What the C50A102 alarm means

The 48 V robot supply failed to reach the acceptable level during the power-up ramp; joints cannot be enabled. We see this alarm constantly when servicing Universal Robots CB3 / e-Series (UR5/UR10/UR16/UR20) systems, and the first on-site pass usually pins down the cause.

Most common causes

  • Weak or failing 48 V power supply unit
  • Excessive inrush load from a partially shorted joint motor
  • Bad power cable between supply and control box
  • Aging supply capacitors
Field note: Based on our handling experience the first cause is the most frequent — use it as your starting point.

Step-by-step field fix

  1. Do a complete reboot sequence (full power-off, wait, power-on)
  2. Check the power cable and connections between power supply and control box
  3. Measure the 48 V output under load
  4. Disconnect peripherals and retry to isolate the load
  5. Replace the power supply if the ramp still fails
Safety first: For Universal Robots c50a102, this alarm family involves the DC bus, which sits at several hundred volts and stays charged for minutes after power-off - touching terminals is a shock hazard. On site you may not carry out the first step yourself: Do a complete reboot sequence (full power-off, wait, power-on). Beyond that it is the DC bus and power stage work - leave it to a professional. On Universal Robots c50a102: checking incoming voltage at the disconnect and looking at breakers and fuses is fine to do yourself; opening the cabinet to touch bus terminals or pulling power modules needs a qualified electrician.

How we diagnose drive alarms on site

Universal Robots c50a102 belongs to the drive & power stage family. On Universal Robots c50a102 the failing stage is joint-integrated drives (motor, drive and harmonic reducer in one joint) on the UR control box (CB3 / e-Series) - the fault sits on the energy chain: mains input, rectification, DC bus, braking resistor, inverter power devices and the drive board. We start reading Universal Robots c50a102 at the PolyScope event and alarm pages; codes carry a C prefix (C15x, C5x, C7x).

The chain involved in Universal Robots c50a102 runs: three-phase mains -> rectifier -> DC bus (bus capacitors) -> brake chopper / braking resistor -> inverter (IGBT) -> motor.

Weak or failing 48 V power supply unit is common on Universal Robots c50a102, but look at the duty cycle: Cells with high-inertia loads or frequent start-stop cycles return far more regenerative energy than a plain transfer cell, and the braking resistor lives in a hot cycle that shortens its life. When we suspect this family we ask what changed: if the takt was raised recently or a heavier fixture went on, the alarm cluster usually lines up with that process change. Replacing the resistor alone will not fix it - the chopper rating and resistor wattage have to be re-checked against the new duty.

Field notes on this page cover Universal Robots c50a102 (48V Power Supply — Voltage Rising Too Slowly); written by the Henghuan repair team, updated 2026-10-01.

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Related Drive & Power Stage alarms

C55A34 — Current Distributor / Safety Relay Error C72A2 — Power Supply Cannot Deliver 48V
Same family: this code belongs to the Drive & power stage alarms family - the page collects all Drive & power stage-family codes we have handled, with the common inspection order and spares.
Other brands: we also keep field-fix libraries for Kawasaki · Staubli · Epson and more — see the complete alarm code hub.

Frequently asked questions

QWhat does the Universal Robots c50a102 alarm mean?

The 48 V robot supply failed to reach the acceptable level during the power-up ramp; joints cannot be enabled. We see this alarm constantly when servicing Universal Robots CB3 / e-Series (UR5/UR10/UR16/UR20) systems, and the first on-site pass usually pins down the cause. Universal Robots c50a102 sits in the drive & power stage family; on our work orders the highest-frequency trigger for this code is "Weak or failing 48 V power supply unit".

QCan I clear a Universal Robots c50a102 alarm myself?

Whether Universal Robots c50a102 clears on its own depends on the outcome of "Do a complete reboot sequence (full power-off, wait, power-on)". Finish the flow with "Replace the power supply if the ramp still fails" to confirm Universal Robots c50a102 - do not skip it. If that step is clean, the second most frequent cause on Universal Robots c50a102 is "Excessive inrush load from a partially shorted joint motor". Two attempts and still there means stop: Universal Robots c50a102 involves the DC bus and power stage, and running Universal Robots c50a102 on can turn a repairable fault into a replacement.

QFor drive/power alarms like Universal Robots c50a102, what spares do you bring on site?

The commonest causes we see on Universal Robots c50a102 are "Weak or failing 48 V power supply unit, Excessive inrush load from a partially shorted joint motor, Bad power cable between supply and control box", so we stock the van against those. For Universal Robots c50a102 specifically, we stock the van along the energy chain: braking resistors, bus capacitors, input fuses, drive-board supply parts and the common power modules for the brand. For Universal Robots c50a102, drive alarms usually stop the whole line, so they sit at the front of our dispatch queue. Before we roll, tell us what Universal Robots c50a102 shows on the PolyScope event and alarm pages; codes carry a C prefix (C15x, C5x, C7x) and what the machine was doing - phone or video triage is free.

The notes on Universal Robots c50a102 were summarized by the Henghuan Robotics repair team from on-site service. The alarm interpretation is our empirical summary for troubleshooting reference and may vary by controller and software version — please check Universal Robots documentation or contact us with questions. Henghuan is an independent service provider, not affiliated with or endorsed by Universal Robots.