Solenoid Turn-Off: Check Clamp Voltage and Release Time

A solenoid turn-off test should establish three separate results: the switching electronics stay within their limits, coil current decays as intended, and the mechanism reaches its required released state in time. A flyback diode can limit the voltage across a switch while allowing current to circulate long enough to delay the movement the product needs.

For a founder reviewing a custom actuator, “the output turns off” is therefore an incomplete acceptance statement. Ask what was measured at the coil and at the moving part. This guide concerns a safeguarded, isolated low-voltage DC development setup. It is not a mains circuit design, a machine-safety assessment or a component-selection recipe.

Define release at the mechanism

Write the mechanical requirement before choosing the suppression component. Identify the released position or contact state, the event that starts the timing measurement, the permitted delay, and whether bounce or reclosure matters. A firmware command, a transistor switching off and a plunger reaching its destination are different events.

Texas Instruments makes this distinction visible in Using Motor Drivers to Drive Solenoids, section 3.2. In one particular fast-discharge demonstration, current reaches zero within about 1 ms while de-actuation takes about 3.5 ms. Those are TI’s example results, not universal timings or MakersGround measurements. Their useful lesson is that an electrical trace alone does not establish mechanical completion.

يوثّق سجلّ Gomicron الهندسي المنشور documents mechanical design, custom electronics, firmware and control within one product. That cross-discipline scope is relevant to this review. The record does not identify a solenoid, suppression circuit or release-time test used in Gomicron; the method below is general engineering guidance.

Draw the current path after switch-off

Start the circuit review with two drawings: the energized path and the path that carries current immediately after turn-off. In the familiar low-side arrangement, a diode across the coil provides a recirculation route when the transistor opens. The diode initially carries the current that was flowing before turn-off. Analog Devices’ demagnetization explanation describes this behavior and the thermal limits of active-clamp alternatives.

A higher opposing voltage can shorten current decay, but it also changes the voltage and pulse-power demands on the circuit. In TI’s illustrated diode-plus-Zener arrangement across the coil, the low-side switch node rises to the supply voltage plus the suppression-path voltage. A clamp across the transistor is a different topology. Do not transfer a voltage or energy calculation between the two without redrawing the discharge loop.

Mark the component that absorbs energy, the polarity of each suppressor, and any connection to the supply during discharge. Also mark the actual current-measurement point. A resistor below the low-side switch can stop seeing current when that switch opens even while current continues around the coil’s recirculation loop. That is a circuit-analysis observation, not evidence that the coil has already released.

A calculated comparison, not a release-time prediction

Hypothetical electrical model: assume a 12 V supply, a fixed 100 mH inductance, a fixed 24 Ω winding resistance and 0.50 A immediately before turn-off. Assume a clamp directly across the coil maintains a constant reverse terminal voltage magnitude U, with no parasitic effects or mechanical movement. These are illustrative assumptions, not a selected solenoid or recommended suppressor.

For this model, L di/dt + Ri + U = 0. The time until the modeled current reaches zero is t = (L/R) ln(1 + RI₀/U). The expression applies only during decay to zero; it does not describe a negative current afterward.

The initial magnetic energy, ½LI₀², is 12.5 mJ in both cases. The higher clamp shortens this modeled decay while increasing initial suppressor power and switch voltage. It does not create additional initial stored energy. The calculation includes winding resistance; simply using LI/U would give a substantially different answer for the low-voltage case.

Actual coil inductance changes with plunger position, and resistance changes with temperature, as discussed in TI’s current-controlled solenoid reference design. Real suppressor voltage also varies with current and temperature. The table cannot predict movement, establish a device rating or replace measurement.

Check the clamp specification, not its label

A “24 V TVS” is not necessarily a component that clamps a pulse at 24 V. The Littelfuse SMAJ datasheet, for example, lists SMAJ24A with 24 V standoff, 26.7–29.5 V breakdown at 1 mA, and a maximum 38.9 V clamping value at 10.3 A. Those are different specifications. The last value is not a prediction at the hypothetical 0.50 A above, and this example is not a part recommendation.

