To save on cost, I wish to buy a 9V DC and a 1.5V DC power supply and connect the positive outputs together to create a 7.5V 200mA supply my Plantronics headset needs. That is, 9 – 1.5 = 7.5 across the two negatives at that point. Problems?
Combine 9V and 1.5V power supplies to make a 7.5V supply
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The 1.5V supply will have current trying to flow in reverse through it from the 9V supply. If it were a battery there may be some problems with some technologies but because it's a power supply and more than likely uses a diode bridge, I don't think you'll find one that works: -
simulate this circuit – Schematic created using CircuitLab