Basic schematic symbols

Basic schematic symbols

The basic elements of circuits

Automatic switches

Automatic switches are actuated by a thermal, pressure or other element
that will cause a change of state for the switch.

The change of state is usually caused by an increase of temperature or
pressure.

The position of the switch blade either under or above the right contact
point shows us that the switch will either open or close when it rises.

Multiple Choice

Automatic switches are commonly activated by a rise in ?

• Temperature

• Pressure

• voltage

• Both a and b

Thermostat closes on rise ( cooling )

The thermal element is below the switch blade and the blade is below the
contact point on the right which shows us if it rises the switch will close.

Multiple Choice

A cooling thermostat will have what action?

• Open on rise

• Closes on rise

Thermostat Opens on rise (heating)

The thermal element is below the blade of the switch and the blade is
above the contact point on the right side which means on an increase in
temperature the blade will rise up and open the circuit.

Multiple Choice

A heating thermostat will have which action?

• Closes on rise

• Opens on rise

Loads

The loads are the current consuming devices in our diagrams and are
often depicted by a circle with a descriptor or an abbreviation to indicate
to us what they are.

​Compressor

​The compressor is shown as many loads are as a circle with the abbreviation Comp for compressor. In this symbol it shows 2 motor windings going to a common connection on the left side and the other two connections on the right. This shows us there are 2 motor windings which indicates a single phase compressor motor,

Relay contact

Shown here is a normally open relay contact, the contact is open until the relay coil is
energized . As soon as the coil is energized the contact will close and allow current
flow.

Relays and Contactors

Relays and contactors have one or more contacts and a coil
that when energized cause the contacts to change state from
open to closed in the case of normally open contacts.

They are used to control high current loads with a low voltage
control circuit usually a 24 volt coil and a 120 or 240 volt load
circuit.

Relays and Contactors

•Relays usually control loads up to 15 amps of current but not much
more than 15 amps.

•Relays often have normally open contacts but sometimes have
normally closed contacts that open a circuit when energized.

•Contactors usually control loads that are higher than 15 amps of
current.

•Contactors are almost always normally open contacts.

Multiple Choice

Relays usually control loads with a current draw up to ?

• 10 amps

• 5 amps

• 15 amps

• 100 amps

Multiple Choice

If a relay or contactor with normally open contacts has the coil energized the contacts will?

• Open

• Close

• Chatter

• Hum

​Transformers

​Transformers have two coils around a magnetic core. The top coil is the primary and is powered by the line voltage and the bottom coil which is the secondary outputs the low voltage for the control circuit. There is no electrical connection between the coils the secondary voltage is generated by the magnetic field of the primary.

Testing transformers

With an ohm meter and the transformer de energized the resistance test of the primary and secondary should give a valid resistance greater than 0 and not more than a few hundred ohms. If they are open or shorted the transformer has failed.
The other method is to test voltage at the primary and secondary windings. If the primary is energized and 24 volts is not present at the secondary the transformer has failed.

Multiple Choice

If we are testing a transformer that is energized and has 120 volts at the primary what should we read at the secondary winding?

• 120 volts

• 24 amps

• 24 ohms

• 24 volts

Multiple Choice

If we are ohm testing a transformer and the meter reads OL on either winding what does this tell us about its condition?

• The winding is shorted.

• The winding is probably okay

• The winding is open and the transformer has failed

• The winding is open but the transformer will work fine.