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Thread: Bringing up IMAI 8080, should I fuse the low voltage supplies?

  1. #1

    Default Bringing up IMAI 8080, should I fuse the low voltage supplies?

    Finally decided to pull the IMSAI 8080 out of storage and see if she'll come up. Although I don't recall doing this (or much else...), when I built this in 1978 or so, I put inline fuses for the 8V, +16V, -16V supplies. I didn't like the soldering job I did on the ground wires, so I pulled the motherboard and power supply. I'd prefer to feed the +8V supply with all 4 leads (per the IMSAI manual), but I'm also partial to the idea of fusing it. I'm looking for yay/nay opinions (with reasoning behind them) as to whether I should or should not fuse them, and if so, single feed each supply, or use lower rated fuses and multiple leads.

    Right now I've pulled the caps out of the PS board and am in the process of reforming them. I really wish I could find a proper capacitor tester that can handle measuring the ESR at 120Hz, the leakage, DF and actual capacitance. Overkill, I know, but I'd like to know well these caps have aged. I know I can do all these with a frequency generator, 'scope, etc, but I like to buy test equipment

    Also really hoping my boot floppy hasn't suffered bit rot...


  2. #2


    If you are thinking of adding fuses you need to consider what it is you are aiming to protect.

    Generally a fuse on the primary side of the transformer protects the wiring leading to the transformer and the transformer windings sometimes if the transformer is overloaded or the primary winding fails or it develops shorted turns. So in the same way the fuse on your domestic breaker box protects the wiring in the walls of your house from fire/burning, the fuse inside your appliance on the primary side protects the wiring in the instrument and possibly the transformer, so generally the fuse in the appliance would have a lower rating than the breaker box fuse, and a lower current rating than the wiring in the appliance that the fuse protects.

    It is possible of course to have an abnormally high current and overheat the transformer, with the standard fuse, because the fuse is often selected with a significantly higher current rating than the running current, so it does not cause nuisance fuse blows with turn on current surges. In general the application of fuses is to protect wiring, not components, but with transformer primaries there is some protection as they are made of copper wire and its high temperatures there that degrade insulation and cause transformer failures. Most conventional fuses are not fast to to protect semiconductor devices, there are special types that can be included in circuits locally that can help, including fusible resistors.

    If you are considering adding a fuse to the secondary circuit, it is impractical and achieves little over the primary side fuse, by adding one on the transformer secondary/rectifier side of the filter capacitor, because the initial surge currents can be massive, especially with a large uF value capacitor typical of those used in vintage analog computer supplies.

    So then it raises the question, what about the output of the power supplies after the filter capacitors, is it worth adding fuses and what would you be protecting?

    The 5V supplies are generally regulated and have overload protection, so these is little risk to any wiring there. For unregulated 8V or 16V supplies, if you added a fuse you would be protecting the wiring leading from the main filter capacitors in the power supply to the card edge connectors. The regulators on the pcb cards do most of the protection, such as the 7805, 7812 etc, have overheating and short circuit protection built in.

    Though there could be an argument for adding fuses if you are doing a lot of experimental work and could accidentally make a blunder shorting out connection

  3. #3


    Few fuses would blow fast enough to save diodes from a hard short.

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