Compare the selected device’s actual voltage-current data, tolerances and temperature behavior with the switch’s permitted stress. Include supply tolerance and wiring overshoot. Keep pulse duration, repetition rate, mounting and starting temperature in the review: a headline surge-power rating describes stated test conditions, not arbitrary repetitive solenoid operation.

Stored coil energy is only a starting point for checking dissipation. In some switch-clamp topologies the supply contributes energy during discharge. TI’s load-driving application note, sections 4.5–4.6, distinguishes stored energy from energy dissipated in the switch and compares the latter with device-specific demagnetization limits. Apply the analysis for the actual circuit rather than assuming every absorbing component receives exactly ½LI².

Capture the command, current, voltage and movement together

Agree the acceptance limits and instrument setup before energizing the prototype. Use an isolated, current-limited low-voltage supply, secure moving parts, and guard pinch points. De-energize before changing suppression components. An unsuppressed turn-off pulse is not a necessary casual baseline.

Use correctly rated probes and verify their reference connections. Never defeat oscilloscope protective earth to measure a floating node; Tektronix’s floating-measurement guidance explains the hazard and appropriate measurement approaches. Our دليلنا لإتاحة نقاط اختبار لوحة PCB covers planning accessible measurement connections before fabrication.

1. Record the starting state. Retain coil current immediately before turn-off, supply voltage, mechanical position, load and temperature. Equal supply settings alone do not establish equal initial current.
2. Measure electrical decay. Observe actual coil current through a suitable measurement path. State the threshold used to mark completion and the measurement resolution; “zero” on a display is not an infinitely precise result.
3. Measure voltage stress. Capture the relevant switch and suppressor voltages with enough bandwidth and a suitable reference arrangement. Preserve overshoot as well as the main clamp interval.
4. Observe the mechanism independently. Use an appropriate position or contact signal, record its detection delay, and measure from the agreed command event to the required released state.
5. Repeat the operating cycle. Test the intended repetition rate and thermal conditions after the initial pulse review. A successful cold single pulse does not establish repetitive operation.

Retain the cases that change the decision

Build coverage around the product’s permitted conditions: supply extremes, cold and warm winding states, the intended harness, mechanical loading and normal duty cycle. Record hardware and firmware revisions with the captures. Stop testing when reviewed electrical, thermal or movement limits are approached rather than pushing through a warning to obtain a faster result.

For relay contacts, add contact-state behavior under the intended switched load. TE Connectivity’s relay-suppression guidance explains why suppression can affect armature speed and contact performance, with qualifications for normally-open and normally-closed contacts. “Faster is always better” is not a defensible general rule.

The handover should contain the discharge-path drawing, component-rating review, synchronized electrical and mechanical evidence, repeated-cycle thermal assessment, and unresolved conditions. For a custom electronics and PCB development project, these records make the decision traceable: the selected suppression arrangement must satisfy both the board’s limits and the mechanism’s release requirement. Neither result substitutes for the other.

المصادر والنطاق

Primary technical sources and linked company material checked on 30 October 2026. The numerical comparison and proposed validation sequence are original engineering analysis, not measured MakersGround project results. Vendor examples apply only to their documented circuits and conditions.

• Texas Instruments: Using Motor Drivers to Drive Solenoids, SLVAE59A, April 2022.
• Analog Devices: Switching Inductive Loads With Safe Demagnetization.
• Texas Instruments: Current Controlled Driver for 24-V DC Solenoid With Plunger Fault Detection, TIDU578.
• Littelfuse: SMAJ Series TVS Diodes Datasheet, revision printed 12/02/25.
• Texas Instruments: How to Drive Resistive, Inductive, Capacitive, and Lighting Loads, SLVAE30E.
• Tektronix: Floating Oscilloscope Measurements and Operator Protection.
• TE Connectivity: The Application of Relay Coil Suppression with DC Relays.