when I say that the IMSAI 8080 PSU has "soda can sized caps", I'm not joking
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@lynn
Color me unimpressed with IMSAI's use of 15VDC rated capacitors (assuming those are factory spec) on a supply rail they rate for nominal 13.5VDC. I never before noticed that IMSAI did that.
Aluminum electrolytics don't need DC bias derating like ceramics, but they should still be specified with some headroom. 15V is only 11% over nominal, and line voltage can easily be vary over time by more than ±11%.
@brouhaha pretty sure this is all factory. It's also the PSU-C, Rev 1, the very first one they did.
I think a few months in they changed a bunch of the design, including replacing the transformer to one that can be wired for many different input voltages, though the caps were all wired from the top, exposed, so a little more dangerous
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@fae @lynn
But if you are wearing a metal watch, ring, etc, and get one of the supply rails shorted through it to ground or another supply rail, the supply will give all of the amps it can (usually much beyond its rated output, at least for seconds) through that metal. It can't electrocute you, but the metal can heat up enough to cause a serious burn. With a ring, potentially serious enough to lose the finger.
Lesson: remove watch and hand jewelry while working inside computers. Even PCs.
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@brouhaha @fae fairly confident that one of the I/O ports was wired up for a parallel keyboard due to the character terminal card installed, but this thing came with neither the keyboard or disk drive, and I have since tried to re-wire the I/O card (I might still have pics of the original config somewhere, but like... good luck finding the right keyboard. i'll just talk to it over serial, somewhere down the line, probably)
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@lynn @fae
Even so...
Most cards other than 8080 CPU cards had their main power draw on the +5V rail. Notable exceptions were dynamic RAM cards using 4K or 16K DRAMs, which drew a lot of current from the +16V (+13.5V IMSAI) rail.
I could see having a 22-slot backplane full of fancy cards that each drew perhaps 3A of +8V. AFAIK, no stock S-100 chassis had a power supply that could handle that, nor could any stock backplane, and you'd need extra cooling fans blowing over the cards. -
@lynn @fae
Even so...
Most cards other than 8080 CPU cards had their main power draw on the +5V rail. Notable exceptions were dynamic RAM cards using 4K or 16K DRAMs, which drew a lot of current from the +16V (+13.5V IMSAI) rail.
I could see having a 22-slot backplane full of fancy cards that each drew perhaps 3A of +8V. AFAIK, no stock S-100 chassis had a power supply that could handle that, nor could any stock backplane, and you'd need extra cooling fans blowing over the cards. -
@fae @lynn
But if you are wearing a metal watch, ring, etc, and get one of the supply rails shorted through it to ground or another supply rail, the supply will give all of the amps it can (usually much beyond its rated output, at least for seconds) through that metal. It can't electrocute you, but the metal can heat up enough to cause a serious burn. With a ring, potentially serious enough to lose the finger.
Lesson: remove watch and hand jewelry while working inside computers. Even PCs.
4/The designer of the Rational R1000 told how his engineering-ring once shorted pins on this backplane.
Those grey metal blocks at the bottom is where you bolt on the AWG 3/0 cables to the power-supplies, each of which is rated for 200A.
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@lynn
And when they turn the power on, it's sure to dim the lamps
With plus or minus sixteen volts and fourteen hundred amps
– Frank Hayes, "S-100 Bus"@brouhaha @lynn OMG I had never heard that https://www.youtube.com/watch?v=bMS6G83NqFQ
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when I say that the IMSAI 8080 PSU has "soda can sized caps", I'm not joking
@lynn As a schoolboy I worked with a cast-off LACE MK II analogue computer.
Which was valve based, and had big power supplies with very large capacitors.
My little brother discovered that if you took a spare one of these, and clamped its terminals in a stand nicked from the chemistry lab, and then put 6.3v AC at many, many amps across the terminals from one of the computer's massive valve heater transformers, the capacitor can would shoot right across the school playground.
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