HOW-TO GUIDE
Why Does a Switching Power Supply Make a Buzzing Noise? What to Check
By the OhmSavvy Research Team ·
Mains supplies can hold a lethal charge after unplugging
- The primary side of an offline switching supply runs at rectified mains voltage, roughly 170 V DC from a 120 V outlet (120 × √2 ≈ 170) and about 340 V DC from 240 V, and its bulk capacitor can stay charged after the plug is pulled.
- Do not open a mains-powered supply unless you are trained to work on one.
- Every check below on the output side can be done with the case closed.
- If you see smoke or flames, unplug only if it is safe to do so, and call 911 (or your local emergency number) if a fire starts.
Causes at a glance
| Symptom | Likely cause | How to check | Fix or who to call |
|---|---|---|---|
| Faint whine only with little or no load | Burst mode at light load | Listen with and without the load connected | Normal; unplug when not in use |
| Steady whine at normal load, output correct | Coil or MLCC vibration (coil whine) | Multimeter shows correct voltage under load | Usually harmless; replace if it bothers you |
| Rhythmic ticking, LED flickers, output low | Overload or short: hiccup protection | Disconnect the load; does output recover? | Fix the load fault or fit a larger supply |
| Noise after adding devices; supply hot | Running at or over its rating | Add up load current vs the label | Reduce load or upsize the supply |
| Buzz plus heat, output sagging over months | High-ESR electrolytic capacitors | Scope for ripple; ESR meter on caps | Replace the supply, or a qualified repairer |
| Noise started after a drop or knock | Cracked core or loosened windings | Visual check of the case; output test | Replace the supply |
| Crackling, popping, smell, sparks | Arcing or insulation breakdown | None: do not test further | Unplug and replace; never reuse |
What actually makes the sound
A switching supply chops current through inductors and a transformer many thousands of times per second. Three physical effects turn that into sound when the energy lands in the audible range:
Magnetostriction
Magnetic cores change size slightly as flux changes, so a coil or transformer can hum or whine.
Piezo effect
Ceramic capacitors (MLCCs) flex with applied voltage and can sing against the board.
Loose windings
A core or winding that is not firmly bonded rattles at the switching or burst rate.
Coil whine is a mechanical side effect rather than an electrical fault, and many supplies make some noise from new. What matters is change: a supply that has always whined faintly is in a different category from one that started buzzing last week, got louder, or now runs hot. Note whether the sound tracks the load, such as rising when a laptop is busy or a motor starts, since that points to the output stage rather than the mains input.
Why you can hear it: the frequencies
Human hearing spans roughly 20 Hz to 20 kHz, and the upper limit falls with age, so many adults hear little above about 15 kHz. Most switching supplies run continuously far above that, commonly somewhere from about 50 kHz to several hundred kilohertz, which is why a healthy supply at a steady load is silent. The sound appears when something slower modulates the switching: burst packets at light load, typically repeating in the hundreds of hertz to a few kilohertz, or hiccup retries a few times per second. The ear is most sensitive around 2 to 5 kHz, right where burst-mode packets often land.
Bars use a logarithmic scale: each step is ten times the last.
1Characterize the noise
Listen before you measure. A continuous high whine is coil or capacitor vibration. A buzz or chirp that appears only with nothing plugged in is burst mode. A regular tick, often with an LED blinking in time, is hiccup mode. A sharp, irregular crackle is arcing, and that ends the investigation: skip to the arcing section below. Note whether the sound changes with the load, the temperature or the time since switch-on, because each of those points to a different cause in the table above.
Pin down where the sound comes from, too. A charger brick that whines from its own body is a different case from a desktop computer whose noise comes from the graphics card's power stage, or a monitor that buzzes only at certain brightness settings. Brightness and load changes shift the converter's operating point, which is why a sound can appear at one setting and vanish at the next. A short phone recording made close to the case, noting what was connected at the time, gives you a baseline to compare against if the noise changes, and it is useful evidence for a repair shop or a warranty claim.
2Change the load
Disconnect everything from the output and listen, then reconnect the load. If the noise appears only unloaded and stops with the device attached, it is burst mode and harmless. If it ticks with the load and stops when you disconnect it, the supply is in overcurrent protection: the load is faulty or too big. If it is the same either way, it is coil whine from the supply itself.
3Measure the output under load
Without opening the case, put a multimeter on DC volts across the output, first with no load and then with the real load or a suitable test resistor connected. Size the resistor with Ohm's law: for a 12 V supply at half of a 5 A rating, R = V ÷ I = 12 ÷ 2.5 = 4.8 Ω, and it dissipates P = V²÷R = 12² ÷ 4.8 = 30 W, so it needs a power resistor rated well above 30 W and it will get hot. The reading should stay within the tolerance printed on the label or datasheet; many supplies hold within a few percent. The full method is in how to check a 12V power supply under load.
