# Make a circuit to make sure a device only gets a maximum of 1A

I'm making my own smart home device with a Raspberry Pi. I will power the Pi on the same circuit as our bell. Now the bell transformer is 8V, 1A and the bell itself uses 0.5A.

I'm thinking to replace the bell transformer with a more powerful one, 12V, 2A or so. But is it possible to safely add the bell to the circuit without giving the device too high a current?

I know when I place two devices in parallel the current is lowered and when you place a resistor in series with the devices it will lower it, but is there not a better way to do this?

In the picture of the circuit the AM1 will be the bell and AM2 the circuit for powering the Pi. Is this safe so that AM1 will not be over 1A?

• You are begging for issues if you are going to share a transformer between the bell and the RPi. The bell's transformer is designed to work with such a bell, the voltage will drop dignificantly when the bell is rung but that is OK, it is designed to behave like that. A 12 V 2 A transformer is overkill for an RPi as you need 5 V DC, 2 A for an RPi. So I would use a DC power adapter (USB charger) for that. Coming back to combining the two on the same transformer: chances are that if the bell is rung, the voltage drop and/or spikes on the bell's connections might reset the RPi. Nov 25 at 14:38
• If you want to detect that the bell is rung with the RPi, I would use an optocoupler which will be easy to use with an RPi. Nov 25 at 14:41
• Your circuit drawing doesn't represent how this would behave. The transformer behaves as a voltage source, not a current source like you have drawn. It is not about dividing the currents to the bell and RPi, the bell and RPi need to have the correct voltage. That can be done (for the bell) with a resistor but that resistor needs to be a high power resistor and it will get hot. Also a transformer outputs AC while an RPi can only work from DC. Nov 25 at 14:45
• For the raspberry pi I will place a AC to DC converter. Nov 25 at 14:54

without giving the device a too high current?

You misunderstand electricity.

voltage is pushed, current is pulled


That is, each load takes as little or as much current as it wants. The transformer doesn't force current onto the loads. No. The loads set the current.

Your drawing shows a current source on the left. That is wrong. A transformer is a voltage source (or darn close to it) not a current source.

Is this safe so that AM1 will not be over 1A?

12 V / 200 Ohm = 60 mA. So, yes, the current will be less that 1 A. Way less.

Raspberry pi.

A Raspberry pi is not a resistor! You must add a 12 Vac to 5 Vdc power supply between the transformer and the Raspberry pi to power it.

I suggest you have a look at the Ring ‘Pro’ Power Kit (https://support.ring.com/hc/en-au/articles/212004006-What-is-the-Pro-Power-Kit-) This is installed across the bell coil to provide extra current to the Ring without the bell being activated. While as a stock unit it may not provide enough current for the Pi, it could be reverse-engineered and modified for your purpose.

Even with this bypass, you may still need to consider a battery at the Pi to deal with peak currents.