Since I’m almost to the point of building my prototype SCELBI, I have to figure out how I’m going to power the SCELBI during initial bring up. While, later on, I intend to put together a linear supply similar to what SCELBI sold, for now I’m going to make do with more modern supplies. First thing to do is to look at requirements.
The SCELBI 8H requires two supplies, +5 volts and -9 volts. Here is what the SCELBI “ASSEMBLY AND INSTALLATION INSTRUCTIONS AND SYSTEM DOCUMENTATION” says about power requirements.
“CONNECTION OF THE POWER SUPPLY
THE SCELBI-SH MINI-COMPUTER REQUIRES A POWER SUPPLY CAPABLE OF DELIVERING +5 VOLTS AND -9 VOLTS (PLUS OR MINUS 2%.) THE CURRENT REQUIREMENTS OF THE SUPPLY DEPEND ON THE AMOUNT OF MEMORY IN THE SYSTEM (AND ANY ADDITIONAL PERIPHERAL UNITS WHICH MAY BE CONNECTED TO THE SUPPLY.) THE AMOUNT OF CURRENT THAT THE POWER SUPPLY MUST BE CAPABLE OF DELIVERING WHILE MAINTAINING ADEQUATE VOLTAGE REGULATION BE DETERMINED FROM THE FOLLOWING DATA.
THE BASIC SCELBI CARD SET CONSISTING OF ONE EACH OF: SCELBI #1100 CPU CARD, SCELBI #1101 DBB & OUTPUT CARD, SCELBI #1102 INPUT CARD, AND SCELBI #1104 FRONT PANEL CARD; REQUIRES A MAXIMUM OF 1.5 AMPS AT +5 VOLTS AND 100 MILLIAMPERES OF -9 VOLTS.
EACH “PAGE” OF MEMORY IN THE SYSTEM (256 WORDS) REQUIRES 200 MILLIAMPERES OF +5 VOLTS AND -9 VOLTS.
THE CURRENT REQUIREMENTS FOR ANY ADDITIONAL PERIPHERAL UNITS UTILIZING THE POWER SUPPLY MUST BE ADDED TO OBTAIN THE TOTAL CURRENT THAT THE POWER SUPPLY MUST PROVIDE.
FOR EXAMPLE: A BASIC SCELBI-8H EQUIPPED WITH 1024 WORDS OF MEMORY
(4 PAGES) REQUIRES: 1.5 A + (4 X .2 AMPS) = 2.3 AMPS OF +5 VOLTS AND
.1 A + (4 X .2 AMPS) = .9 AMPS OF -9 VOLTS.”
Now, as far as the -9 volt supply goes, this rating appears to be very conservative, at least on paper. There are only two ICs that use -9 volts on the SCELBI 8H, the 8008 processor, and the 1101 SRAM. Reading the spec sheet shows that the 8008 consumes a max of 60 milliamps and the 1101 SRAM a max of 19 milliamps per chip. Typical values are 30 milliamp for the 8008 and 13 milliamps for the 1101.
If the spec sheets are correct, then the base board set would typically draw 30 and max of 60 milliamps. Each page of SRAM would typically draw 13×8 = 104 and max of 19×8 = 152 milliamps. My initial bring up will involve the 4 base board set and 1 SRAM card with 4 banks populated. Max value for this setup is calculated as 152×4 = 608 + 60 or 668 milliamps. The 900 milliamps SCELBI recommends for this config, would give plenty of margin, and just to make choosing a supply easier, I’ll round up to see if I can find a minimum of a 1 amp supply for the -9 volts. Since typical values of a system with 1K might be around 104×4 + 30 = 446 milliamps, I may actually be able to power 2K of SRAM with a 1 AMP -9 volt supply.
The same calculation on the 5 volt supply is much more involved, since it literally involves counting current consumption of every component in the system, so I’ll go with SCELBI recommendations of 2.3 AMPS, but for sanities sake I’ll round up to 3 AMPs as a minimum for the 5 volt supply.
