Printer restarts when heater kicks — PSU sag test (12V/24V, bed & hotend)
Joe B.
Expert ★★★★★ 27,853 satisfied customers
Specialities: 3D printers, PSU sizing, heated bed wiring, brown-out resets, Marlin/Klipper/RRF tuning
3D printer reboots on heat — brown-out, undervoltage, voltage drop under load, bed/heater inrush & EMI
Technician
• Printer & board: model, 12V or 24V, PSU wattage/brand (e.g., Mean Well 24V/350W).
• What reboots first — the screen, the USB host (OctoPrint), or the whole MCU? Any “brown-out reset” or watchdog in logs?
• Does it happen only when the bed heats, or also when the hotend cycles (PID)? Any smell of hot plastic at the bed terminals / XT60 / screw blocks?
Technician
Technician
1) Safety: Power off, remove mains if you need to access terminals. Set your DMM to DC volts.
2) Clip probes to the controller VIN (board + and –) or the PSU 24V output. Ensure the probes are secure — no slips.
3) Power on. In your host (screen/OctoPrint), preheat bed only to 60–70 °C. Watch the meter at heater switch-on and during PID pulsing.
4) Healthy 24V systems should stay near 23.5–24.2 V with tiny ripple. Dips below ~22.5 V (or below the MCU’s BOD threshold) indicate PSU/wiring problems.
5) Repeat with hotend only. If hotend is stable but bed isn’t, the bed circuit is the culprit (PSU current headroom, bed wiring, terminals, MOSFET/SSR).
Technician
• Terminals: Power off, inspect the PSU and mainboard screw blocks, bed terminals, XT60s. Look for browning, melted plastic, or loose screws. Reseat and re-torque gently.
• Wire gauge & length: Use at least AWG16–14 for the bed on 24V (AWG14–12 on long runs/12V). Replace crispy or tinned wire ends with ferrules.
• Bed input connector: Many boards burn these over time. Consider an external MOSFET module or SSR to offload current from the mainboard if the connector shows heat damage.
• PSU capacity: Bed (e.g., 220–300 W) + hotend + motors + electronics should be below ~80% of PSU rated wattage for headroom.
Technician
• Run the bed from a dedicated PSU or AC silicone bed + SSR (observe mains safety).
• If staying DC, ensure line voltage selector (115/230V) is correct and the PSU isn’t a counterfeit/weak unit.
Technician
• Temporarily switch bed to bang-bang and re-test. If resets stop, address PSU/wiring then return to PID.
• Keep brown-out detection enabled; it protects your board. Update firmware only after electrical fixes.
• Route bed leads away from endstop/thermistor lines; add ferrite on LCD/USB if you suspect EMI.
Technician
• Marlin LCD often shows V in the info screen; some builds report it via host.
• On RepRapFirmware,
M122 reports VIN min during the session (useful to catch sags).• OctoPrint plugins and some Klipper setups can log supply voltage. Still, a DMM at the board is the gold standard for fast dips.
Technician
• Run a bed PID tune after hardware fixes. Keep bed bang-bang hysteresis modest if you prefer less ripple.
• Re-crimp with ferrules, zip-tie strain relief, and re-check torque after a few heat cycles.
• If using an AC bed + SSR, choose a zero-cross triac SSR rated for the load, on a heatsink, in an enclosed, grounded box.
Technician
Quick answer: the accepted fix
Accepted fix when the printer reboots as the bed/hotend turns on: measure VIN during heat to confirm a PSU sag, tighten/replace bed power connectors, use correct wire gauge/ferrules, consider an external MOSFET/SSR, and upgrade to a quality PSU with 20–30% headroom. Re-tune PID and keep brown-out detection enabled.
Why heaters trigger resets (current draw, brown-out & connectors)
Heated beds draw large inrush/current. Weak or counterfeit PSUs, loose screw terminals, undersized wiring, or burnt XT60/screw blocks cause voltage drop. The controller’s brown-out detection resets the MCU to avoid corruption. PID pulsing can accentuate ripple; PSU regulation and solid wiring eliminate the dip.
How to verify the fix: repeat the sag test
With the new PSU/wiring, repeat the sag test at the board VIN. Expect <3% drop from idle to full bed load and stable voltage during PID. Log VIN if your firmware/host supports it; on RepRapFirmware, check VIN min after a heat cycle.
Power stability — quick troubleshooting tips
Most sudden reboots are undervoltage from PSU headroom or connectors.
Quick path: run a PSU sag test → re-terminate bed power with ferrules → check terminal heat/browning → upgrade PSU (quality 24V/350W typical) or move to AC bed + SSR with proper mains safety → re-tune PID and keep BOD enabled.
Disclaimer: Mains wiring can be hazardous. If you’re not comfortable with AC work, consult a qualified technician. For device-specific guidance, chat with a verified expert on Whizz-Tech.
Getting Started Is Easy
Ask for help, 24/7
Members enjoy round-the-clock access to 12,000+ verified Experts, including tech support, mechanics, vets, and more.
Expert will respond in minutes
We match you with an Expert who specializes in your situation. Talk, text, or chat — whichever you prefer.
Save time & money
No scheduling hassles or expensive consults. Membership can save you significant time and money each month.
FAQ
Why does the printer restart when the heated bed turns on?
The bed draws high current; weak PSUs, loose or undersized wiring, or burnt connectors cause a VIN drop. The controller hits its brown-out threshold and resets.
Will a bigger PSU always fix it?
Not if the connectors are bad. First fix wiring/terminals; then choose a quality PSU with 20–30% headroom. AC beds with SSR are another option (mains safety required).
PID vs bang-bang — which is safer for marginal power?
Bang-bang has longer on/off cycles and can hide minor sags, but proper fix is stable PSU + sound wiring. After hardware fixes, PID gives steadier temps and better prints.
How can I log undervoltage events?
Some firmware reports VIN on the LCD/host; RepRapFirmware shows VIN min via diagnostics. External logging with a DMM or meter module captures fast drops best.
Sources
- Marlin — Troubleshooting (power supply, wiring, and random halting)
- Prusa Knowledge Base — Random crashes & freezes (power issues)
Filed under: Printer won’t print
Related articles
- Printer says “Offline” and won’t print — what should I check first?
- My printer “forgot” my Wi-Fi — the saved network is gone
- Printer Wi-Fi settings reset by itself — why did it forget my network?
- My printer keeps popping up “Set up Wi-Fi” again — why won’t it stay connected?
- My printer Wi-Fi setup keeps failing — what am I doing wrong?
- Printer Wi-Fi setup is confusing — why won’t it connect to my network?
- Printer Wi-Fi setup is too complicated — what do I check first without messing it up?
- Printer setup fails on Wi-Fi — why won’t it connect during setup?