Power transformer question

[mikefamig]

I had a recent experience that got me thinking about power transformers and how they behave when you apply a load to them while they are energized.
 
I think that I may have damaged a video doorbell button while installing it. It was working fine until I removed the power leads from the 16vac transformer and then reconnected them. After that the unit would reboot every time you ring the doorbell. Lucky for me that Nutone has very good support and was happy to replace the device.
 
My question for those of you who understand AC circuitry - how does current behave when you first apply a load to a transformer? Is there a surge of current as the coil reacts to the load that may have damaged the doorbell?
 
Mike.
 
 

 

[RAL]

I don't think there would be any surge when you connected the doorbell to the transformer.
 
A transformer is an inductor, and inductors resist changes in current.  Before you connect the load, the output voltage is 16VAC, and the current in zero.  At the moment you connect the load, the voltage would drop, because Ohm's law must hold. With zero current, the voltage across the load must also be zero. Thus, the load would pull the voltage down. That is just for an instant, though, and then over the next few microseconds/milliseconds, the current will gradually increase and the voltage will rise back up again. 

 

[BLH]

Some transformers when not loaded. Can have a higher voltage output than when loaded.
So it is possible the 16V was higher. When reconnected.

 

[mikefamig]

There is a diode installed across the chime terminals  in the doorbell circuit as part of the camera installation in order to complete the circuit to deliver current to the doorbell button/camera even when the doorbell button is not pressed. It appears to create a path for a small amount of current to travel to power the button without activating the chime magnets?. Look at step two in the following link.
 
http://www.nutone.com/docs/knock-video-doorbell/dcam-quick-start-guide.pdf
 
I don't feel that I explained this well but the link might help. I have read about inductance when studying transistor radio tuner circuits but have trouble understanding it. This leads me to think that the transformer could have damaged the doorbell camera circuit.
 
Mike.

 

[RAL]

I'm not sure of the primary purpose of the diode.   Without it, the camera would draw current through the solenoid's coil, and if that current is large enough, it could cause the solenoid to buzz when the doorbell button is not pressed.  By placing the diode across the coil, that would allow the current to bypass the coil on one half of the AC waveform.  That might be enough to keep the solenoid from buzzing.
 
Another reason to place the diode there might be to make the solenoid run on 1/2 wave DC.  When the doorbell button stops being pressed, that would normally result in a voltage spike due to the collapsing magnetic field.  Diodes are usually used to suppress those spikes.  Reducing those spikes would help protect the camera circuitry. Though I would expect a good design to include some MOVs in the camera for that purpose.
 
Since the installation instructions say "it is recommended to turn off power," they don't seem overly worried about damaging the camera if power isn't shut off.  If they were, I'd expect them to say you MUST turn off power.

 

[mikefamig]

RAL
 
Thanks. It makes sense that the diode is there to complete the circuit and deliver half wave dc pulse to power the electronic doorbell button bypassing the relatively large resistance in the chime coils.I hadn't thought about the current causing the coils to hum.
 
I have noticed that the chime doesn't ring as loud and clear with the diode as it does without the diode but is sounds good.
 
I feel better that you don't think that removing and re-attaching the transformer would harm the cam. It ran for a few days and when I changed the transformer for a new one the camera started malfunctioning. Maybe just coincidence.
 
I'm happy to say that Nutone was very good about it and tech support was very helpful. It's a neat gadget now that it's it's working.
 
Mike.

 

[RAL]

I have noticed that the chime doesn't ring as loud and clear with the diode as it does without the diode but is sounds good.

 
I was going to ask you about that... with the solenoid only getting power for 1/2 of the AC cycle, it's not going to strike the chime with as much force.  I wondered if it was noticeable.

 

[mikefamig]

It is a compromise but it allows the device to work without batteries which is the main reason that I chose this one. That and the fact that it woks with the mechanical chime. I prefer the old fashioned ding-dong over an electronic sound.
 
Mike.