Bistable Relay DPDT (Single Coil)

One stored relay state switches independent 5 V and 3.3 V contact circuits together. Compare the coil currents and both outputs during pulses and hold intervals.

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, or between poles A and B.

VA supplies pole A with 5 V, and VB supplies pole B with 3.3 V. Each pole has separate NO and NC loads. The A loads are 1 kΩ; the B loads are 2.2 kΩ. Both poles follow one stored relay state. The chart shows both NO outputs; the NC branches select the opposite state.

Run and observe

VDRIVE sends +5 V set pulses at 5–10 ms and 45–50 ms, and a −5 V reset pulse at 25–30 ms. Positive coil current sets the relay; reverse current resets it.

Every edge takes 0.1 ms. Each run starts reset with COM connected to NC on both poles. 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.

Expected values

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

The coil 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. The reverse reset threshold is −20 mA.

Selected contacts have 0.1 Ω resistance and open paths have 1 GΩ resistance. NO A approaches 4.9995 V and its load draws about 5 mA. NO B approaches 3.29985 V and its load draws about 1.5 mA. Their different voltages demonstrate electrical separation while their switching times demonstrate shared state.

Try it

• Reduce the positive VDRIVE pulse levels to 2 V in Time/Voltage Pairs. The current approaches only 16 mA, so those pulses cannot set the relay.
• Remove the negative 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.
• Change VB from 3.3 V to 9 V, or change one B load from 2.2 kΩ to 4.7 kΩ. Pole A retains its own voltage and loading while both poles continue changing together.

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.