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How do you choose the right type and size of relay and timer for your project?

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Relay logic and timers are essential components of electrical control systems, but how do you choose the right type and size for your project? In this tutorial, you will learn the basics of relay logic and timers, how to select the appropriate relay and timer for your application, and how to wire them correctly. By the end of this tutorial, you will be able to design and implement simple relay logic and timer circuits for various purposes.

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1 Relay logic basics

Relay logic is a method of controlling electrical devices using electromechanical relays. A relay is a switch that can be turned on or off by applying a voltage to a coil. When the coil is energized, it creates a magnetic field that pulls a movable contact to close or open a circuit. Relay logic uses relays to create logical functions, such as AND, OR, NOT, and timers, by wiring them in different configurations. Relay logic is simple, reliable, and easy to troubleshoot, but it can also be bulky, slow, and consume more power than other methods.

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Matthew Roman

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Relays are slow because they are mechanical devices. They require movement to actually make or break contact. This delay can be up to 200ms per relay. That can stack along a large circuit and drastically affect timing. In addition to this, they are typically an inductive load that will force current back through the previous portions of the circuit. This creates noise, a big problem for low-voltage inputs and information processing.

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2 Timer types and functions

A timer is a device that delays or measures the time between events. Timers are often used in relay logic to create time-dependent functions, such as ON delay, OFF delay, pulse, and interval. There are two main types of timers: mechanical and electronic. Mechanical timers use gears, springs, or pneumatic elements to create time delays. Electronic timers use capacitors, resistors, or integrated circuits to generate timing signals. Mechanical timers are more robust and accurate, but they are also more expensive and require more maintenance. Electronic timers are more compact and flexible, but they are also more sensitive and prone to errors.

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Matthew Roman

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I would also add in counters, those are just as important as timers. For the counter section if you choose to add it. They will typically have a DC supply on the counter that is used to actually count provided you do not have a DC source in your system. I made this mistake my first year of engineering and never forgot that counters and ac signals do not mix. Now I know why, because of zero crossings. But that is another subject.

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Matthew Roman

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You should also include the time base variants, Some timers 1.0 - 5s while others can be 100 to 500ms. You must spec in the timers that you use for a very specific purpose. The play in the settings gives you wiggle room but if you get it wrong you will be buying a new timer.

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3 Relay and timer selection

When choosing the right relay and timer for your project, there are several factors to consider, such as the voltage and current ratings of the coil and contacts, the number and type of contacts, and the mounting and wiring options. Additionally, you should look into the timing range and accuracy to ensure that the timer can provide the desired time delay or measurement with the required precision and repeatability. Furthermore, make sure that the relay and timer can handle the power supply and load without overheating or damaging any components.

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4 Relay and timer wiring

To wire a relay and timer correctly, you must first identify the terminals of the relay and timer. Generally, the coil terminals are marked with A1 and A2, and the contact terminals are marked with numbers or letters such as 11, 12, 14, or NO, NC, COM. Then, you should connect the power supply to the coil terminals, making sure to match the polarity of the coil with the power source or use an alternating current (AC) supply. After that, connect the load to the contact terminals in series with either NO or NC and connect the common terminal to either the power source or ground. Finally, wire the timer according to its function and diagram and connect it to either the relay coil or contact that you want to time, as well as connecting it to the power source if needed.

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5 Relay logic and timer examples

To illustrate how relay logic and timer circuits work, here are some examples of common applications. A motor starter circuit uses a relay to start and stop a motor, while a timer provides an ON delay. A flashing light circuit employs a relay and timer to create a flashing light effect, with the timer configured as a pulse generator. Lastly, a staircase light circuit uses a relay and timer to keep a light on for a certain period after a switch is pressed. The timer provides an OFF delay so that the light stays on for a set time after the switch is released.

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Matthew Roman

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All of your examples can be supplemented by PLC controls, it would be helpful to include ladder diagrams that will help people understand the logic behind the relays/timer/counters.

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How do you choose the right type and size of relay and timer for your project?

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1 Relay logic basics

2 Timer types and functions

3 Relay and timer selection

4 Relay and timer wiring

5 Relay logic and timer examples

Electrical Controls

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