Current
There are two types of current……
Direct current (dc) in which the current travels in one direction only
Alternating current (ac) in which the current alternates
(changes direction) 50 times a second
Transformers – How they work
Iron core
Primary coil
Secondary coil
An alternating current in the primary coil
Induces a magnetic field through the core which…
Induces an ac current in the secondary coil
Transformers – changing voltage &
current
Primary
side
Secondary
side
More turns on the secondary coil
means it is a step up transformer
50 turns
100 turns
VOLTAGE UP, CURRENT DOWN
More turns on the Primary coil
means it is a step down transformer
VOLTAGE DOWN, CURRENT UP
100 turns
50 turns
Same number of turns on each side
Means……..
It is an isolating transformer.
50 turns
50 turns
Example would be a shaver. If water
gets on the mains side it stops you
getting a shock as the wires aren't
touching
The live parts will not touch the
Earth so a large current cant flow
through you
Calculations – easy if you follow the steps
You are always given three of the four variables
Vp = 12V
Vs = ?
Primary
side
Secondary
side
Tp50 =turns
Ts150 =turns
1. Voltages one side, number of turns on the
other
2.
Put in the numbers
Vs
12
=
Vs
=
Vp
150
150 x 12
50
50
Ts
Tp
=
36V
This time you need to find VP
You are always given three of the four variables
Vp = ?
Vs = 12
Primary
side
Secondary
side
Tp50 =turns
Ts150 =turns
1. This time you put the primary side on the top
Vp
=
Vs
2.
Put in the numbers
Vp
12
=
50
50 x 12
150
150
Tp
Ts
=
4V
Transformers and the National Grid
The Grid transmits electricity throughout the country at a high voltage
This is because it results in a low current which reduces the energy
loss as heat
The power loss as heat can be found using the formula P = I2 R
Where R = resistance in ohms
Diodes & Capacitors
A diode changes alternating current into direct current
?
A.C turns into ………
This is called half wave rectification
This wastes half the current
Four diodes connected the correct way result in full wave rectification
Charging a capacitor
+
-
+
-
M
Charge can be stored on
a capacitor
The capacitor has a
voltage across it
When connected the
capacitor discharges
The current will be large
at first but drop quickly
Because the voltage across
the capacitor drops
Capacitors and diodes in a circuit together
What do they do??????????
Smooooooooooth out the current