The 24V Supply That Dropped: Sizing the Control Transformer
A 24V DC power supply feeding 20 sensors, 10 relays, and 5 solenoid valves. Under load, the voltage dropped to 21V. The sensors started dropping out. The relays chattered. The power supply was rated for 5A. The actual load was 4.8A. It was running at 96% capacity — barely. This is about power supply sizing, not about the supply being defective.
The current calculation
| Load | Current draw | Quantity | Total |
|---|---|---|---|
| Proximity sensor | 0.05 A | 20 | 1.0 A |
| Relay coil (24V) | 0.02 A | 10 | 0.2 A |
| Solenoid valve (24V) | 0.3 A | 5 | 1.5 A |
| PLC (CPU + I/O) | 1.0 A | 1 | 1.0 A |
| HMI | 0.5 A | 1 | 0.5 A |
| Total | 4.2 A |
The supply is rated 5A. The load is 4.2A. That’s 84% capacity. But inrush current (solenoids turning on, relays energizing) spikes to 6A for a few hundred milliseconds. The supply’s hold-up time is short. The voltage dips. The sensors see 21V. The PLC’s 24V input (minimum 20V) is close to the edge. When multiple solenoids turn on simultaneously, the voltage drops further.
The sizing rule
A 24V power supply should be sized at 60-70% of its rated current. If the load is 4.2A, use a 6A or 8A supply (not 5A). The 6A supply runs at 70% capacity. The voltage stays stable under inrush. No dips.
| Supply rating | At 4.2A load | Margin |
|---|---|---|
| 5 A | 84% loaded | Too tight (our problem) |
| 6 A | 70% loaded | Good |
| 10 A | 42% loaded | Plenty of margin |
What we did
We replaced the 5A supply with a 10A supply. The voltage stayed at 24V under all loads. The sensors didn’t drop out. The extra capacity costs $50 more than the 5A supply. The downtime from a brown-out was costing $1000 per incident. The 10A supply paid for itself in one incident.
The inrush
Solenoid valves and relay coils draw inrush current when they first turn on. The inrush is 2-5x the steady current for 50-100 ms. If multiple devices turn on at once (at machine start), the supply sees a spike. A larger supply handles the spike without voltage drop. A small supply browns out. The 10A supply handles the 6A inrush easily.
The distributed power
For large machines, use multiple power supplies: one for the PLC and I/O (sensitive electronics), one for the valves and relays (high-current, noisy). The valve supply doesn’t share its dips with the PLC. If the valves draw 1.5A and cause a dip, the PLC supply (separate) doesn’t see it.
The 24V rule: Size the supply at 60-70% of rated current. If the load is 4A, use a 6A or 10A supply. Don’t run at 90% capacity. Separate PLC power from valve power. A brown-out that drops sensors isn’t a sensor problem — it’s an undersized supply.
