It is true that high-side switches are more expensive, but only 2x the cost part-for-part. Relays also require sockets, drive circuitry, and typically a fuse. Relays are also more expensive in other ways:

* They take up more space - one cubic inch (!!) versus 0.08 cubic inch
* They dissipate significant power due to low coil resistance
* They are susceptible to mechanical vibration and heat
* They have finite contact life

For me, though, the primary reason for eliminating relays is because the target car will be used for racing in desert conditions. The particularly high heat and high vibration have been known to cause relays to fail. Some racers have even taken the precaution of replacing relays with switches to prevent this failure.

Solid state switches have several added benefits. They have current limiting and thermal shutdown, so fuses can be eliminated along with relays. The current limiting is also more accurate than standard fuses. They also have diagnostic ability. In my design, there is an LED for each solid state switch. It is a green "OK" LED. If it ever goes out then there is a fault - open load, short circuit, undervoltage, etc. This is very helpful. You can tell if everything is OK at a glance.

Is that sufficient detail? I will try to post my previous solid state power distribution schematic tonight. I built it and it is currently running in the 2003 University of Toledo Formula SAE racecar. The next design will be a significant refinement of this design.

- Lee