Many beginners in electrical control get stuck on one question:"When the switch is closed, the output lamp lights; when the same switch is opened, the lamp goes out."Which of these four common control circuits does this belong to — the momentary circuit, the latching circuit, the open circuit, or the interlock circuit? This article explains the answer first, then builds it for real usingan AC contactor + push button + indicator lamp — a three-piece set — so the logic is built for real, including a component selection list, a terminal-by-terminal wiring table, and the complete operating sequence.
1. The conclusion first: this is a momentary (jog) circuit
The behavior described in the question has a very clear place in electrical control:Momentary circuit, and the standard term used on the job site isjog control circuit (Jog / Inching Circuit).
Quick answer
Momentary circuit (jog circuit, Jogging / Inching Circuit)
Key criterion:The output state follows the input exactly, with no "memory" in between.Hold the button and the contactor coil is energized and the output lamp lights; release the button and the coil de-energizes and the lamp goes out instantly. "Momentary" means the actuator makes only a tiny, controlled movement — the moment the operator's hand leaves the command device, the action must stop immediately.
Why not the other three options?
| Candidate answer | Why it does not hold |
|---|---|
| Latching circuit (self-holding / Latch) |
Latching is defined as"having memory": after the command device is pressed once and released, the outputremains held, and must be reset by another device (usually a stop button). But the question saysthe same switch is opened and the output goes out, which shows there is no holding stage — and that is exactly the watershed between jog and self-latching. If this were a latching circuit, the lamp would stay on after the switch is released. |
| Open circuit （Open Circuit） |
An open circuit is not a control logic but acircuit state — a state where the loop is broken somewhere and current is zero. In an open state the output lamp will not light at all, which directly contradicts the question's "the lamp lights when the switch is closed." |
| Interlock circuit （Interlock） |
Interlocking addressesthe mutual restraint between two or more circuits (typically the forward/reverse contactor interlock that prevents phase-to-phase short circuits). The question has only one input and one output, with no "two mutually exclusive paths," so interlock does not apply. |
| Momentary circuit ✓ | Input and output are synchronized, there is no self-holding stage, no third device is involved — it satisfies the entire description. |
2. The four circuits, clarified in one table
Put these four side by side and you will find that theiressential difference comes down to just one thing: whether there is "holding" and "restraint."
| Circuit type | Behavior | Key component | Typical field use |
|---|---|---|---|
| Momentary circuit Jog / Inching |
Moves only while held, stops the instant you let go;no memory | one NO push button + contactor coil | crane fine positioning, machine tool tool-setting, conveyor jog correction, test-run direction check |
| Latching circuit Self-latching / Latch |
After one press to start,runs continuously, requires a stop button to reset | start button + stop button + contactorauxiliary NO contact | fans, water pumps, air compressors, conveyors that must run continuously |
| Open circuit Open Circuit |
Loop broken,no current, a fault/state description | None (a blown fuse, loose wiring, or burned contacts can all cause it) | Acts asa troubleshooting target, rather than a design solution |
| Interlock circuit Interlock |
Two or more circuitsmutually block, prohibiting simultaneous operation | the other contactor'sauxiliary NC contactwired in series into this circuit | motor forward/reverse, two-speed switching, two machines as mutual primary/backup |
One-sentence rule:Look at the drawing —check whether the start button has the contactor's own auxiliary NO contact wired in parallel across it. Yes → latching; no → momentary (jog). Then check whether someone else's NC contact is wired in → that is an interlock.
3. Build it with an AC contactor + push button + indicator lamp
3.1 Principle and structure
The whole system is divided intotwo circuits, the prerequisite for understanding all contactor circuits:the main circuit drives the load, the control circuit powers the coil, and the two are coupled through the contactor's electromagnetic mechanism.
