powering a device with AC or DC
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. AC/DC device powered by AC source.







simulate this circuit


  How is this possible? 



The polarity question is easy: If the device shown in Figure 1 is powered from a DC supply two of the diodes are permanently reverse biased and are effectively removed from the circuit as shown in Figure 2. If the DC supply polarity is reversed the other two diodes will be used.
The voltage regulation may be a bit more complicated. Linear voltage regulators are cheaper than switched mode voltage regulators but suffer the disadvantage that they burn off the excess voltage as heat. If, for example, our device required 1 A at 12 V and we fed it with 24 V then we would have to burn off 12 V x 1 A = 12 W. This presents a cooling problem. The solution is to use a switched mode power supply which is efficient but more complex or use the linear regulator but specify an AC or DC input sufficient to run the regulator without excessive heat dissipation.



  And why isn't it more common? Are there reasons that more manufacturers don't follow this route?


As explained above, it's certainly more idiot proof but does increase complexity and hence the cost.