Intermittent Voltage Sag on a 24 V Control Bus
A technician maintains a 24 VDC control cabinet feeding eight solenoid valves through a shared 18 AWG bus. The supply is rated 5 A with foldback current limiting. Valves are rated 12 W each. During commissioning, the technician measures 24.1 V with no valves energized, 23.6 V with one valve energized, and 21.2 V with all eight energized. A digital multimeter in DC mode reads these values, but a scope on the same bus shows a 1.5 V peak-to-peak ripple at roughly 100 Hz when all valves are on. One valve occasionally fails to pull in, and the fault moves when valves are swapped. Bus length from supply to the far valve is 22 m.
1. Compute the total load current and the expected voltage drop given the measured values. Is the supply current limit a plausible contributor?
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Each valve draws P/V = 12 W / 24 V = 0.5 A, so eight valves = 4.0 A. That is below the 5 A limit but close to it, so the supply is loaded to about 80 percent and any inrush could momentarily trip foldback. The observed drop from 24.1 V to 21.2 V is 2.9 V at 4.0 A, implying about 0.73 ohm of series resistance in the bus and connections. For 18 AWG at roughly 0.021 ohm/m, a 22 m run (44 m of conductor) would be about 0.92 ohm, so the measured drop is consistent with conductor resistance rather than a failing supply. Conclusion: the voltage sag is dominated by bus resistance, not current limiting, though the 80 percent loading leaves little margin.
2. The 100 Hz ripple and the moving valve fault: what is the most likely explanation, and how would you confirm it?
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100 Hz on a DC bus fed from a full-wave rectified 50 Hz supply points to inadequate bulk capacitance or a failing filter capacitor, so the supply is not holding the bus stiff under pulsed solenoid current. The moving fault suggests the marginal valve is the one seeing the lowest voltage, not a defective valve. Confirm by scoping the supply output under a resistive load to see if the ripple persists without valves, then scoping at the far end of the bus with all valves energized. If ripple is present at the supply with a dummy load, the filter is the problem; if it appears only at the far end, the bus impedance and load stepping are the cause.
3. Recommend a corrective action that addresses both the sag and the ripple without replacing the supply unnecessarily.
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First verify the supply filter with a resistive load; replace the capacitor or supply only if ripple persists unloaded. For the bus, either increase conductor size (e.g., 14 AWG lowers resistance roughly fourfold) or move the supply closer, and add local bulk capacitance at the far end to absorb solenoid switching. Also add flyback diodes across each solenoid coil if not present, since unsuppressed inductive kick aggravates ripple and can stress the supply. Re-measure loaded voltage at the far valve; target staying above the valve's minimum pull-in voltage with margin.