An_Introduction_to_Linear_AC-DC_Power_Supplies

1. By Solid State Workshop
An introduction to
Linear AC-DC Power Supplies

2. What is an AC-DC power supply?
● An AC-DC power supply converts AC
electricity from a wall outlet into DC
electricity that a sensitive electronic
device can use.

3. Desktop PC Power
Supply
Laptop Power
Adapter
Phone Charger
Examples of AC-DC Power Supplies

4. Using an Oscilloscope
● An oscilloscope is
an instrument which
allows us to view
changes in voltage
over time.

5. Using an Oscilloscope
● Time is on the x-axis.
● Voltage is on the
y-axis.
● Each horizontal or
vertical line is an
increment of either
unit.
Time
+ Voltage
- Voltage

6. Direct Current (DC)
1.5𝑉
∆𝑡 = 0𝑠
● The direction of
current is always
the same because
the voltage always
remains greater
than 0.
1.5𝑉
∆𝑡 = 3𝑠
1.5𝑉
∆𝑡 = 7𝑠

7. 0𝑉
∆𝑡 = 1𝑠
2𝑉
∆𝑡 = 0𝑠
−2𝑉
∆𝑡 = 2𝑠
Alternating Current (AC)
●The direction of
current is constantly
changing because
the voltage is
constantly passing
through 0V.
(+ to -, - to +, etc.)

8. The First Step
● The voltage available at a wall outlet is
much too high and is always changing
polarity, making it useless for sensitive
electronics.
● The first step is to reduce the voltage using
a transformer.

9. ● A transformer is a device that allows us to
convert an AC voltage to a higher or lower
level.
● Transformers can only be used with AC.
● They operate on the basis of an
electromagnetic theory called mutual
inductance.
Transformers

10. 120 VAC
60 VAC
2 : 1
Transformer Operation

11. Effect of a Transformer
● This is the effect of
a step-down
transformer.
● Note that only the
amplitude (the
voltage) has been
reduced, not the
frequency.

12. Examples of Transformers
High Frequency
E-Core Toroidal

13. The Second Step
● Though we have successfully reduced the
voltage, we still have the problem of the
voltage fluctuating from positive to negative.
● To “force” the current to only flow in one
direction we construct a bridge rectifier
circuit, using 4 diodes.

14. The Bridge Rectifier
From
transformer

15. Effect of a Bridge Rectifier
● A bridge rectifier
basically takes the
absolute value of the
AC waveform.
● Now, instead of the
current constantly
reversing, it now has
only one direction.

16. Examples of Bridge Rectifiers
High power bridge
4 discrete diodes GBU-type bridge

17. The Third Step
● The fluctuation in voltage can have ill effects on the
circuit at hand. Semiconductor devices have minimum
voltages needed to turn them on. (Typ. 0.7V)
● In the current waveform, we can observe that the
voltage constantly dips to very low voltages (including
0V), which is problematic.
● To fix this problem, we use filter capacitors.

18. From
transformer
The Filter Capacitor

19. Effect of a Filter Capacitor
● As the capacitor
discharges its voltage
also decreases.
● By using a higher
capacitance, the
capacitor can provide
a greater voltage for a
longer time.

20. Examples of Filter Capacitors
Solid Polymer
Electrolytic Bulk
(Electrolytic)

21. Looking back
● This power supply is called an unregulated power
supply.
● The voltage it produces depends on the demand of
the load and the capacitance provided.
● Also, the output voltage is highly dependent on the
input voltage across the transformer.

22. The Fourth Step
● Ideally, we want the output voltage of the power
supply to remain constant regardless of the load, and
fluctuations from the voltage source.
● To accomplish this, we can use integrated circuit
called a “voltage regulator” which does exactly what
its name implies.

23. From
transformer
The Voltage Regulator
Voltage
Regulator
out

24. The Voltage Regulator
Voltage
Regulator
in out
gnd
out
Vref
Out
in
gnd
Base
Emitter
Collector

25. Effect of a Voltage Regulator
● Let’s say we need a
steady 5.0V supply
● The voltage regulator
will take care of any
fluctuations in voltage,
provided the input
voltage never dips below
the desired output
voltage (Vout + 2V or so).

26. Examples of Voltage Regulators
TO-92
TO-220 TO-3

27. Examples of Voltage Regulators
Some part names of common
voltage regulators:
-LM78xx -LM723
-LM317 -L200
-LM338 -LT1085

28. Trans-
former
Bridge
Rectifier
Filter
Capacitor
Voltage
Regulator
Mains
Input
Basic Power Supply Topology

29. I’d love to hear your feedback!
Thanks for watching!