Wednesday, 27 November 2013

Transformers - voltage & coil ratios.


To investigate how the ratio of number of turns on the primary and secondary coils affects the potential difference across them.


laminated iron C cores and holder
Insulated wire for winding
Power supply (12V a.c.)
Digital Multimeter or a.c. voltmeter
Connecting leads & crocodile clips


Wind each side of the C core with 50 turns of wire.
Connect one side to the a.c. power supply set to 6V - this is the primary coil.
Connect the other side to the multimeter (set to measure a.c. voltage) or the a.c. voltmeter - this is the secondary coil.
Join the C cores using the holder.
Switch on the power supply and record the reading on the meter, then turn off the power supply again. Repeat the reading twice more.

On the side connected to the multimeter UNWIND 5 turns.
Take another three readings for the voltage.
Continue unwinding and taking readings until you have only 5 coils on the secondary side of the transformer.

Risk assessment

Wires may become hot if current is allowed to flow for too long due to low resistance so the power supply should not be left on. If it is left on too long wires should be allowed to cool after the power is shut off before moving the equipment.

There is a risk of electric shock with this experiment.
It is very important to start with the maximum number of turns on the secondary and then unwind it. This prevents any stepping up of the voltage which could cause sparking.
The current will increase as the voltage is stepped down so all changes should be made to the secondary coil without the power being turned on.
The equipment should not be touched when turned on.
Anyone with a heart condition or who has a pacemaker should be especially careful.

Tansformer power measurements

This experiment will allow you to perform a basic investigation of the power transfer & efficiency of a simple transformer.


Power supply (12V a.c )
12V bulb
Variable resistor
Two a.c Voltmeters
Tw a.c Ammeters (multimeters could be used)
Connecting leads with croc clips
Wire for winding (laminated or insulated)
Iron rod 15 - 20 cm long


From the power supply attach the variable resistor and one ammeter in series. Create a primary coil of fifty turns around one end of the iron rod using the winding wire and use croc clips to connect this pimary coil in series with the ammeter and variable resistor back to the power supply. Connect one voltmeter in parallel across the primary coil.

Create a secondary coil at the other end of the iron rod, again using fifty turns. Using crocodile clips connect the bulb and the second ammeter in series with the secondary coil and connect the voltmeter in parallel to teh secondary coil.

With the variable resistor turned completely one way not the readings on all of the meters.

Turn the variable resistor and take readings in five to ten different positions until the variable resistor reaches the other limit of rotation.

Repeat twice more, using the variable resistor to give your the same values for current and voltage on the primary coil each time.


Find the mean readings for each step and then use the equation P = I V to find the power on the primary and the power on the secondary.

Plot a graph of the input and output power and describe any relationships you notice. Why is there a discrepancy in the power between each side?