This is for initial bring up. Eventually I’ll want to test a fully populated system, with accessories. Once I have bring up done, I can measure actual consumption and increase power supply requirements based on measured values. Just in case I can start with with power supplies that will handle a full system at initial bring up, I’ll use SCELBI’s recommendations to calculate requirements for a full 4K, 8H system.
That would be 16 pages x 200 milliamps of 5v and -9v for memory, plus the base of 100 milliamps of -9 and 1.5 amps for 5 volts. Total would be 16 pages x.2 amps = 3.2 amps + .1 = 3.3 amps for -9 volts. For 5 volts, 16 pages x .2 amps = 3.2 amps and +1.5 amps = 4.7 amps. I’ll round up the 3.3 AMPs of -9 volts to 4 AMPs and 4.7 AMPS of 5 volts to 5 AMPS.
Ok, so now I know requirements for 8H power supply for bring up and and have a target for full system testing.
Minimum (bring up) for -9 volts is 1 AMP and full system is 4 AMPs
Minimum (bring up) for 5 volts is 3 AMPs and full system is 5 AMPs
What is the most practical way to provide +5 and -9 volts, at these power levels.
I actually have all the components in my Mimeo parts stash, except the -9 regulator, to build a linear supply that would meet this requirement. I actually have the -9 volt regulator, but it would have to stripped off my 1101 SRAM tester.
There are some downsides to this approach as it uses up some hard to find Mimeo parts, it would not be sufficient for a 4K 8H system, and it would take some time to put together and test.
To me, the best answer, is to use off the shelf variable “bench” power supplies. These will be useful for a number of other projects down the road and decent ones provide readouts on current consumption, which a basic homemade supply will not provide. I did a little investigation and there is an easy way to supply both positive and negative voltages to a circuit, if you have a couple of bench supplies with “floating” outputs. Here is a link to BK Precision’s description about how to do it.
For initial bring up, I need 2 bench power supplies (or a single dual supply) with floating outputs for bring up. I already have an old Micronta 22-123 1 AMP 0-25 volt variable DC supply. I’ve had this supply so long, that I don’t even know when or where I got it. It’s been through the wars, having been repaired a number of times. For a long time it operated intermittently, but a year or two ago, I reflowed the solder on the power transistor pins, and it’s been solid, since. Since it will supply 1 AMP, it should work as the -9 volt supply for bring up, assuming that it has floating outputs. Even though I’ve had it a long time, I’ve always run it independently of other power supplies, so I wasn’t sure about that.
After failing to find a manual online, I checked for connectivity between the AC plug and the terminals and there was none. This was a good sign. To prove that the outputs were floating I connected the plus side to the ground on an Apple II motherboard. I connected this through a 5 ohm, 5 watt power resistor to the negative terminal of the power supply. Since the resistor was rated at 5 watts, I could run 1 AMP through the 5 ohm resistance when the power supply was set to 5 volts. This would give me a good indication that I could supply the SCELBI at -9 volts at 1 AMP with this power supply. Next, I connected a voltmeter on each side of the resistor, so I could measure current and voltage.
Before turning on the Apple II or power supply, I cranked the voltage on the power supply all the way down. Now I turned on the Apple II and started running a 2102 SRAM test, that I already had it set up to run. I made sure that it was running correctly and turned on the Micronta 22-123 power supply with crossed fingers. Nothing happened, as I hoped. I made sure the Apple II continued to run the SRAM test, before starting to turn up the voltage. Everything performed as expected, the supply providing 1 AMP of current at -5 volts with no problems, at all. The Apple continued to run the SRAM test, with no glitches. It appears that this supply will work fine as the bring up -9 volt supply. Half the bring up power supply problem is solved.
The other half of the power supply problem is deciding upon a new bench supply to support 5 volts. I’ll blog about my decision process in an upcoming post.