ORIGINAL: Andrew

Erik

If you metered the voltage between the 5v rail and the 12v, it would read ~7v. The reason you can get 17v with -5v and +12 is because DC voltages are measured both by voltage differential and by which pole is considered to be ''more positive''. For negative voltages, DC ground could be thought of as being ''more positive'' and the voltage differential between -5v and ground is 5 volts. The diffenential between ground and +12v is 12 volts, consequently the diffential between -5v and +12v will be ~17. If you drew out a number line and marked places for -5, 0, +5 and +12, then counted the units between each, the units between +5 and +12 is 7. The +12 is thought of as being ''more positive'' than +5 since it is further from the ground plane of 0 and when metered, you would use your negative probe on +5 and the positive probe on +12. It's a bit more complicated, but voltage is only the differential between two points, taking into account polarity.

My inclination would be to initially use all 4 resistors is series for a total of 4 ohms. Adding load (reducing the resistance) when not needed only adds heat and consumes power from the PSU. Many of the newer PSUs will latch without a load, but will have a more stable output voltage on the 12v rail with a load. Your choice of resistance should be the highest ohm rating possible for good performance. If it powers your charger satisfactorily with 4 ohms, there is no gain in going any lower.

Keep us posted on how your conversion works out.

andrew