When you look at a schematic diagram a double pole throw relay (commonly known as a DPDT relay), you are looking at two completely isolated circuits mapped onto a single physical component. The control side (the electromagnetic coil) operates independently from the load side (two separate sets of changeover contacts). Understanding this galvanic isolation is the key to safely switching high-voltage or high-current loads using low-voltage microcontrollers or PLCs.
A standard DPDT relay schematic features a coil symbol and two sets of contact symbols (Common, Normally Open, and Normally Closed). In this guide, we will break down the IEC 60617 schematic symbols, map the physical pinouts, detail the critical load derating matrices, and cover bench-testing procedures to verify your circuit before applying mains power.
Decoding the Schematic: Coil vs. Contact Side Wiring
The schematic is divided into two distinct zones. The coil side is your control circuit, typically represented by a rectangle labeled with the coil voltage (e.g., 24V DC) and pins A1/A2 (or 7/8 on an 8-pin octal/flat base). The contact side represents your load paths, shown as two parallel switch mechanisms. Each 'pole' has a Common (COM) terminal that physically moves between a Normally Closed (NC) and Normally Open (NO) terminal when the coil is energized.
For a standard 8-pin DPDT relay (like the ubiquitous Omron MY2 or Finder 55.32 series), the physical pin mapping translates to the schematic as follows:
- Coil: Pins 7 (Positive/Hot) and 8 (Negative/Neutral)
- Pole 1: Pin 1 (COM), Pin 2 (NC), Pin 3 (NO)
- Pole 2: Pin 4 (COM), Pin 5 (NC), Pin 6 (NO)
Relay Rating Table: Which Column Governs Your Load?
A common beginner mistake is looking at the '10A' printed on the relay cover and assuming it can safely switch any 10A load. The contact rating on a schematic or datasheet is actually a matrix. Which rating column governs your specific load depends entirely on the load's power factor and inrush characteristics. Inductive and motor loads generate severe arcing upon contact opening, requiring massive derating compared to purely resistive loads.
Below is the rating matrix for a standard industrial DPDT relay (e.g., Omron MY2N DC24, typically $6 to $12 in 2026 pricing):
| Parameter | Specification | Notes / Conditions |
|---|---|---|
| Coil Voltage | 24V DC | Must operate between 80% and 110% of nominal (19.2V - 26.4V) |
| Coil Resistance | 650 Ω | Draws ~36mA; easily driven by standard logic-level MOSFETs |
| Contact Rating (Resistive) | 10A @ 250V AC / 24V DC | Applies ONLY to heaters, incandescent bulbs, and pure resistance |
| Contact Rating (Inductive) | 3A @ 250V AC / 24V DC | Applies to solenoids, contactor coils, and transformers (cos φ = 0.4) |
| Breaking Capacity (Max) | 10A @ 250V AC | Absolute maximum fault current the contacts can safely interrupt |
Selection Decision Path by Load Type
Use this decision tree to determine which column governs your application and how to size your relay:
| Load Type | Examples | Governing Column | Derating Factor & Action |
|---|---|---|---|
| Resistive | Space heaters, dummy loads | Resistive Rating | 1:1. Use nominal rating (e.g., 10A). |
| Inductive (AC) | AC contactor coils, solenoids | Inductive Rating | Derate to 30%. A 10A relay handles ~3A. Add an RC snubber across the load. |
| Motor (AC/DC) | Fans, pumps, conveyors | Motor / Inductive Rating | Derate to 20-30%. Motors draw 5x-7x Locked Rotor Amps (LRA) on startup. Size relay for LRA, not FLA. |
| Tungsten / LED Drivers | Incandescent arrays, LED PSUs | Inrush / Tungsten Rating | Derate to 10-15%. Massive cold-filament inrush can micro-weld contacts on the first closure. |
Bench Testing: Dead and Live Verification
Before wiring a DPDT relay into a live panel, you must verify its mechanical and electrical integrity on the bench. Here is the exact sequence using a standard digital multimeter (DMM).
1. Dead Testing (De-energized)
Set your DMM to Resistance (Ω) or Continuity mode.
