The 24V Supply That Dropped (Undersized)
We used a 24V power supply (5A) for a machine. The load (sensors, valves, PLC) drew 4A. On the bench, it held 24V. On the floor, when all the valves switched (the inrush current), the supply dropped to 18V (the PLC browned out, the sensors reset). The problem: the supply was rated for 5A (continuous), but the inrush (when the valves energized) was 7A (peak). The supply couldn’t handle the peak (it dropped). We upsized to a 10A supply. The peak (7A) was within the 10A rating. The voltage held. The mistake was sizing the supply for the average (4A) without the peak (inrush).
24V power supply sizing includes the peak (inrush). This article covers the method.
The Load (Continuous vs Peak)
The 24V load has two parts:
- Continuous: The sensors (each draws 10–20 mA), the PLC (a few hundred mA). Steady. Sum: 4A.
- Peak (inrush): The valves (each solenoid draws a peak, article 90). When all the valves switch at once, the inrush is high (7A, briefly).
Step 1: Sum the Continuous
Sum the continuous load (sensors + PLC). For 20 sensors (15 mA each) = 0.3A, the PLC = 0.5A, the valves (steady) = 3A. Total continuous: 3.8A.
Step 2: Add the Peak (Inrush)
The valves’ inrush (when they energize) is 2× the steady (briefly). For 8 valves (steady 0.375A each), the peak is 0.75A each (briefly). All at once: 6A peak. Plus the continuous (3.8A). The peak total is ~7A.
The 24V rule: Size the supply for the peak (inrush), not the average. The supply that dropped was 5A on a 4A average (but 7A peak). Use a 10A supply (the peak is within). Keep 50% margin (the peak × 1.5).
Step 3: The Margin (50%)
Add a 50% margin (for future, article 185). For a 7A peak, the supply must be 10A (7 × 1.5). A 10A supply handles it. The supply doesn’t run at 100% (it’s rated 10A, peak 7A). It lasts.
Step 4: Redundancy (Critical)
For a critical machine (a food line, a 24/7), use a redundant supply (two supplies, in parallel). If one fails, the other takes over. More expensive, but no downtime (article 100). For a general machine, a single supply (with margin) is fine.
| Load | Continuous | Peak | Supply |
|---|---|---|---|
| Small (10 sensors, 4 valves) | 1.5A | 3A | 5A |
| Medium (20 sensors, 8 valves) | 3.8A | 7A | 10A |
| Large (50 sensors, 20 valves) | 10A | 18A | 20A |
A 24V Checklist
- Sum the continuous? (Sensors + PLC?)
- Is there a peak? (Valve inrush?)
- Size for the peak? (Not average?)
- Add 50% margin? (Future?)
- Is the supply rated? (A?)
- Does the voltage hold? (24V? Test?)
- Is there a fuse? (Short circuit?)
- For critical: redundancy? (Two supplies?)
- Is the supply cooled? (Article 168?)
- Is the 0V grounded? (Article 169?)
The Bottom Line
24V power supply sizing uses the peak (inrush), not the average. The supply that dropped was 5A on a 4A average (7A peak). Size for the peak, with 50% margin. The supply that held the voltage wasn’t the cheapest one — it had peak margin.