3.2 Component list and selection recommendations
The specifications below targetsingle-phase 220V, pure indicator load (a few to tens of watts)teaching/demo scenarios; if you switch to a real motor, all main-circuit components must be re-calculated to the motor's rated current.
| Code | Name | Selection notes & recommended specs |
|---|---|---|
| QF | Miniature circuit breaker | As main isolation and short-circuit protection, it is recommended to use DZ47-60 2P C6(C6 is fine for control demos; for a motor, choose type C at 1.5–2× rated current) |
| FU | Fuse | to protect the control circuit separately, it is recommended to use RT18-32X base with a 2A fuse element. Its role is to limit fault current to the low-power branch and avoid burning the contactor coil |
| KM | AC contactor | coil voltage must match the control supply voltage, choose CJX2-1810 (coil AC220V 50Hz)."10" means it includes one built-in auxiliary NO contact — use this for the self-latching upgrade; when buying, do not mistakenly pick 1801 (that is NC) |
| SB | Push button switch | must beself-resetting NO type; recommended: LA38-11 or NP2 series, greenindicates start.Never use a toggle switch, rocker switch, or a knob without self-reset, otherwise the jog function loses its meaning |
| HL | Signal indicator lamp | AD16-22DS or ND16,rated AC220V, red. When selecting, verify the lamp/LED module voltage rating; a 380V lamp on 220V will be dim or not light at all |
| — | Auxiliary materials | 35mm standard DIN rail TH35, terminal block, wire markers. Control-circuit wire: BV 1.0～1.5mm², main circuit BV 1.5～2.5mm² |
Convention for contact numbering(varies slightly by brand; always follow the marking on the actual device):
Main contacts are usually marked 1L1/3L2/5L3(line) and 2T1/4T2/6T3(load); auxiliary NO is usually 13-14, auxiliary NC is usually 21-22 or 11-12; the coil is always A1、A2.
3.3 Terminal-by-terminal wiring table (follow it screw by screw)
Always start wiring fromthe control circuit, and connect the main circuit last;work de-energized throughout. Each step in the table is "from which terminal → to which terminal."
| Step | From | To | Note |
|---|---|---|---|
| 1 | QF output terminal (line side) | KM main contact 1L1 | main circuit, route on the left |
| 2 | KM main contact 2T1 | HL indicator terminal X1 | main-circuit output |
| 3 | HL indicator terminal X2 | supply N (neutral) | main-circuit loop closed |
| 4 | QF output terminal (take the same phase) | FU base input terminal | the control supply must be taken after QF, same as the main circuit, otherwise the control circuit stays live even when QF is open |
| 5 | FU output terminal | one end of SB's NO contact | the control circuit |
| 6 | the other end of the SB button | KM coil A1 | critical series node |
| 7 | KM coil A2 | supply N | control-circuit loop closed |
| 8 | external L / N supply | QF input terminal | connect last; after wiring, do not re-energize yet |
3.4 Complete operating sequence after power-on
- Rest state.QF is closed, FU intact. At this moment SB is released, its NO contact is open, the KM coil A1—A2 loop is open, the contactor armature is held in the released position by the return spring, and main contact 1L1—2T1 is open,the HL does not light.
- Press SB(within ~0.1 s). The NO contact closes and a current path forms: L → QF → FU → SB → KM coil A1 → A2 → N. The coil energizes and produces electromagnetic attraction.
- Contactor pulls in.The armature overcomes the spring force and is pulled down, driving the contact carrier, and main contact 1L1—2T1 closes instantly; the normally-open auxiliary contact also closes at the same time (in this design the auxiliary contact is not wired, so it does not affect the logic).
- Indicator lamp lights.Main circuit conducts: L → QF → KM main contact → HL → N, the HL receives its rated 220V across its terminals and the lamp lights.At this point SB must be held down continuously.
- Release SB.The contact returns to open, the coil loop is cut, electromagnetic attraction vanishes, the armature immediately releases under spring force, the main contact opens accordingly, and the HL loses power and goes out.The whole process takes only tens of milliseconds, and the lamp goes out immediately.
If you express this logic as a Boolean equation, it is surprisingly simple:
Compare it with the latching version below and you will immediately see where the difference lies.
