Control Power Supplies

What the supply feeds

Almost everything in a modern panel runs on 24 VDC: the controller, the I/O modules through the backplane, the network switch, the loop-powered transmitters, the interposing relays, the pilot lights, the radio or cellular modem, and the touchscreen. A single supply can feed all of it in a small panel, and in a small panel it often does. That is the arrangement that produces the call where a shorted float switch cable took the whole station off line.

Separating loads

The supplies in a panel are separated by what fails together. The controller and its backplane are the highest value load and the one that should never lose power because of something in the field. The field devices are the ones exposed to wet junction boxes, chafed cables, and technicians with meters.

Controller supply

Feeds the processor, the backplane, and usually the network switch and the HMI. Nothing that leaves the panel is on it.

Field and I/O supply

Feeds the input wetting voltage, the loop-powered transmitters, the interposing relay coils, and the solenoids. It is the one that trips.

Communication supply

On sites that report over radio or cellular, the modem and radio are often on their own backed-up supply so the station can report the outage that took everything else down.

Sizing

1. 1

   List every load with its current.

   From the datasheets: the backplane draw of the chassis, each transmitter at 20 mA plus HART margin, each relay coil, each pilot light, the switch, the HMI, and the radio. Use maximum values, not typical ones.

2. 2

   Add the inrush.

   Relay coils, solenoids, and anything with a capacitor draw several times their running current for a moment at power-up. A supply that cannot cover the simultaneous inrush at power-up will fold back and the panel will not come up.

3. 3

   Derate for temperature.

   A supply rated at its full output to 40 or 50 degrees C delivers less above that. A panel in the sun in summer is above it. Apply the derating curve from the datasheet at the enclosure design temperature.

4. 4

   Add margin.

   Twenty to thirty percent above the derated total. The margin covers the loads that were not on the list and the ones added later.

5. 5

   Check the voltage at the far end.

   A transmitter at the end of a long run sees the supply voltage minus the drop in the wire. The loop voltage budget on the 4-20 mA pages and the voltage drop calculator cover the arithmetic.

Protection

A DC supply into a short circuit does not blow a fuse the way a transformer does; most fold back their output current to a limit and sit there, dropping the voltage on everything they feed. Each branch off a supply therefore gets its own fuse or DC-rated breaker, sized so that a short on that branch opens the branch protection before the supply reaches its current limit. Fusing the field wiring separately from the panel wiring, and fusing each remote device group separately, keeps a field short from browning out the rest of the panel.

Electronic fused terminal blocks and DC circuit breakers with indication show which branch tripped from the front of the panel. A glass fuse in a holder shows nothing, and finding the open one in a row of twenty is a familiar way to spend an hour.

Grounding the common

The negative side of the 24 VDC system is normally bonded to the panel ground at one point, which references every signal to a known potential and lets a ground fault on the positive side be detected. Bonding it at more than one point creates a ground loop between the bonds, and leaving it floating lets the whole system drift with whatever leakage exists. The one bond, its location, and whether the field supply shares it with the controller supply are decisions that belong on the drawings.

Backup power

A station on utility power loses it several times a year. The question is what should keep working through the outage. The pumps will not, unless there is a generator. But the controller can, and the radio can, and that means the station can report that it is down, hold its retentive data cleanly, and restart without a technician.

AC UPS ahead of the supplies

A small uninterruptible power supply feeding the DC supplies. Simple, and it also rides through the momentary dips that otherwise reboot the controller. The battery is lead-acid or lithium and needs replacing every few years.

DC UPS with a battery module

A 24 VDC supply with a battery on its output, so the DC bus itself is held up. No inverter, higher efficiency, and the battery is monitored by the module. The UPS runtime calculator sizes the battery.

Generator with a transfer switch

For the pumps. The controller still needs a UPS to cover the seconds between utility loss and generator transfer, which is exactly when the controller is needed to manage the restart.