-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Poul-Henning Kamp Sent: Tuesday, July 21, 2009 10:45 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Where does 28V come from?
In message <[email protected]>, Robert Atkinson write s: >The newest standard for cars (Automobiles) is 42V bus with > 36V (3x12V) battery standard, they are jumping 28/24V completely. That one has run into all sorts of trouble. It was sort of predicated on people with microwave ovens and dishwashers in their hummers and that entire market has more or less evaporated. 42V was chosen over 56V because the latter is "high voltage" according to various countries codes. ---------------------------------------------- Higher voltages are being driven by a couple factors, independent of dishwasher installation.. There is a desire to reduce the wiring harness mass and cost. Higher voltages let you use smaller wires, so you spend less money on copper and insulation. For things like electric motor driven power steering and airconditioning, higher voltages are better (smaller wires, lower mass, better efficiency) and would allow doing away with hydraulic pumps and belt driven compressors. Wires are a lot easier to move around than belts and pulleys, so that makes internal layout easier for the styling folks. There is a also a desire to do point of load switching (which also reduces mass.. fewer wires). Historically, there was a problem getting semiconductor switches with appropriate parameters to do high side switching. A Vds-on of 1 Volt is awfully big in the context of a 12V device (that might actually be running at 9V during cranking). It's less of a problem in the context of a 50V bus. Working against this is that semiconductors sort of start at about 60V ratings. A 42 volt bus (still being a multiple of 6, for historical reasons.. darn those Babylonians) gives you 18V margin against the 60V. (especially if you consider that 14.4V is the actual max voltage for a 12V system... 42=50.4V) And then there's the 50V threshold for "low voltage systems" that Poul mentions. I think the latter is starting to be less important, as more hybrid cars have high voltage battery packs show up in the field and we haven't seen dozens of firefighters electrocuted by inadvertently cutting the wires, or shop technicians dying by putting a screwdriver where they shouldn't. You're still stuck in the cost trade between higher voltage semiconductors and lower copper and actuator costs with higher voltages, and even more, between the cost of things like transient suppression. I'll note that spacecraft have been going to higher voltage buses for this kind of reason (except for science spacecraft, which tend to stick with the venerable 28V avionics power, since it's familiar, and there tends to be a lot of scrounging of surplus and reuse of proven designs). ISS uses, I think, 100V DC as the bus, and commercial comsats use bus voltages of 70-90V. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