4. If what you really want is "stays lit after release" — upgrade to a latching circuit
In real engineering, many people say "switch controls the lamp" but actually mean "press once and it stays lit." That need is no longer a momentary circuit, buta latching (self-holding) circuit. The good news is:you only need to add one short jumper wire and one stop button on top of Fig. 1.
How to modify
- In the control circuitconnect in seriesoneNC (normally closed)push button SB1 (red, as stop);
- take the contactor's built-inauxiliary NO contact (13-14)and with a piece of wireconnect it in parallel across SB2 (the green start button).
Complete logic equation of the latching version
The latching circuit additionally gives you two important protections(the momentary circuit does not have):
- Loss-of-voltage (no-voltage) protection: a sudden power loss during operation releases the contactor and cancels the self-latch; after power returns it will not start by itself and must be re-pressed by a person — preventing the equipment from moving unexpectedly when unattended.
- Undervoltage protection: when voltage drops below about 85% of rated, the electromagnetic force is no longer enough to overcome the spring and the contactor releases, avoiding the motor stalling and heating at low voltage.
What if you want both "jog + self-latching"?
A common need in the workshop: self-latching continuous run in normal operation, but jog one-step-at-a-time during setup. There are three proven approaches:
| Approach | Method | Recommendation |
|---|---|---|
| Selector-switch method | Use ajog/runtwo-position rotary switch wired into the self-latching branch. Flipping to "jog" physically cuts the holding loop, so you can only step with the button | ★★★★ Simple and reliable, suited to a fixed operation station |
| Intermediate-relay method | the start button drives intermediate relay KA to self-latch, and KA's NO then drives contactor KM;the jog button drives the KM coil directly, bypassing KA | ★★★★★ industry's first choice, thoroughly eliminating contact competition |
| Dual-contact button method | jog button uses one NO pair to drive the coil and one NC pair in series into the self-latching branch | ★★ carries contact-competition risk (colloquially "false latching"), which may cause accidental holding after release,not recommended |
5. How to write the same thing in a PLC / soft logic
If this control is implemented in a PLC, the contactor and buttons on the hardware side stay the same — only the logic moves into the program:
Although the ladder diagram changed its medium, thejudging criteria are exactly the same: on the rung where the output coil sits, check whether its own NO contact is wired back in parallel. If yes to latching; if no to jog.
An easily-overlooked safety principle:stop button and overload protection contactmust always be wired NC. This way, if a wire breaks or a terminal loosens, the circuit naturally opens and the equipment stops — that is "fail-safe." If you mistakenly wire the stop button as NO, a broken-wire fault would instead leave the equipment unable to stop.
6. Field troubleshooting table
| Symptom | Most likely cause | Remedy |
|---|---|---|
| Lamp never lights when button pressed | control-circuit open: FU blown, terminal loose, A1/A2 not seated, button terminal wired to the NC pair | after de-energizing, use a multimeterin resistance mode, measure section by section: first check FU continuity, then whether SB conducts when pressed, finally whether the coil is open |
| lamp off but contactor buzzes | insufficient coil voltage or wrong rating (e.g. a 380V coil on 220V), or oil/dirt or jamming on the armature pole face | verify the nameplate coil voltage; after de-energizing, clean the pole face and check the moving parts for jamming |
| lamp lights but stays on after releasing the button | mistakenly wired as self-latching (auxiliary NO shorted into the control circuit), or the wrong button was chosen —non-self-resetting type | remove the self-latching jumper and replace it with a self-resetting button |
| trips as soon as you press | both ends of the control circuit were taken to different phases other than L and N, or N was shorted to a phase by the button | re-check: it must bephase into button → button output into coil A1 → A2 back to N, never let the NO contact bridge between two phases |
| lamp dim, flickering, or burns out quickly | indicator rated voltage does not match the supply; a neon indicator was wrongly placed in series in the AC circuit | replace with an indicator of the same rating as the supply; the indicator mustbe in parallelacross the load, never in series |
| button occasionally "sticks" and will not reset | mechanical jam at the button head, failed spring, or ingress of dust | Replace the button.in jog control, button reliability is the safety baseline, never cut corners |
7. Safety red lines (must read before you start)
- You must perform lockout/tagout (LOTO) — de-energize, verify, tag — before touching any wire.Even with the control circuit open, the contactor's main-circuit incoming terminals remain live.
