High-Performance Power ICs and Hall-Effect Sensors

	Contents
		Introduction
		Input Voltages
		Relay Contacts
		Activating Relays
		Solid State Relays

Relay Contacts
A relay contact that is open when not energized is called a normally open (NO), make, or form A contact. Figure 1 shows the graphic symbol used to depict a Form A contact. On the left is the symbol representing the coil with its two input wires, and on the right is the open contact. The schematic symbol, figure 2, is shown as two open plates that face one another, and will be most commonly seen on circuit diagrams of control cabinets.

If the contact were closed when not energized, and open when energized, the term normally closed (N/C), break or form B contact are used (Figure 3). The contacts are electrically connected with one another, and Figure 4 depicts the contact set that can be interrupted, or opened, when energized.

Some control applications call for a combination of a NO contact with a NC contact, and a common termination for power. The relay changes the state of both the contacts simultaneously, and is referred to as form C, make after break, transfer or changeover contact. The form C contact in Figures 5 and 6 combines the form A and form B contacts.

Any relay description should include the number of contacts. If the relay only has one NO or NC contact, we refer to it as a single-pole, single-throw relay. If there are two sets of NO or NC contacts, the designation is double- pole, single-throw.

The form C contact is a double throw, and depending on the number of contact sets is called a single pole, double throw or double pole, double throw as shown in Figure 7.

Input Voltages

Activating Relays