Bistable Relay SPDT (Dual Coil)

Separate positive SET and RESET pulses select a changeover contact. Compare winding currents and the output to see the stored state between pulses.

Read the schematic

K1 is centered, with its coil connections on the left and loaded contact outputs on the right. The sources are grouped below. Matching named net labels connect each source to its winding or common terminal; RETURN labels connect winding returns to the bottom reference bus. The relay has no internal electrical connection between its windings and contacts.

VCONTACT supplies 5 V to COM. Separate 1 kΩ NO and NC loads show the changeover function. The chart shows NO voltage alongside both winding currents; NC is selected whenever NO is off.

Run and observe

VSET sends +5 V pulses at 5–10 ms and 45–50 ms. VRESET sends a +5 V pulse at 25–30 ms. Positive current into SET+ selects NO; positive current into RESET+ selects NC. Each winding has its own return and can be driven without reversing polarity.

Every edge takes 0.1 ms. Each run starts reset with COM connected to NC. After the first set pulse, NO remains selected until the reset pulse. After reset, NC remains selected until the final set pulse. Compare the outputs with coil current: the contact position persists during the long zero-current intervals.

Coil interaction: drive one winding at a time for ordinary operation. Reverse current in a winding does not operate it. If both windings exceed their operating thresholds, this model gives RESET priority. Simultaneous subthreshold RESET current opposes SET drive.

Expected values

RDS and, where fitted, RDR represent 0.1 Ω of series driver resistance. Their coil-side wires provide the current measurement points.

Each winding has 125 Ω resistance and 50 mH inductance, giving L/R = 0.4 ms. Including the series driver resistance, a 5 V pulse approaches 39.97 mA, exceeding the 20 mA operating threshold. SET and RESET currents are measured positive into their + terminals.

Selected contacts have 0.1 Ω resistance and open paths have 1 GΩ resistance. The selected load receives about 4.9995 V and draws about 5 mA. The unselected load stays near 0 V.

Try it

• Reduce the VSET pulse levels to 2 V in Time/Voltage Pairs. The current approaches only 16 mA, so those pulses cannot set the relay.
• Remove the RESET pulse by changing its 5 V entries to 0 V. The set state should persist after the first operating pulse.
• Increase Set current to 30 mA. The 5 V set pulse still works, but switching occurs later in the current rise.
• Increase Set coil inductance to 100 mH. SET current rises and falls more slowly; its settled value stays 40 mA.

About the model

This generic educational relay retains state within a run and initializes reset for every fresh simulation. Its animated symbol follows the simulated latched state. It omits contact bounce, mechanical switching delay, arcing, heating, and magnetic saturation. Values are adjustable model settings rather than manufacturer ratings. The ideal waveform sources absorb returned coil energy.