Stepping up and Stepping Down

Power supplies mounted on a correctly sized holder…

I’m building something to make an ancient (1976) telephone more interesting. I’m putting a Raspberry Pi inside it. It’s very important to me that the Pi is able to ring the telephone bell. To do this I need a 35 volt supply. I also need a 5 volt supply too. The device itself will be supplied with 12 volts. The device at the top right of the picture is my “stepper up” which makes 35 volts from 12 volts. The one at the bottom right is my “stepper down” which makes 5 volts from 12 volts. Both of these devices work in the same way. They have a little transformer coil which is driven by the chip you can see on each. The blue trimmers have a little screw you can use to adjust the output voltage.

You might think you could use two copies of the same device and just adjust one to produce 35 volts and the other to make 5 volts but this is not the case. The step-up ones can’t produce a voltage which is lower than the power input (in this case 12 volts) and the step-down ones on the can’t produce a voltage which is higher than the input. You can buy a bunch of these for not much money and it is useful to have both types in stock. Search your favourite suppliers for “buck converter”.