# Transformers and Electromagnetism

mortymoose

## Homework Statement

An electric doorbell requires a 12 V 60Hz ac supply
a)What turn ratio is required on a transformer to operate from a 120V 60Hz ac supply?
b)Step-up or step-down?
c)If the transformer is connected the wrong way around the circuit what voltage is supplied to the doorbell?

## Homework Equations

Ns/Np =Vs/Vp
Vs>Vp ---stepup
Vs<Vp ---- step down

## The Attempt at a Solution

a) so for this I have the Vs=12V and the Vp=120V... 12V/120V= 0.1 giving a 1:10 ratio
120V is too high for the 12V doorbell so it would need to be connected as a step down and convert its high voltage to a low voltage
b) then that makes this a step-down transformer because Vp is greater than Vs
c) This is the part that confused me a little.. So if it was connected backwards would that turn it into a step-up transformer changing the ratio to 10:1
Vs/Vp=Ns/Np
120V/12V=10turns/1turn
Vs=Vp*(Ns/Np)
Vs=120V(10/1)
Vs=1200V
and then 120*10=1200V supplied to the doorbell? And that would be way to much voltage for the doorbell to handle and damage it?

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cnh1995
Homework Helper
Gold Member
120*10=1200V supplied to the doorbell?
Yes, and that would burn the doorbell.

mortymoose
mortymoose
Yes, and that would burn the doorbell.
Okay thank you!

Merlin3189
Homework Helper
Gold Member
But don't try reversing a power transformer, even without attaching the doorbell.
The low voltage winding will have far too few turns, the core will saturate and large primary current will flow until (very quickly) something burns out (supply fuse or primary winding or connecting wires.)

cnh1995