Posted by Sherman in Idaho [72.47.120.154] on Tuesday, June 02, 2015 at 20:43:05 :
In Reply to: Re: Relays? posted by John B [68.69.26.160] on Tuesday, June 02, 2015 at 00:00:04 :
Manufacturers these days are under so much pressure to increase their CAFE numbers that they have reduced the wire sizes to an absolute minimum, usually well below what electrical codes would allow in buildings. They are just big enough to not melt, but that's about it. On top of that, there's no room for extra wires behind the dash any more, and in most new vehicles it's really a computer that controls the various electrical functions any way, with the driver's "controls" just being a request to the computer to please think about maybe doing such and such if it isn't too much bother. A tiny switch that sends a signal to a computer weighs less and takes up less room than a conventional switch that must handle full load current.
Put all that together, and relays make a lot of sense. Minimizing the length of heavy wire outweighs (as it were) the weight of the added relays. Relays can be put close to loads, which is usually not behind the dashboard. And with computers controlling high-power loads, relays would be needed anyway, so even the weight of the relay isn't really a penalty. I'm sure eventually MOSFETS and SSRs will replace electromechanical relays to save a little more weight, but for now relays are cheap and reliable and provide isolation for free.
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