A relay wiring diagram shows how the coil and its contact sets connect to a control circuit and a load. To read it, identify the coil terminals, the contact configuration (common, normally open, normally closed), and how the control path and the load path are wired together.
Reading relay diagrams involves recognizing standard symbols, terminal numbers, and the form of the contact arrangement. This guide explains the common symbols, how to interpret coil and contact sections, and provides practical examples to help you troubleshoot or design circuits safely.
Key symbols and terms
Before you can interpret a diagram, familiarize yourself with the most frequent symbols and terminology you will encounter. The list below summarizes what you will typically see on relay diagrams.
- Coil: The input that, when energized, changes the state of the contacts. It is usually labeled with terminal numbers like 85/86 on DC coils.
- Common (C): The terminal that connects to either NO or NC when the coil is energized or de-energized.
- Normally Open (NO): The contact that is open (disconnected) when the coil is de-energized and closes when the coil is energized.
- Normally Closed (NC): The contact that is closed (connected) when the coil is de-energized and opens when the coil is energized.
- Form A, Form B, Form C (contact configurations): Common naming for SPST (Form A or B), SPDT (Form C), and more complex arrangements like DPDT.
- SPST, SPDT, DPDT: Single Pole Single Throw, Single Pole Double Throw, and Double Pole Double Throw, indicating how many independent circuits the relay can switch and how many throw options exist for each.
- Terminal numbers (examples): 85/86 for coil, 30 for common, 87 NO, 87a NC on many automotive and industrial relays; other formats exist for non-automotive relays.
- Electromagnetic vs Solid-State Relays (SSR): Electromagnetic relays have physical contacts; SSRs use semiconductor switches and opto-isolation, which affects leakage current and switching behavior.
- Diode or snubber across the coil: A protection device in DC coils to suppress voltage spikes when the coil is de-energized.
Understanding these terms helps you quickly identify how the relay will behave when the coil is energized and which circuit paths will be opened or closed.
How to read the coil and contact sections
Follow these steps to interpret the coil and contact portions of a relay diagram. The list below outlines a practical sequence you can apply to most diagrams.
- Identify the coil terminals and their supply voltage. Look for the coil symbol and terminal numbers (commonly 85/86 in DC coils). Determine how the control signal energizes the coil.
- Locate the contact sets and note the terminal numbers for each set (for example, 30 as common, 87 NO, 87a NC in SPDT relays).
- Determine the type of contact configuration (NO, NC, or both). Check whether the contact path is closed when the coil is de-energized (NC) or open (NO).
- Trace the control (coil) path separately from the load (switching) path. The coil circuit should be isolated from the load circuit, except for the drive signal that energizes the coil.
- Look for protective components such as a diode across the coil in DC circuits or snubber networks to prevent voltage spikes.
- Note any multiple contact sets (SPDT, DPDT, etc.) and how they are wired in parallel or series with different loads.
- If the diagram includes a solid-state relay (SSR), identify input and output sides, and be aware that the output leakage and response differ from mechanical relays.
Interpreting these elements will let you determine how energizing the coil affects the connections between common and the NO/NC contacts, and how the load is controlled.
Interpreting common relay configurations
Different configurations offer different control possibilities. The following list highlights typical relay arrangements you’re likely to encounter, and what they enable in a circuit.
- SPST (Single Pole Single Throw) – one switchable circuit, normally open or normally closed. Simple on/off control of a single load.
- SPDT (Single Pole Double Throw) – one common terminal that can connect to NO or NC; provides a simple changeover function, useful for selecting between two loads or directions.
- DPDT (Double Pole Double Throw) – two independent SPDT circuits operated by a single coil; can switch two loads or be used to reverse polarity in a motor control scenario.
- Form C vs Form A/B terminology – Form C is a SPDT arrangement (one common with both NO and NC contacts), Form A is NO, Form B is NC in SPST terms.
Knowing the configuration helps you map a diagram to a real circuit and understand how energizing the coil will affect multiple load paths simultaneously.
Practical examples
Example 1: Simple light circuit controlled by a relay
In a basic setup, a relay coil is energized by a control switch, and the relay’s NO contact powers a light. The coil terminals (for example 85 and 86) connect to a 12 V control circuit, with one side switched to ground and the other side connected to +12 V through the control switch. The relay contact common (30) is connected to +12 V, and the normally open contact (87) feeds the light, which then returns to ground. A diode across the coil protects against voltage spikes.
Example 2: Motor control using a DPDT relay
A small DC motor is controlled by a DPDT relay to allow forward and reverse operation. The coil terminals energize from a control circuit (e.g., 12 V). The DPDT contact arrangement provides two switchable paths: one pair connects the motor terminals in one polarity (forward) and the other pair reverses polarity (reverse) when the coil is energized, enabling bidirectional rotation. In practice, one side of the motor may be permanently grounded while the other side is driven through the relay contacts. As with DC coils, include a flyback diode or a snubber across the coil for protection.
Safety and testing tips
Working with relays requires basic electrical safety and careful verification. Use these guidelines to test and troubleshoot diagrams safely.
- Always de-energize the circuit before inspecting or modifying wiring to avoid shocks or unintended actuation.
- Verify coil resistance with a multimeter to confirm you have the correct coil type and voltage rating.
- Test the control path and the load path separately to ensure the coil energizes reliably and the contact set switches as drawn.
- Check for protective components like flyback diodes on DC coils; ensure proper orientation to avoid short circuits or damage.
- When dealing with SSRs, be aware of leakage current and slower switching characteristics compared to mechanical relays.
Following these practices will help you interpret diagrams accurately and avoid miswiring or component damage.
Summary
Relays act as controllable switches with coil-driven contacts. By identifying the coil terminals, recognizing common/NO/NC contacts, understanding the form (SPST, SPDT, DPDT), and tracing control versus load paths, you can read most relay wiring diagrams with confidence. Real-world diagrams may include protective components, multiple contact sets, and, in the case of SSRs, different switching behavior. Practice with a few simple circuits, and you’ll be able to diagnose and design reliable relay-based controls.