The coil is the input side of a contactor or relay, and understanding coil faults — open, shorted, and burnt out from the wrong voltage — is essential, because a coil fault stops the device from operating and is often the answer when a device is fed but will not pull in. A coil can go open (no operation), short (draw excess current), or burn out (from wrong or excess voltage). Understanding how to test a coil and recognize these faults lets you diagnose the coil side confidently. This chapter covers coil faults.
Testing a coil by resistance
The direct way to test a coil is by its resistance, and understanding what the resistance tells you lets you diagnose the coil confidently. With the circuit isolated and dead, measure the resistance across the coil terminals (A1–A2). The reading tells you the coil’s condition: a normal value (matching the coil’s datasheet, typically some hundreds of ohms to a few kilohms for a control coil) means the winding is intact and good; an infinite reading (open circuit, ‘O.L.’) means the coil is open — a broken winding, so it can never energize, and it must be replaced; a near-zero or very low reading means the coil is shorted internally, which also requires replacement. So the coil resistance directly reveals an open or shorted coil versus a good one. Understanding this test lets you confidently confirm or rule out a coil fault: rather than guessing, you measure the coil and read its condition. A coil that reads normal is good (so if it will not operate, look to voltage or mechanism); one that reads open or shorted is the fault. So understanding testing a coil by resistance — normal, open, or shorted — lets you diagnose the coil confidently. Understanding how to test a coil by resistance — measuring across A1–A2 on an isolated dead circuit, where a normal value means an intact winding, infinite means an open coil to replace, and near-zero means a shorted coil to replace — lets you diagnose the coil confidently, so that you confirm or rule out a coil fault by measurement rather than guessing (a normal reading meaning the coil is good, so look elsewhere; an open or shorted reading meaning the coil is the fault), which is the direct and definitive test of the coil at the heart of diagnosing the input side of a contactor or relay.
The wrong-voltage burnout
The classic coil fault is burnout from the wrong voltage, and understanding it — why it happens and how to recognize it — explains a common failure, especially after a replacement. A coil is made for a specific voltage and type; feeding it more than it is made for overheats and burns out the winding. This happens when a coil is fed too high a voltage (a 230 V coil on a 400 V supply), or a DC coil is put on AC (or vice versa, causing wrong current and heating), most often after someone replaces a coil or contactor with the wrong one. The result is a burnt-out (open) coil — and, tellingly, one that may burn out repeatedly if the wrong coil keeps being fitted or the supply is over-voltage. Understanding this explains the fault and directs its diagnosis: a burnt-out coil, especially a repeat burnout, points to a wrong coil voltage, wrong type, or an over-voltage supply, not just a random failure. So you check the coil’s marking against the actual control voltage. Understanding the wrong-voltage burnout — a coil overheated by too much or the wrong type of voltage — explains a common failure and directs you to check the coil against the supply. So understanding the wrong-voltage burnout directs you to check the coil marking versus the actual voltage. Understanding the wrong-voltage burnout — a coil overheated and burnt open by being fed more voltage than it is made for or the wrong AC/DC type, most often after a mismatched replacement, and prone to repeat if the wrong coil or an over-voltage supply persists — explains a common coil failure and directs its diagnosis, so that you recognize a burnt-out coil (especially a repeat burnout) as a sign of a wrong coil voltage, wrong type, or over-voltage supply rather than a random failure, and you check the coil’s marking against the actual control voltage, which addresses the classic wrong-voltage coil burnout at its cause rather than merely fitting another coil to burn out in turn.
AC coils, DC coils, and inrush
A nuance worth understanding is the difference between AC and DC coils and the inrush of an AC coil, because it explains some coil behaviors and faults. An AC coil and a DC coil are built differently: an AC coil is designed for alternating current, and when first energized it draws a high inrush current that drops to a lower holding current once the armature is seated (because the seated armature changes the coil’s impedance); a DC coil draws a steady current. This inrush behavior of an AC coil matters: it means an AC coil draws much more current at the instant of energizing than when held in, which stresses the control contacts switching it and can cause problems if the armature cannot seat (the coil stays in the high-inrush state, overheating — which is why a contactor that cannot seat, from dirt or binding, may buzz and overheat). Understanding AC versus DC coils and inrush explains these behaviors: the high switching current of AC coils, and the overheating of an AC coil that cannot seat. So understanding AC coils, DC coils, and inrush explains coil behaviors like high switching current and overheating when unseated. Understanding AC coils, DC coils, and inrush — the AC coil drawing a high inrush current that drops once the armature seats, versus the DC coil’s steady current, and the overheating of an AC coil that cannot seat and so stays in its high-current state — explains some coil behaviors and faults, so that you understand why AC coils stress their switching contacts with inrush and why an AC contactor that cannot seat fully (from dirt, binding, or low voltage) buzzes and overheats, which adds the AC/DC distinction and inrush to your understanding of coils beyond the basic resistance test and explains the overheating and switching-stress behaviors particular to AC coils.
