FAQ
TL;DR: RS232 relay “chatter” here is caused by motor inrush and inductive spikes disturbing control power/logic—“it took 6 or 7 tries before the pump finally started up” (a measured stat). [Elektroda, Steven Furick, post #21664940]
Why it matters: This FAQ helps makers and technicians fix 12 V pump/relay dropouts in PC- or Bluetooth-controlled systems.
Quick Facts
- Startup surge: peak ≈11 A was measured with a 1 Ω shunt during pump start. [Elektroda, Steven Furick, post #21664941]
- Inductive kick: observed spikes about +95 V to −200 V on the pump line. [Elektroda, Steven Furick, post #21664940]
- Separate supply didn’t cure dropout, so noise/logic upset remains likely. [Elektroda, Steven Furick, post #21664935]
- Board auto-retry: pump often starts after 6–7 attempts (N=10 retries set). [Elektroda, Steven Furick, post #21664940]
- Battery health: internal resistance, not just Ah rating, drives voltage sag under load. [Elektroda, Steve Lawson, post #21664937]
Why does my relay close and instantly open with a 12 V pump?
DC motors draw high inrush current at start. The resulting voltage sag and inductive noise can drop the relay coil below its hold voltage, so it releases. If the battery is weak or wiring adds resistance, the sag worsens and the controller may glitch. “An electric motor draws a much higher current than normal when it is starting up.” Measure supply at the relay coil during start to confirm. [Elektroda, Frank Bushnell, post #21664932]
How do I confirm the battery is causing the dropout?
Watch battery voltage during start with a scope or fast DMM. A sharp dip indicates high internal resistance or poor connections. Old or undercharged batteries sag more. If possible, power the relay/control electronics from an independent regulated supply and repeat the test to isolate the load from logic. If the dropout vanishes, the shared supply path was the trigger. [Elektroda, Frank Bushnell, post #21664932]
What inrush current should I expect from a small 12 V pump?
Expect several times the run current. In this thread, a 2.1 A pump showed a peak near 11 A with a 1 Ω series shunt. That surge can momentarily pull the supply or wiring low, unlatching a 5 A relay and upsetting logic. Plan wiring, fusing, and relay contact ratings with that peak in mind. [Elektroda, Steven Furick, post #21664941]
How do I suppress the inductive kick that’s upsetting the controller?
Add a flyback diode across each relay coil (observing polarity) to block back‑EMF into drivers. For the motor line, use a snubber network or TVS diode rated for the observed spike. Keep high‑current loops short and separated from logic traces. If drivers were stressed before diodes, replace any marginal parts. “Add the diodes, one per relay coil.” [Elektroda, Steve Lawson, post #21664936]
Do I really need flyback diodes on relay coils?
Yes. The relay coil is an inductor. When de‑energized, it generates a high‑voltage transient that can punch into the driver or microcontroller. A reverse‑polarity diode across the coil clamps that spike, protecting hardware and reducing EMI that can flip logic states or corrupt serial data. [Elektroda, Steve Lawson, post #21664936]
What is “inrush current” in this context?
Inrush current is the initial, much higher current a motor draws before it spins up. The stalled rotor looks like a low resistance, so current spikes until back‑EMF builds. That surge can sag supplies, drop out relays, and create mechanical chatter. It’s expected; design for it. [Elektroda, Frank Bushnell, post #21664932]
How should I wire the system to reduce noise and false trips?
Use opto‑isolators on RS232/logic inputs, short and separated traces for relay/motor currents, and solid decoupling on drivers. Keep the logic supply highly stable and filtered. Add mains and DC input filtering to power supplies. Validate cable lengths and shielding to prevent serial data corruption. [Elektroda, Mark Harrington, post #21664943]
Can RS232 or the Bluetooth link be corrupted by the pump starting?
Yes. Inductive spikes and ground bounce can upset the controller, causing missed or retried commands. The board vendor warned that inductive collapse can interfere with logic, and enabling auto‑retry lets the pump start after several attempts. Separate logic power and add suppression to reduce retries. [Elektroda, Steven Furick, post #21664938]
How can I systematically diagnose this dropout?
Try this quick 3‑step:
- Scope the relay coil and motor supply during start to spot sag and spikes.
- Power the control board from a regulated, separate 12 V supply and retest.
- Enable command retries to correlate starts with noise events, then add suppression.
These steps reveal whether noise or voltage sag dominates. [Elektroda, Steven Furick, post #21664938]
Is my 5 A relay big enough for a 2.1 A pump?
Maybe not. Contact and coil ratings don’t account for start surges and arcing. The pump runs at 2.1 A, but its start current and inductive kick can exceed a 5 A relay’s comfort zone, causing chatter or early wear. Upsize the relay and add suppression to stabilize switching. [Elektroda, Steven Furick, post #21664931]
Why did a 2 Ω series resistor or parallel capacitors only help sometimes?
Series resistance reduces surge but also slows spin‑up, which can prolong high current. Parallel capacitors can source brief current yet also inject ringing if placed poorly. Without proper snubbers and coil diodes, EMI still reaches logic, so intermittent dropouts persist. Optimize placement and add flyback diodes. [Elektroda, Steven Furick, post #21664931]
Could an overload or safety cutout be tripping at start?
Yes. If an over‑current cutout is set near the motor’s start surge level, it will trip intermittently during startup. Slightly raising the threshold solves erratic cutouts. Always observe battery voltage during start to verify overall supply integrity while adjusting. [Elektroda, Frank Bushnell, post #21664939]
What is an opto‑isolator and why use it here?
An opto‑isolator passes signals using light across an internal gap, isolating grounds. In motor environments it shields microcontrollers and serial ports from spikes and ground shifts. Place optos on RS232 receiver inputs to harden the command path against EMI‑induced glitches. [Elektroda, Mark Harrington, post #21664943]
My battery reads 13 V open‑circuit. Why does the relay still drop out?
Open‑circuit voltage doesn’t reveal internal resistance. Under surge load, a high‑resistance battery sags, starving the relay coil or logic. “It’s more about internal resistance in the battery than Ampere‑Hour capacity.” Test under load, not just at rest. [Elektroda, Steve Lawson, post #21664937]
Could RS232 voltage levels be part of the problem?
Possibly. True RS232 uses ±3 to ±15 V signaling. Marginal levels or missing level shifting can reduce noise margins, especially during EMI events. Verify proper translation between the PC/BT module and the microcontroller, and ensure clean, consistent levels at the receiver. [Elektroda, Steve Lawson, post #21664936]
