Introduction: The drive circuit for a 100A latching relay changes with coil topology, because one winding needs polarity reversal while two windings use separate set and reset commands.
Junior technicians often see two latching relay variants with the same 100A contact rating but different coil terminals. The contact side may look identical, yet the control side decides how the driver board is wired, how many PLC outputs are used, and whether an H-bridge is needed. For a 100A latching relay, this difference matters because the coil still draws a pulse current even though it holds state without continuous power. this guide explains how single coil and dual coil drive circuits are organized, using YC602 published coil data as a practical example.
How Single Coil Latching Relays Reverse Polarity to Change State
A single coil latching relay has one coil. To set the contacts, current flows one way. To reset them, current flows the other way. The relay itself is bistable, so it stays in position after the pulse ends, but the coil needs a driver that can reverse the direction. In practice, that means an H-bridge or a dual-polarity driver, not a simple low-side switch. The H-bridge applies voltage across the coil in one direction for the set pulse, then reverses the voltage for the reset pulse. For a 100A relay, the coil is not a low-power signal coil. YC602 lists 3W rated coil power, and its single coil data includes 12V at about 235mA and 24V at about 117mA. The driver must handle that pulse current cleanly. The H-bridge also changes the control logic. A microcontroller or PLC does not usually connect directly to the coil. Instead, it drives transistor gates or a relay driver IC. The firmware needs a direction signal plus an enable or pulse command. If both high-side and low-side devices turn on at the same time, the bridge can short the supply. The payoff is simpler coil wiring: only two coil terminals are needed, and the same winding handles both states. TI application material on pulse driving and energy-efficient coil control supports this approach for latching coils, where a short controlled pulse is better than continuous coil power. YC602 published data also includes a minimum pulse and a 70% operate voltage, which helps a driver board size its supply and switching stage.
What Changes When a Latching Relay Uses One Coil Instead of Two
Moving from a dual coil design to a single coil design is not just a terminal-count change. It changes the driver board, the control firmware, the wiring harness, and the way a technician tests the relay during commissioning. The contact side can still switch the same 100A load, but the coil side behaves differently. The list below shows the practical differences that matter on a 100A switching circuit.
- Coil terminals and wiring: A single coil relay uses one winding, so the two coil wires must carry current in either direction. A dual coil relay uses two separate windings, usually one for set and one for reset, and each winding has a fixed polarity. Dual coil wiring needs more conductors, but the terminal labels are more direct.
- Driver circuit: A single coil needs an H-bridge because current direction must reverse. A dual coil needs two independent drivers, often two low-side stages, because each coil works in one direction. Both must handle pulse current; YC602 dual coil data lists 12V at about 231mA with two 26Ω windings.
- Control logic and I/O: A dual coil relay maps naturally to two PLC or MCU outputs, one for set and one for reset. A single coil relay needs a direction input plus a pulse or enable signal. That can save I/O in some designs and use more in others, depending on how the control board is organized.
- Testing and fault behavior: With two coils, a technician can pulse set and reset separately and see which side responds. With one coil, a failed bridge or a reversed driver connection can affect both states. Dual coil designs add more driver components and PCB space, while single coil designs concentrate the switching function in one bridge.
How Dual Coil Latching Relays Separate Set and Reset Commands
A dual coil latching relay uses two windings inside the same package. One winding is pulsed to set the contacts, and the other is pulsed to reset them. The polarity across each winding stays fixed, so the driver does not need to reverse current. This is why dual coil drive circuits often look like two separate outputs rather than one bridge. Each output can be a transistor, a MOSFET, or a driver channel that connects one end of a coil to ground while the other end sits at the supply rail. The control board sends a pulse to the set coil or the reset coil, then turns that output off. The separation is useful in industrial control panels where set and reset commands come from different logic conditions. For example, a PLC might use one output for an “on” command and another for an “off” command, with neither output needing to know the polarity of the other. The relay still holds its state after the pulse, so neither coil needs continuous current. YC602 supports both single coil and dual coil configurations. Its published coil data covers 6V, 9V, 12V, and 24V DC, with separate current and resistance values for each topology. The 48V DC option appears in the ordering information, but the detailed coil current and resistance are not shown in the same data table, so the exact 48V winding values should be checked with the relay supplier before finalizing a driver design. Dual coil does not automatically mean better. It uses more coil terminals and more driver channels. Single coil saves winding and terminal count but requires a bridge and careful polarity control. On a 100A relay, the contact rating is the same for both; the choice is about the control side. A driver board that already has an H-bridge may favor single coil. A board with spare low-side outputs and simple firmware may favor dual coil. The best fit depends on the controller, available I/O, and how the panel is wired. Microchip and NXP application material describe pulse-actuated latching relay drive circuits in general terms, which supports this comparison without tying either topology to a universal recommendation.
Conclusion
Single coil and dual coil latching relays do the same job on the contact side: they switch a 100A load and hold position without continuous coil power. The difference is on the drive side. A single coil relay needs current reversal, usually through an H-bridge, so the control logic must manage direction. A dual coil relay separates set and reset into two windings, so two independent outputs can drive it without polarity reversal. Neither topology is universally better. YC602 supports both configurations, and its published coil table gives 6V, 9V, 12V, and 24V values that help match the driver to the available supply. For a final design, the exact variant, wiring diagram, and 48V coil details should be checked against the published product information and the relay supplier’s engineering data.
FAQ
Q：What is the main difference between a single coil and a dual coil latching relay?
A：A single coil latching relay uses one winding and changes state when the current direction reverses. A dual coil latching relay uses two separate windings, usually one for set and one for reset, so each command has its own coil and fixed polarity. The contact side can be identical, but the drive circuit and control logic differ.
Q：Does a single coil latching relay require polarity reversal to switch states?
A：Yes. A single coil latching relay needs the coil current to flow in one direction for set and the opposite direction for reset. That is why the driver is usually an H-bridge or another bipolar circuit. Without polarity reversal, the same coil cannot produce both set and reset actions.
Q：Can the YC602 100A latching relay be driven with either single coil or dual coil configuration?
A：Yes. The YC602 is listed with both single coil and dual coil options. Published coil data covers 6V, 9V, 12V, and 24V DC with current and resistance values for each topology. The 48V DC option appears in the ordering information, but its detailed coil current and resistance are not shown in the same table, so the exact 48V variant should be checked with the supplier.
Sources / References
Energy Efficient Relay Drivers and Coil Pulse Management
Smart Energy and Prepayment Disconnect Relays
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