Coil current can also vary from one system to another. One reason is that some relays are activated by electronic circuits, such as common collector transistors and their equivalent within an integrated circuit. Most of the relays that operate in this fashion, however, are installed on the motherboard already so you don’t have to worry about it.
Most of the decision-making process when it comes to relays involves adding relay modules in order to control multiple loads or providing more than one signal path, or both. To determine coil current using the voltage and resistance specification given by relay manufacturers, you must use Ohm’s Law: Voltage (E) = Current (I) X Resistance (R), or I = E/R.
Output Contact Requirements
Perhaps the most important aspect of relay selection involves the output contacts. Because the list of applications for which auxiliary relays are applied is virtually endless, so are the requirements of the output contacts that technicians use.
An example of where the relay contact specifications are important is in an access control system. Most access control systems of any size employ a centralized power supply, or perhaps a number of centralized power supplies positioned throughout the facility.
Each electrically controlled door lock is powered by these power supplies. An addressable relay module can be placed within each power supply. A nearby SLC is then extended to include each of these relay modules. When the fire alarm system signals these relays of a fire condition via the SLC, each of them will open their NC contacts, thus removing power from these associated electronically-operated door locks.
In the case of a 2.5A-rated power supply, the contacts of the relay must be capable of supplying a current of at least 20-percent more, or 2.5 X 0.20 = 0.5A. The total contact rating is computed by adding these two values together, or 2.5 + 0.5 = 3A. Another way to do this is 2.5 X 1.2 = 3A.
Most relay contacts come with two specifications: AC and DC. Be sure to carefully scrutinize these specifications so you purchase the right relay for your application.