- Coil Check: Place probes on pins 7 and 8. You should read the nominal coil resistance (e.g., ~650Ω for a 24V DC coil). A reading of 'OL' (Open Loop) means the internal fine wire is snapped; the relay is dead.
- NC Contact Check: Place probes on COM (Pin 1) and NC (Pin 2). You should read less than 0.5Ω. Repeat for Pole 2 (Pins 4 and 5).
- NO Contact Check: Place probes on COM (Pin 1) and NO (Pin 3). The meter must read 'OL'. If it reads continuity, the contacts are micro-welded shut from a previous overload.
2. Live Testing (Energized)
Apply the rated coil voltage (e.g., 24V DC to pins 7 and 8). You should hear a distinct mechanical click.
- Contact Transition: With power applied, COM-to-NC should now read 'OL', and COM-to-NO should read less than 0.5Ω.
- Voltage Drop Test (Under Load): Pass your actual load current through the NO contacts. Set your DMM to DC/AC Volts (mV range) and place the probes directly on the COM and NO solder terminals. A healthy contact will show a voltage drop of less than 50mV. If you read >100mV, the contacts are pitted, oxidized, or failing, and will overheat under continuous load.
When to Repair vs. Replace
For standard PCB, DIN-rail, or octal DPDT relays (under 15A), always replace. Never attempt to file, sand, or burnish the silver-alloy contacts. Filing removes the thin cadmium or tin oxide suppression layer, leading to immediate micro-welding upon the next use. Furthermore, opening a sealed relay case destroys its arc-quenching atmosphere. Repair (such as replacing arc chutes or main contact tips) is strictly reserved for heavy-duty industrial contactors rated above 50A.
Frequently Asked Questions
What is the difference between a double pole single throw and double pole double throw schematic?
A Double Pole Single Throw (DPST) relay acts like two standard light switches on one yoke; it only has COM and NO (or COM and NC) terminals, meaning it can only turn two circuits on or off. A DPDT relay adds the 'double throw' mechanism, providing both NO and NC terminals for each pole. This allows the DPDT to route power between two different paths, making it essential for applications like motor reversing or transfer switching.
Why does my DPDT relay schematic show a diode across the coil?
That is a flyback (or freewheeling) diode. It is mandatory on DC-driven coils to protect the switching semiconductor. When the driving transistor turns off, the relay coil acts as an inductor trying to maintain current flow, generating a massive reverse voltage spike. The diode provides a safe, localized loop for this stored magnetic energy to dissipate as heat, clamping the spike to roughly 0.7V above the supply rail.
Can I use a DPDT relay to reverse a DC motor direction?
Yes, this is a classic DPDT application. Wire your DC power supply's positive to one COM and negative to the other COM. Then, cross-wire the NO and NC terminals to the two motor leads (COM1-NO to Motor+, COM2-NC to Motor+; COM1-NC to Motor-, COM2-NO to Motor-). Energizing the coil swaps the polarity across the motor, reversing its direction. Ensure the relay's contact rating exceeds the motor's stall current.
How do fuses and breakers interact with relay contact ratings?
You cannot treat fuses and miniature circuit breakers (MCBs) as interchangeable when protecting relay contacts. A relay's 'Breaking Capacity' dictates the maximum fault current it can interrupt without welding. If a short circuit occurs, a standard Type C MCB requires 5 to 10 times its rated current to trigger the magnetic trip (e.g., a 10A breaker might not trip until 50A-100A). During the milliseconds it takes for the breaker to clear, a 50A fault will instantly weld a 10A relay's contacts shut, causing a fire hazard. Fuses, however, have a specific I²t let-through energy curve and clear high-magnitude faults in microseconds. Therefore, relay contacts must always be backed up by a fast-acting fuse sized below the relay's breaking capacity, or an MCB with a verified let-through energy lower than the relay's withstand rating. For deeper reading on protective device coordination, refer to the International Electrotechnical Commission (IEC) standards on low-voltage switchgear and All About Circuits' guide on relay circuits.