A multimeter shows an averaged DC value, so it can't reveal switching spikes, ripple or the on-off cycling of hiccup mode, though a Min/Max recording mode catches dropouts. For the waveform itself, see how to measure ripple voltage with an oscilloscope and measuring a switching power supply waveform.
4Check the load against the rating
A supply that started buzzing after you added devices may simply be overloaded. Add up the current each device needs and compare it with the label. Example: three devices on a 12 V, 5 A (60 W) supply drawing 1.5 A, 2 A and 2 A total 5.5 A, which is 5.5 ÷ 5 = 110% of the rating. The supply will run hot and may tick in and out of protection. A common rule of thumb is to keep the continuous load at or below about 80% of the rating, here 0.8 × 5 = 4 A.
| Load on a 12 V, 5 A supply | Current | Share of rating | Verdict |
|---|---|---|---|
| One LED strip | 2.0 A | 40% | Comfortable |
| Strip + controller + fan | 3.8 A | 76% | Fine, within 80% |
| Three devices (example above) | 5.5 A | 110% | Overloaded: expect heat and hiccup |
If an LED driver is shutting off rather than buzzing, the same arithmetic applies; see why an LED power supply keeps shutting off.
5Check heat and ageing capacitors
Electrolytic capacitors dry out with heat and time, and their equivalent series resistance (ESR) rises. The supply then has to work harder to regulate, ripple increases, and the control loop can become unstable enough to buzz. A widely used rule of thumb says electrolytic life roughly doubles for every 10 °C drop in operating temperature, so a capacitor rated 2,000 hours at 105 °C might last on the order of 2,000 × 2 = 4,000 hours at 95 °C and 8,000 hours at 85 °C. Heat is what ages a supply, which is why a buzzing supply in a closed cabinet deserves attention.
Bulging tops or leaked electrolyte are visible signs, but a capacitor can fail by ESR alone while looking normal. A multimeter's capacitance range does not measure ESR; an ESR meter does. For what a standard meter can check, see how to test a capacitor with a multimeter. Give the supply open air around it, and replace a sealed adapter that buzzes and runs hot rather than opening it.
Crackling, popping or a burning smell means unplug it
Meters for supply checks
A dependable DC voltage reading under load answers most buzzing-supply questions, and Min/Max recording helps catch a supply that drops out. These picks come from our verified multimeter list.
Meters for load tests and dropout checks
True RMS, CAT III 600V, 6,000-count backlit display, with capacitance and frequency. Hands-on Tested.
True RMS, CAT IV 600V, 6,000-count with analog bar graph and Min/Max-Average recording for catching dropouts.
22,000-count True RMS with ±(0.05%+5) DC volts, frequency to 220 MHz and a USB PC link.
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Frequently asked questions
Is a buzzing switching power supply dangerous?
A steady, faint whine with normal output voltage and no heat or smell is usually just coil whine and not a safety issue. Crackling, popping, a burning or hot-plastic smell, scorch marks or visible sparks are different: they point to arcing or insulation breakdown. Unplug the supply at once and replace it, or have a qualified technician inspect it before any further use.
Why does my power supply only buzz when the device is idle?
Many supplies drop into a burst or skip mode at light load to save energy. Instead of switching continuously at tens or hundreds of kilohertz, they fire short packets of pulses with gaps between them, and the packet rate can fall to somewhere in the hundreds of hertz to a few kilohertz, well within hearing. The noise often fades once the load increases.
Can I stop coil whine by gluing the inductor?
Manufacturers sometimes varnish or pot inductors for exactly this reason, but opening a mains supply to do it exposes you to the charged primary side, and the wrong adhesive can trap heat. For a sealed adapter, replacing it is the safer option. Changing the load, for example by not leaving a charger plugged in with nothing attached, sometimes moves the noise out of the audible range.
Will a multimeter tell me if the supply is failing?
It tells you whether the average output voltage is correct with a real load attached, which rules a lot in or out. A 12 V supply that reads 12.1 V unloaded and 11.9 V at full load is regulating fine. A multimeter can't show ripple spikes or the fast restart cycle of hiccup mode, though Min/Max recording can catch dropouts. An oscilloscope shows those directly.
Why did my power supply start buzzing after I added more devices?
The extra load may be pushing it near or past its rated output, so it runs hotter, regulates less comfortably, or cycles in and out of overcurrent protection. Add up the current the connected devices need and compare it with the rating on the label. As a rule of thumb, keep continuous load at or below about 80% of the rating for headroom and a longer life.
Why does my laptop charger make a high-pitched noise?
Most laptop and phone chargers are small switching supplies that enter burst mode at light load, so a faint squeal when the laptop is fully charged or unplugged is common and usually harmless. If the noise is new, loud, comes with heat or a smell, or the laptop reports charging problems, stop using that charger and replace it with one from the laptop maker.