Comparing coil resistance to the datasheet
A practical refinement to the coil resistance test is comparing the reading to the expected value, because ‘normal’ means matching the coil’s specific rating, not just being non-zero and non-infinite. Each coil has an expected resistance determined by its voltage and construction: a low-voltage coil (24 V) typically has a lower resistance than a high-voltage one (230 V), because the higher-voltage coil has more turns of finer wire. So ‘normal’ is the value for that specific coil, from its datasheet or a known-good identical coil. A reading much lower than expected suggests a partially shorted coil (some turns shorted together, lowering the resistance and the pull), even though it is not fully shorted; a reading much higher suggests a partial open or a poor internal connection. Understanding this refines the test: you compare the reading to the expected value, not just check for open or dead-short, catching partial faults. So understanding comparing coil resistance to the datasheet refines the test to catch partial coil faults. Understanding comparing coil resistance to the datasheet — recognizing that ‘normal’ means the specific expected value for that coil (lower for a low-voltage coil, higher for a high-voltage one), so a reading much lower suggests partially shorted turns and much higher suggests a partial open — refines the coil test to catch partial faults, so that you compare the measured resistance to the coil’s expected value rather than merely checking it is not open or dead-shorted, which catches the partial coil faults (some shorted turns weakening the pull) that a crude open/short check would miss and makes the resistance test a more discerning diagnosis of the coil’s condition.
Scenario: the resistance test that settled it
A scenario shows the coil resistance test settling whether the coil is the fault. A contactor would not pull in, and the coil had its rated voltage across it (measured live), yet nothing happened — so the fault was the coil or the mechanism. To settle which, the technician isolated the circuit and measured the coil’s resistance. It read infinite — an open circuit. So the coil was open (a broken winding): it had voltage across it but could not conduct, so no current flowed and no magnetic pull was produced. That settled it — the coil was the fault, not the mechanism. He replaced the coil (or contactor), and it pulled in normally. The resistance test had definitively confirmed an open coil, distinguishing it from a mechanical problem without any guesswork. Understanding the coil resistance test — infinite means open — settled the diagnosis cleanly. This scenario shows the coil resistance test confirming an open coil. Understanding that infinite resistance means an open coil let the technician confirm the coil as the fault. It reinforces that measuring coil resistance settles whether the coil is open, shorted, or good. The scenario reinforces the coil test: with rated voltage present but no pull-in, the technician measured the coil resistance and read infinite, confirming an open coil rather than a mechanical fault, illustrating how the resistance test definitively settles whether the coil is the fault, distinguishing an open coil from a mechanism problem without guesswork and directing a confident replacement.
When the flyback diode fails
A nuance worth understanding is the failure of a coil’s flyback diode, because it can cause faults that look like coil or circuit problems. A DC coil often has a flyback (freewheeling) diode across it to suppress the voltage spike when the coil de-energizes. This diode can fail: if it fails open, the suppression is lost (the switch-off spike returns, potentially damaging the switching device over time, though the coil still works); if it fails shorted, it shorts the coil — so the coil cannot energize (current bypasses it through the shorted diode), making the contactor fail to pull in, and drawing excess current that may blow a fuse or damage the driver. Understanding this explains faults where the coil itself measures fine but the contactor will not operate (a shorted flyback diode shorting the coil) or where a switching device is being damaged (a failed-open diode losing suppression). So a coil circuit fault may be the flyback diode, not the coil. Understanding flyback diode failure explains these coil-circuit faults that can masquerade as coil problems. So understanding when the flyback diode fails explains faults that look like coil problems. Understanding flyback diode failure — a failed-open diode losing spike suppression and a failed-shorted diode shorting the coil so it cannot energize — explains faults that look like coil or circuit problems, so that when a coil measures fine but the contactor will not pull in you consider a shorted flyback diode shorting the coil, and when a switching device is being damaged you consider a failed-open diode losing suppression, which adds the flyback diode to the possible causes in a DC coil circuit and explains the coil-circuit faults that can masquerade as coil or driver problems.
The coil as the simplest fault to confirm
To close, it is worth appreciating that the coil is one of the simplest faults to confirm definitively, because a single resistance measurement settles it. Unlike some faults that require inference, a coil’s condition is directly measurable: its resistance tells you plainly whether it is open (infinite), shorted (near zero), or good (normal value). This makes the coil one of the most satisfying parts of the diagnosis to nail down — no ambiguity, just a measurement and a verdict. Understanding this encourages you to confirm the coil early and definitively when it is in question: a quick resistance check settles whether the coil is the fault, letting you move on confidently to the mechanism or the feed. So the coil’s simplicity to confirm — one measurement, a clear verdict — makes it a solid, satisfying anchor in the diagnosis. Understanding this makes you use the coil test to settle its part of the question cleanly. So appreciating the coil as the simplest fault to confirm encourages settling it early and definitively. Understanding the coil as one of the simplest faults to confirm definitively — a single resistance measurement plainly showing open, shorted, or good — encourages you to settle it early and cleanly when in question, so that you use the quick, unambiguous resistance check to nail down whether the coil is the fault (no inference needed, just a measurement and a verdict) and move on confidently to the mechanism or feed, which makes the coil a solid, satisfying anchor in the diagnosis and one of the most definitively confirmable of all contactor and relay faults.