Why does a printer's inrush current double for 240V vs 120V?

Question

In this answer, the inrush current for a laser printer is listed as follows:

Inrush Current: (Duration: significantly < 1 second)    
  Model A  (120V): 23 A peak (20 deg C, from cold start) 
  Model AB (240V): 40 A peak (20 deg C, from cold start)

The question posed by Dan Neely in the comments piqued my interest and makes me question my understanding of electrical theory. I would have expected the current draw to be the same, or possibly even half as much, for the higher-voltage model. I should note that I'm basing this assumption on past experience in building out racks in a data center, where we could typically put more 240V servers in a rack than 120V since their current draw was significantly smaller.

So, please school me: why does the 240V model have nearly twice the inrush current as the 120V model?

Answer 1

The operating current should be inversely proportional to the voltage as you expected, but inrush is a different issue. The inrush current probably comes from charging up the reservoir caps immediately after the full wave bridge in power supply. Those capacitors are fixed, but get charged up proportional to the line voltage. The current eventually drawn from them will be inversely proportional to the line voltage, but the inrush only sees the capacitance immediately connected to the power line. More voltage on the same size cap means more Coulombs, which means more current over the same short amount of time.

Answer 2

Inrush generally flows through a resistive device such as a thermistor, and will charge up any capacitors on the mains-side of any power supplies in the device. This is why a larger input voltage means a larger inrush current - the capacitors are like a short-circuit at the moment the power is applied, so it's only the thermistor and the mains voltage that define the maximum inrush current.

The thermistor limits the current charging the capacitors to a safe level, preventing component damage and nuisance trips (blown breakers, fuses, etc.). The thermistor can also help limit the short-circuit current in the event of a shorted capacitor.

Inrush is usually characterized with a programmable AC source or a triac circuit that applies the mains voltage starting at the one of 90 degree peaks, to maximize the peak voltage (and the inrush current).

Cold start refers to the unit being off for sufficient time to ensure that all of the capacitors in the unit are discharged to their absolute minimums. (usually 24 hours).