Built and tested crowbar circuit, corrected issue in power supply and verified it works exactly as it should

CROWBAR CIRCUIT IS A MODERN ADDITION TO THE POWER SUPPLY

A crowbar circuit and card was designed to add to the power supply, forcing a dead short if the output voltage exceeds 6.2V. The powers supply has a fuse added into the secondary circuit, which will blow if the crowbar trips. 

The PCB set I bought had a power supply board but not a crowbar board. I used some perfboard to solder together the circuit, which is only a Zener diode, capacitor, resistor and two SCRs in parallel. The way it wires into the power supply is odd, with very thin traces supporting the short circuit current if the crowbar were to trip. 

Furthermore, the power supply board is designed to only work when cabled to the missing crowbar card, but the schematics are out of date and don't show the connectors involved. For example, the connector bringing the full 11V DC to the crowbar card is two wires, but they are only bridged on the crowbar card. Without it connected, the power supply will not work properly. The connector bringing the +5V to the crowbar card is also two pins, only bridged on the crowbar card, and without them connected the power supply will not work properly. 

VOLTAGE LOW IN POWER SUPPLY DUE TO INCORRECT ZENER DIODE CHOICE

The original design from the Electronics Australia articles had a 6.2V Zener diode and a resistor divider to establish the reference voltage for a 5V output, but the new schematics and PCB specified a 5.6V type. This resulted in an overly low output of the supply. I put in the proper part and was satisfied that it works as specified now. 

TESTED CROWBAR WITH BENCH POWER SUPPLIES

I set up the crowbar card on the bench, with one supply feeding 11V under current limiting so that when the crowbar activates it only draws 1A. Another supply fed the voltage that is sensed by the circuit, which I could carefully dial up and down.

The crowbar circuit is set up to trigger when the output reaches 6.2V. That was exactly the behavior I observed. 

WIRED TOGETHER AND RAN THE POWER SUPPLY UP TO FUSE CAPACITY

I soldered the crowbar circuit leads into the power supply board and plugged in the power cord. I had an electronic load on the output, set up at 100ma initially and which I could then step up to higher loads. The output voltage was 5.12V with the low load and sagged only a bit, remaining above 5V up to the 4A load where the output suddenly went away.

The fuse I had put into the holder was a 4A unit, which blew when the load got that high. I put in a 5A fuse and verified that the supply was working again. Even the 4A would have been sufficient for the EDUC-8 system since it was originally designed for 3A supply to the CPU and another 3A for the worst case demands of peripherals. 

With the change to 74LS logic with its much lower power needs, which will also be used in peripherals, the expected load is going to be less than 4A. In operation, the EDUC-8 consumes than 1A, not 3. 

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