Doorbell camera installation in series on mains

[sairfan1]

I often noticed that door bell cameras are installed with existing wire setup, like removing door bell switch and install camera, and i believe we do not do anything with the bell it stays as is.
I want to know then how does camera bell gets enough current from wire? like how does this work?

[jwet]

The doorbell is a magnetic solenoid that draws real current when it rings- like 200 mA or more.  The doorbell camera has a small battery in it and draws less that 50 mA to trickle charge its battery.  This trickle current isn't enough to pull in the doorbell.  When the doorbell button is pushed, full current goes though the switch and rings the bell.  In normal mode, the camera keeps it battery topped off and when it senses motion, it turns on, does it thing, talks over wifi etc.  It then replenisheds it charge and sits idle.

LCD thermostats often work in a similar way.  They need a little bit of power (very little).  They have a small battery internally, in the past nicads but today more likely an alkaline (non rechargeable)- cadmium is a heavy metal and it didn't make sense to have 100 milliion thermostats entering the waste stream.  They generally draw some current out of the signal loop (like the camera).  They draw low enough power that they can rob some without activating the relay in the furnace.  When they want to call for heat, they short out the lines and the relay closes.  They use their battery while the thermostat is closed to stay litup, etc.  With nest thermostats, etc, this has gotten less common but is still done.

There are a lot of industrial sensors that measure pressure, temps, etc  and return data over an analog two wire current loops, usually 4-20 mA for zero to full scale.  Their circuits run on the 4 ma and they shunt more current as the measured parameter changes.  In industrial, its called "loop powered".  In consumer, it usually called "phantom power".  Just one of the many clever things you can do.

[jwet]

Sairfan- Does this make sense? I can draw a circuit and elaborate a bit otherwise.

[sairfan1]

thanks a lot for detailed answer, helped me to understand.

[al777]

Also, some doorbell cameras are wired and do not have battery (e.g. some Reolink models). Those come with a jumper wire you supposed to install across the existing bell, otherwise camera may not function properly (would be starved of power due to bell winding being inline) and/or the bell may start making funny noises due to camera power consumption having spikes - the bell may want to ring randomly :-) ). The wired-but-with-battery doorbell cameras used to have another trick up their sleeve - they would actually use the existing electromechanical bell to ring when someone at the door pressed the button - those cameras would temporarily switch to battery only power and short the two wires, causing reliable bell ringing. I am not sure if current models still use this design pattern, as most seem to come with their own wall-wart style doorbell which is wirelessly controlled by the doorbell camera, like those Reolink models I have mentioned. The only thing with shorting the old bell is to not forget to do it last, after the old mechanical button is replaced with the doorbell camera. Otherwise one button press and you may need a new doorbell transformer :-). Also, if ever reversing the setup (like when selling the house) - remove the shorting link.

[amyk]

On the other hand, some electronic doorbells will behave strangely with these cameras, since they aren't just a solenoid and draw much less current than mechanical ones.