Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 09:31 PM 4/26/2007, Tesla list wrote:
Original poster: David Speck <Dave@xxxxxxxxxxxxxxxx>Basic reason -- solar cells make DC current. If you want something else, then you convert it at the user end.
But there is a lot of engineering that goes into the selection of the voltages used.
older (and scientific) spacecraft use 24 VDC, so they can use designs developed back in the days when you needed a 24V battery to start a truck or piston plane engine. Science spacecraft are always low budget (in a relative sense, nothing in space is cheap) so you try and use things and designs you've used before.
In the commercial world, (e.g. geosynchronous satellites with DBS TV), the economies of scale are different so they tend to use higher bus voltages (50, 70, etc.) depending on semiconductor technology (gotta have margin on those FET breakdown voltages.. getting a space qualified 1200V FET is a challenge.. getting 150-200V is easier.. )
As you get over 100V, corona and breakdown become bigger issues (FWIW, more than one spacecraft has been lost from this kind of thing...)
ISS chose 120V as a sort of "as high as you can get without going to herculean efforts for insulation" sort of thing.
Davethe International Space Station, which uses 120V DC power everywhere. Why? -Phil LaBudde