because the internal contact to the button light is broken.

I hope this explains it OK, Good luck.

I hope this helped you out, if so let me know by pressing the helpful button.

http://diyhousehelp.com/how-to/doorbell-...

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How is the light on the bell lightened up?

Doesn't a bulb need to have positive & Negative In order to light up?

The wire that comes from the transformer to the button only provides one I.E. Positive (& than it travelers to the chime when the button is pressed)

@lpfaff1 @abrahamyaris do not forget that this is an AC circuit, same with your thermostat. Let's not focus on the light but on the fact that you measure 24V on the wires of the doorbell switch. I have a couple AC transformers and will see if I can prove it tomorrow so stand by ;-)

Here is how the light stays on when the switch is in the open position ....

http://sloanbooks.com/wiring/utilitech-d...

Look down about 3/4 of the way down the page to see the diagram of a typical lighted doorbell switch.

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That LED will go out when the switch is closed however, as the switch has created a short and all current flows through the switch.

Guys,

You should be getting a DC voltage not AC. This powers interior wired fire alarms and bells. Most circuits that have interrupt switches/ buttons / remotes use DC power. Think everything with a battery and button combination. All LEDs use DC power.

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David, I'm sorry you need to study electronics a bit. There are so many incorrect assumptions here I don't know where to start.

Low Voltage systems are DC, but old school door bells are low voltage AC.

An LED is a diode. You could run an LED from low voltage AC as long as the proper current limitation is there (a resistor). The diode would be on during the positive portion of the waveform and off on the negative swing. In other words it will be on in the forward bias (conducting) when it sees the positive swing of the AC.

In commercial and residential intrusion and fire alarm systems circuits are typically DC.

However, most residential doorbells are 24vac. An ac transformer is simpler and cheaper to make. As you can see the light and the doorbell switch are in parallel with each other. When the switch is idle, current runs through the light, illuminating it, resistance is high through this circuit limiting the current that would pass through the bell, not enough to make it ring. When the button is pushed, the circuit is completed with no resistance through the switch, allowing the bell to draw the current it needs to ring.

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Your door bell is being fed from a step down transformer. Somewhere in your house is a small transformer that is getting 120VAC (primary) and stepping it down (secondary) to a lower AC voltage between (typically) 16 to 24VAC. I have seen the transformers mounted near the breaker panel and sometimes in ceilings and walls. 120 is never wired to the momentary switch (door bell push button), it's a shock hazard.

120VAC > transformer > switch > bell or chime.

When they say one wire interrupted, it's because the switch is in line on the same wire or in series, breaking and making a connection on the send or return wire, it doesn't matter which, you are making and breaking one side of a loop. Not across the send and return, that would create a short when you push the switch.

you are metering voltage at the wires because you have an open circuit. If the switch is wired and you push it closed you will read no voltage because both leads of the meter will be at the same level. If you move one lead to ground then you will read a voltage.

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