The short answer
Failed Power Supplies
The 24 volt supply feeds the controller, the I/O, the instruments, the relays, the network switch, and the radio, so its failure looks like a dead panel or, worse, a panel that half works. It fails in four ways: completely dead, with no output and usually no indicator; low, delivering less than its rating under load; noisy, with ripple or oscillation that upsets analog inputs and communications; or cycling, shutting down on overload or overtemperature and restarting. Each has a measurement: the output voltage under load, the output current against the rating, the ripple on a scope or a meter that reads AC, and the input voltage during the failure. Most supplies that fail did so from heat and age, or because the load grew past the rating as devices were added over the years; the fix is a supply sized for the real load with margin, mounted where it can cool, with a monitored output and, where the panel matters, a redundant pair.
Key points
- Measure output voltage under load, output current against the rating, and ripple; the three numbers classify the failure.
- A supply that cycles is overloaded or overheating, not necessarily failed.
- Loads grow: sum the actual current draw of everything on the bus before blaming the supply.
- Capacitors age with heat; a supply at the top of a hot panel has a short life.
- Specify monitoring, margin, and, for critical panels, a redundant pair with diode coupling.
Classify the failure
| Measurement | Result | Meaning |
|---|---|---|
| Input voltage at the supply terminals | Absent | The problem is upstream: fuse, breaker, transformer, wiring |
| Input present, output voltage | Zero, indicator dark | The supply has failed, or its internal fuse opened on a downstream short |
| Output voltage with the load disconnected | Normal | The supply works; the load is the problem, either a short or an overload |
| Output voltage under load | Low, drooping | Overload, or a supply that has lost capacity with age |
| Output current | At or above the rating | Overloaded; the load grew or a device is drawing fault current |
| Ripple on the output | Well above the specification | Aging capacitors; the supply is failing and disturbing the analog signals |
| Output cycling on and off | Every few seconds | Overload shutdown and restart, or thermal shutdown |
| Input voltage during a motor start | Sags below the supply minimum | The supply drops out on sag; input problem, not supply |
Working it
- 1
Input
Voltage at the input terminals; if absent, work upstream: the fuse, the breaker, the transformer, the disconnect.
- 2
Output unloaded
Disconnect the output and measure. A dead supply with good input has failed; a supply that recovers when unloaded is overloaded or shorted downstream.
- 3
Sum the load
Add the nameplate draw of every device on the 24 volt bus, including the instruments powered from it and the relay coils, and compare with the rating. Then measure the actual current with a clamp meter.
- 4
Section the bus
With the supply disconnected, measure the bus resistance to find a short; with the supply connected, remove branches one at a time to find an overload.
- 5
Ripple and noise
A meter on AC volts across the output, or a scope; ripple above a few hundred millivolts on a supply rated for tens is a failing supply.
- 6
Temperature
The supply case temperature and the panel temperature at its location; a supply derates above its rated ambient and a hot supply fails young.
- 7
Input sag
A recording meter on the input during motor starts and generator transfers; a supply with a wide input range or a longer hold-up time survives what a narrow one does not.
Why supplies fail
- Heat
- Electrolytic capacitors lose capacity with temperature; a supply mounted above a transformer or a drive, or in a panel in the sun without cooling, ages in a few years. Mount it low and clear, with the panel heat calculation done.
- Load growth
- A supply sized for the original panel with no margin, then loaded with a radio, a switch, a touchscreen, and six more instruments. Sum the load at every change.
- Downstream shorts
- A shorted field device or cable holds the supply in current limit or cycles it; the supply is fine and the field is not.
- Input quality
- Sags, surges, and generator transfers on a supply with narrow input tolerance; the supply drops out, restarts, and looks like it failed.
- Age
- A quality supply lasts a decade or more in a cool panel; one at its rated temperature for ten years is due.
Redundant supplies
Two supplies with their outputs combined through a redundancy module or diodes share the load and, when one fails, the other carries it. The arrangement needs each supply rated for the whole load, a monitoring contact on each so that a failed supply is alarmed rather than discovered when the second one fails, and inputs from separate breakers, and ideally separate sources, so that one upstream fault does not take both. Without the alarm, redundancy is a way to have a failed supply for years without knowing.
Specifying the next one
- Rated for the measured load plus a third, at the panel maximum temperature with the derating applied.
- Wide input range and a hold-up time of at least twenty milliseconds where the site sees sags.
- A monitoring contact, wired and alarmed.
- Mounted low in the panel with the clearances the manufacturer specifies.
- Redundant pair with a redundancy module on panels whose failure stops the process or blinds the SCADA.
- A spare of the same model on the shelf.
Frequently asked questions
- The supply reads 24 volts but the controller keeps rebooting.
- Measure under load during the reboot: the voltage may sag briefly when a load switches, or the ripple may be high. A supply near its limit holds the average voltage and loses it in transients. Reduce the load or replace the supply.
- Can I add a second supply in parallel to get more current?
- Only with supplies designed for parallel operation and a proper redundancy module or load-sharing setting, and each rated for what it will carry. Two supplies wired together without that fight each other.
- The supply is hot to the touch. Is that a failure?
- A supply near its rating is warm; one too hot to hold is overloaded, poorly ventilated, or in a hot panel. Measure the current and the ambient, and act before it fails.
- Why does the supply fail only on generator power?
- The generator waveform or frequency, or the voltage during load steps, is outside the supply input range. A supply with a wider input range, or a small UPS ahead of it, fixes it.
Related topics
- Panel Power SuppliesThe 24 V DC supply behind everything in a modern panel: sizing with real inrush and duty, single versus redundant with diode modules, how a switch-mode supply behaves under a short, distribution and fusing per branch, monitoring, and the failure modes that take out a whole panel at once.
- Power Supply FailureA 24 volt DC supply whose output is gone, low, cycling, or noisy: an overload that has put it into current limit, a supply killed by heat and age, input voltage out of range, or surge damage. One test tells an overloaded supply from a dead one.
- How to Size a Control Panel Power SupplyAdd up the real load, account for inrush and simultaneity, add headroom, and check the heat you just added to the enclosure.
- PLC PowerPowering the controller section of a panel: the control transformer and 24 volt supplies, separating controller power from field I/O power, sizing with margin, the UPS and what rides through, protection and monitoring, the DC ground, and restart sequencing.
- Enclosure Heat CalculationsEstimating the temperature inside a control panel from the power its contents dissipate and the surface that carries it out, what to do when the answer is too hot, and why drives change everything.
- Voltage Sag on Motor StartLights dim, contactors drop, and controllers reset every time the big pump starts: the starting current of an across-the-line motor pulling down a weak source. Measuring the sag, checking the source, and fixing it with a soft starter or ride-through.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.