# Differentiation etc.

## Inductors and Transformers

V = L dI/dT

L is the inductance in henries. Putting a voltage source across an inductor causes the current to rise as a ramp.

1 volt across 1 henry produces a current that increases at 1 amp per second.

An inductor is normally formed from a coil of wire which may be wound on a core of magnetic material.

Danger! If you try to stop a current going through an inductor you will generate a high voltage. This will not be enough to kill you (unless it is a very big inductor, but will be enough to kill a transistor (electronic switch) that has the temerity to attempt the feat. Some way of dissipating this energy is required. Even an ordinary mechanical switch will have its life shortened by being expected to switch an inductive load.

For a DC circuit, use a diode clamp circuit to limit the voltage across the switch to 0.6volts more than the supply (but make sure that the supply can take it - with an electronic power supply you may merely be wrecking your power supply), or RC "snubber".

## Transformers

Transformers - multiply an AC voltage by the turns ratio. They are quite efficient - power out is only a little less than power in.

The existence of transformers is the main reason why AC power is used.

Transformers may be used in the first stage of producing low voltage power for electronic circuits.

### Imperfections in Inductors

Inductors are imperfect. For a start the coil has finite resistance, and significant capacitance. Where there is a magnetic core, there are imperfections associated with this. Firstly the magnetic material More seriously, a solid metal core can act as a one turn transformer.

For low-frequency work a core of soft iron is used, which may be laminated. For high

[Incidentally, "core store" uses high hysteresis

Real choos and washing machines use induction motors.

## Switches

Switches are classified in terms of the number of poles and number of throws.

Common types are SPST DPDT SPDT, sometimes with centre-off position.

Use of DPDT switch as reversing switch. Switch tree for selecting power source.

Push button switches.

Note that contacts bounce for about a millisecond after closing. This is not noticable to people, but it is noticable to logic circuits which can respond in nanoseconds.

Switches are rated for current and voltage.

## Relays

These are electro-magnetically operated switches. So the input circuit behaves as an inductor with some loss (the energy required to operate the switch as well as the normal resistance) and the output circuit behaves as a switch.

Relays take milliseconds to operate, and can only manage a few million operations in their lifetime, but they can take a lot of abuse, unlike electronic switches which can die from a very brief overvoltage.

• They operate slowly by electronic standards - taking tens of milliseconds.
• The input is electrically isolated from the output, a property that is hard to achieve electronically (though it can be done readily by optoelectronics)
• One input can operate several pairs of contacts. DPDT is common, 4PDT and other configurations are available.
• In common with all mechanical switches, the contacts bounce.