when I say that the IMSAI 8080 PSU has "soda can sized caps", I'm not joking
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@gsuberland you get unregulated +/-8V and +/-16 rails
you’re supposed to do the regulation on each s100 card separately depending on your needs 
(more from page 87 if you're interested in further details
https://dn790007.ca.archive.org/0/items/imsai-user-manual/IMSAI_User_Manual.pdf) -
@xconde “just” 0.095 farad
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when I say that the IMSAI 8080 PSU has "soda can sized caps", I'm not joking
@lynn Wouldn’t want grab ahold of those leads when charged
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when I say that the IMSAI 8080 PSU has "soda can sized caps", I'm not joking
@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" -
when I say that the IMSAI 8080 PSU has "soda can sized caps", I'm not joking
@lynn the forbidden energy drink
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when I say that the IMSAI 8080 PSU has "soda can sized caps", I'm not joking
@lynn feels like these could double as a smoke machine in a jiffy
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when I say that the IMSAI 8080 PSU has "soda can sized caps", I'm not joking
@lynn I want to sip the power juice
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@lynn feels like these could double as a smoke machine in a jiffy
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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" -
@fae @lynn
The S-100 bus power supplies were nominally +8V, +16V, and -16V, filtered but unregulated. Each card was expected to have linear voltage regulators from +8V to +5V, and, if needed, ±16V to ±12V.
The more cards in your system, the more amps of each supply you needed, though the "1400 amps" in the song is gross exaggeration.
Early Altair 8800 systems, and some others, had small backplanes with few slots, and were provided with wimpy power supplies.
1/ -
@fae @lynn
The S-100 bus power supplies were nominally +8V, +16V, and -16V, filtered but unregulated. Each card was expected to have linear voltage regulators from +8V to +5V, and, if needed, ±16V to ±12V.
The more cards in your system, the more amps of each supply you needed, though the "1400 amps" in the song is gross exaggeration.
Early Altair 8800 systems, and some others, had small backplanes with few slots, and were provided with wimpy power supplies.
1/@fae @lynn
The most common configuration IMSAI 8080 provided 22 slots and a beefy power supply, which resulted in MITS and other S-100 vendors having to offer similar configurations.
If you really did fill up a 22-slot backplane (e.g., with CPU, a few I/O cards, and sixteen 4K RAM.cards), you really needed a lot of amps. I'm sure that large S-100 system configurations were the inspiration for the song.
2/ -
@fae @lynn
The most common configuration IMSAI 8080 provided 22 slots and a beefy power supply, which resulted in MITS and other S-100 vendors having to offer similar configurations.
If you really did fill up a 22-slot backplane (e.g., with CPU, a few I/O cards, and sixteen 4K RAM.cards), you really needed a lot of amps. I'm sure that large S-100 system configurations were the inspiration for the song.
2/@fae @lynn
While almost everyone is aware of the dangers of AC line voltage, it is uncommon for people to be aware of the potential danger of low voltages. Skin contact with the +/-16V DC in an S-100 system is incredibly unlikely to cause any harm. You can grab the +16 with one hand, and -16 with the other, and your skin has high enough resistance that nothing happens.
3/ -
@fae @lynn
While almost everyone is aware of the dangers of AC line voltage, it is uncommon for people to be aware of the potential danger of low voltages. Skin contact with the +/-16V DC in an S-100 system is incredibly unlikely to cause any harm. You can grab the +16 with one hand, and -16 with the other, and your skin has high enough resistance that nothing happens.
3/@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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when I say that the IMSAI 8080 PSU has "soda can sized caps", I'm not joking
@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%.
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@fae @lynn
The most common configuration IMSAI 8080 provided 22 slots and a beefy power supply, which resulted in MITS and other S-100 vendors having to offer similar configurations.
If you really did fill up a 22-slot backplane (e.g., with CPU, a few I/O cards, and sixteen 4K RAM.cards), you really needed a lot of amps. I'm sure that large S-100 system configurations were the inspiration for the song.
2/ -
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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.
4/ -
@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)
