Calculating voltage - Higher
The ratio of voltages on the transformerAn electrical device that increases, or decreases, the voltage (p.d.) of an alternating current. coils equals the ratio of the numbers of turns on the coils.
The equation can be used to calculate what the output voltage might be from a particular transformerAn electrical device that increases, or decreases, the voltage (p.d.) of an alternating current. , or to work out how to design a transformer to make a particular voltage change.
\(\frac{secondary~voltage}{primary~voltage} = \frac{number~of~turns~on~secondary}{number~of~turns~on~primary}\)
\(\frac{V_s}{V_p} = \frac{N_s}{N_p}\)
This is when:
- Vp = voltage across the primary coil in volts, V (the input voltage);
- Vs = voltage across the secondary coil in volts, V (the output voltage);
- Np = the number of turns on the primary coil;
- Ns = the number of turns on the secondary coil.
In a step-up transformer, VS is GREATER THAN VP.
In a step-down transformer, Vs is LESS THAN Vp.
Question
A 230 V, a.c. transformer, has 11,500 turns on its primary coil and 600 turns on its secondary coil.
Calculate the voltage obtained from the secondary coil.
\(\frac{V_s}{V_p} = \frac{N_s}{N_p}\)
Vp = 230 V
Np = 11,500
Ns = 600
\(\frac{V_s}{230} = \frac{600}{11,500}\)
Vs = 12 V
The output voltage from the secondary coil is 12 V.
This is an example of a step-down transformer.
There are fewer turns on the secondary coil, and there is a smaller voltage across the secondary coil.
Voltage has been stepped-down.