1

DC power supplies

CHM6158C - Lecture 13

Notes from Teach Yourself Electricity and Electronics, by Stan Gibilisco

2

Converting AC to DC

On what principle does a transformer operate?

3

Inductance

P

S

P

S

NN

VV

=

No. of coils

What about the power?P = IV

Unit: Henry

sC

sJsCsCJ

AsVH ⋅Ω=

⋅=

⋅=

⋅= 2

2

//

Many ways to express this unit

1 H = rate of change of current in a circuit is one Ampere per second, then the resulting electromotive force is one volt

(Self-)Inductance = property in an electrical circuit, where a change in the electrical current through the circuit induces an electromotive force that opposes the change in current

Due to creation of magnetic field

4

Rectifier circuits

More efficient, since current goes through same secondary circuit.

Disadvantage: 4 diodes, which increases ……….

“Center-tap”

“Full-wave bridge”

5

Voltage doubler circuitNotice number of winds

Large capacitors and low resistors are used

To build up high DC voltage

To prevent diodes from burning out

Concern that diodes cannot handle high currents

6

Filters

Remember RC circuit to filter voltage?

What would the voltage profile look like?

7

Inductance instead of resistance

For a minimum of ripple:Also works because self-inductance in coil acts as resistance

8

Surge current

High spikes in ac can be suppressed by capacitor: e.g. due to thunderstorm

9

Safety considerations

Fuses

Current-limited: e.g. 13A

10

“Bleeder” resistor

Build up large voltages in power supplies – need to discharge