________________________________ There is/was an IEEE standard for the S-100 bus; why not hew to that, regarding supply voltages? If I were designing new "retro" boards today, however, I'd go with one of the switching regulators that can be had in TO-220 packages. Basically the same appearance as the veneered and generated 7805, but far more efficient. Cheers, Chuck -----REPLY----- Hi Chuck! Yes, the S-100 crates I have at home all use +8V and 18V/-16V rails from the linear power supplies. I understand where the power standard came from and agree the standard values would be the ideal solution. However I am working on a low cost small S-100 backplane project and I'd like to know what flexibility I have with regards to power supplies in order to keep costs low. 8V SMPSUs aren't exactly common these days and you'd need at least one of those and another 18/-18V SMPSU for a S-100 backplane to work properly. Practically speaking, the linear power supplies are unregulated and generate a range of voltages probably in the 8-10V range as Allison mentioned. I am thinking a 9V and a pair of 15V SMPSUs might be just good enough to work if not a long term solution. Since my goal is to keep costs low I am trying to find less expensive options and still have a useable system. The intent of the project is a small bench unit for testing and repair of S-100 boards but not a full blown system. I agree with Barry and Dwight to keep the voltage rails just about 3V above the 7805/7812 SCRs output for efficient regulation without excessive heat. However, 9V SMPSUs are really common and cheap on eBay and could be readily converted if not optimal, at least they'd be useable for a while without destroying anything. Add a pair of 15V SMPSUs and for less than $50 you'd have a usable S-100 PS system. I don't think you could make a S-100 linear PS for that low cost. Maybe I could put a high current silicon diode in line with the 9V SMPSU and drop it to like 8.4V? That'd be closer to the 8V standard. I posted a schematic and PCB layout on the N8VEM wiki but whenever I post the URL to CCTALK it never makes it to the list. I suspect it's being spam filtered or something. I believe the actual useable ranges of the S-100 unregulated power supply is about 8V to 10V, 15 to 20V, and -15 to -20V. For current capacity since the PCB is only 4 slots, I am thinking on the order of 1V per slot should be sufficient. There are some older S-100 boards that will exceed that limit like those massive 8K SRAM boards but if you are working on one of those probably not all the slots would be used. At any rate, I am swamped with my current N8VEM related projects and the S-100 backplane project is on slow simmer for a while. Thanks and have a nice day! Andrew Lynch