- Coil voltage must strictly match the control supply voltage.A 220V coil on 380V burns out instantly; a 380V coil on 220V pulls in weakly and the contacts chatter and arc.
- Always distinguish AC coils from DC coils.An AC coil relies on the shading ring; on DC it over-currents and burns, and vice versa.
- The indicator must be wired in parallel across the load, never in series in the load circuit — otherwise it will not indicate properly and will cause abnormal voltage division.
- For demo or teaching scenarios, strongly consider adding a control transformer(e.g. 380V/220V → AC 110V or AC 24V), driving the button and control circuit at low voltage, in line with the safety philosophy of GB/T 5226.1 and giving peace of mind during maintenance.
- Before energizing, re-check with a multimeter in resistance mode, confirming the A1—A2 loop is open when the button is not pressed, to avoid auto-start on power-up.
8. Quick Q&A
Q: Are the momentary circuit and the jog circuit the same thing?
Yes. Both are Chinese renderings of "Jogging / Inching Circuit" — "momentary" is more colloquial, "jog" is the engineering convention. Some sources restrict "jog" to brief full-voltage running and "momentary" to low-speed fine adjustment, but in Chinese electrical terminology the two are essentially interchangeable.
Q: Does using a plain wall switch to control a lamp count as a momentary circuit?
Logically equivalent, but engineering does not call it that. The specific meaning of a momentary circuit is "using a self-resetting command device to control the load through a contactor," emphasizing safe and controllable operation. A wall switch directly controlling a low-power lamp is usually just called a simple on/off control circuit.
Q: Why use a contactor for such simple jog logic? Can't you just put a switch in series?
Technically you can when the load is small. But the contactor offers three irreplaceable benefits: first,strong/weak isolation, the operator only touches the low-voltage control circuit; second, it providesLoss-of-voltage (no-voltage) protectionloss-of-voltage protection, so it will not auto-start after power loss; third,remote operation and expandability, making it easy to later add self-latching, interlock, and PLC control.
Q: What is the fastest way to tell whether a diagram is jog or self-latching?
Look for just one thing: does the start button have the contactor's own auxiliary NO contact wired in parallel across it?If yes, it is self-latching; if not, it is jog.Then check whether another contactor's NC contact is wired in — that is an interlock.
Q: If you replace the indicator with a real load (e.g. a single-phase motor), what wiring changes?
The topology is unchanged — just put the load where HL was — but all main-circuit components must be re-selected to the load's rated current: the QF setting, the KM main-contact rated current, and the conductor cross-section all need recalculation, and the motor circuit also needsa thermal relay (overload protection), whose NC contact is wired in series at the end of the control circuit (before the KM coil A2). This is the biggest single leap from a jog circuit to real engineering.
Q: Can a latching circuit be implemented electronically, without contactor auxiliary contacts?
Yes — an intermediate relay, a flip-flop, or a PLC internal holding bit can all realize "memory." But note: the benefit of hardware self-latching is independence from the control system, so even a PLC crash will not affect the hold; pure software latching depends on the controller running normally, and in critical situations it is usuallyboth are used together..
Written at the end
Back to the opening question: the key to judging is not "whether the lamp is on," but inwhether the output has memory..
- It lights only while held, and goes out the moment you release. → input and output are synchronized, no holding stage → Momentary circuit (Jog circuit)
- Press once and it stays lit; it goes out only when another button is pressed → the output powers itself → Latching circuit (Self-latching circuit)
Building it with an AC contactor, a button, and an indicator is not hard: put a self-resetting NO button in series in the control circuit to drive the KM coil, and let the KM main contacts drive the indicator in the main circuit — a standard jog control is done. To later turn it into a self-latching circuit, you only need toadd one auxiliary NO jumper wire connected in parallel across the start button.
Electrical Control BasicsAC contactorJog and Self-latchingWiring in PracticeLadder Diagram
