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Cannot tell what the instructions say.


Go to solution Solved by Electronics Wizardy,

3/8 a is the current of the fuse, not length.

I am building a SAP-1 Hobby computer, set to run on Intel 8086 instruction set. Though, there is a detail on the (spotty) parts list. The power supply requires a fuse, as listed in the picture below. I can read the text, but I do not know what it means. Could one of you help we with that? I do not know if the 3/8-A slow blow means the filament length (in) or the rated Amperage.


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A bit more help:


LM340T-5 is a fixed output voltage, 5v linear regulator in TO220 package. 

It's a bit expensive because it's an old part number and produced in fewer factories and in smaller quantities.


You can safely use the 100% compatible 7805 linear regulators, they have the same pinout , same fixed output voltage, same maximum voltage, same current etc

Here's a short link for them: https://www.digikey.com/short/p7tb0w


1n4001 is a really generic, basic diode. It's part of the 1n400x family of diodes, pretty much the only difference between them is the maximum voltage they support. 1n4001 is the lowest "quality" ones, capable of maximum 50v, while the highest ones in the family 1n4007 can handle up to 1000v

So the point I'm trying to make is you can buy 1n4001, 1n4002, ... 1n4007, all will work just fine.

For future messing around, it would make more sense to buy 10 or 25 of 1n4004 or higher, since the 1n4004 can handle up to 400v.  If you'll ever want to play with high voltage, you would be able to reuse the diodes.


Transformers :


If it's not clear, the transformer has to be CENTER TAP, that means the transformer must have 3 or 4 wires on the output, 3 for a center tap and 4 if there's two secondary windings. 

If the transformer has two separate secondary windings, you can connect the middle wires together and that becomes your center tap. This splits the 12.6v ac into 2 x 6.3v AC , and that goes through the diodes and then through the 5v linear regulator and you get a smooth 5v output to the chips. I don't see a linear regulator for the negative rail, and I don't see even a smoothing capacitor for that - either the other winding is not used, or it's used as negative for some chips and those probably don't care about smooth negative voltage, as long as it's negative.

12.6v may be hard to find... I think it's safe to buy 15v AC with center tap, if you can't find 12.6v easily.

The screenshot says 12.6v ac , 1.5A - that's 12.6x1.5= ~19VA  - so you're looking for a transformer rated for 20VA or more. It's safe to use a higher VA rated transformer, but if the value is significantly higher, you may have to use a fuse rated for more current, otherwise the fuse will pop when you turn on the circuit..


Here's some suggestions:

with center tap : https://www.digikey.com/short/p7t58w

with two separate windings (can join to make center tap, more versatile) : https://www.digikey.com/short/p7t5j3

(look for the ones that are in stock, not factory stock)


The transformer suggested by the picture is this one, at around 13$ ...


There are slightly cheaper ones in the links above.




The datasheet (much later edit: i mean picture, and for some reason I saw 2A instead of corrected value) says 2A  3/8 A (0.375A) slow blow fuse - btw slow blow is pretty much another term for "time delay"  - you may find fuses that have those words instead of "Slow blow" . The fuses themselves will have a T printed on them.


I don't know where they put the fuse in the circuit - since it's 2A I assume it's on the secondary winding, before the  bridge rectifier or between the bridge rectifier and linear regulator. If the linear regulator gets damaged and pulls more than 1A or 1.5A from the bridge rectifier, or one of the diodes in the bridge rectifier dies, then a fuse would protect the secondary winding of the transformer and the transformer in general from being damaged.


The fuse is slow blow because for a very brief period when that 1000uF capacitor charges up with energy, it will pull a lot of current from the transformer, for a few milliseconds the capacitor is like a short circuit. So if you use a regular fuse, the fuse may blow up because you could exceed 2 A of current draw as that capacitor charges up with energy. A slow blow / time delay fuse will not blow up for some period of time, I don't now  the typical period, 50-100ms, maybe a bit more. 


It would make sense to also have a fuse in front of the transformer so that if anything goes wrong with your transformer, you won't pop the fuses of your house.  You can estimate the fuse value by dividing the VA rating with the mains voltage with ... so for example if you have a 20VA transformer, then 20va / 115v = 0.17A or 170mA ... so I'd use a 250mA or 300mA time delay fuse  (again time delay because when you plug in the transformer the first time, that capacitor will charge up pulling a bit more current, so you don't want to blow the fuse at that point).  250mA at 115v is 28VA, almost 50% more than standard va rating, so it's enough margin. if your transformer works like that for a period long enough to blow the fuse, then it's good, the fuse did its job.


The most common are  5mm / 5.1mm x 20mm-ish (or wider)  and 6.35mm (0.25") x 32mm - ish ( 1.25" )

It wouldn't hurt to get some fuse holders for them, unless you buy fuses with leads pre-soldered to them (more expensive), here's a filtered list with fuse holders: https://www.digikey.com/short/p7t593


If you want, attach the circuit schematic or the pdf and maybe we can give you more advice.


Good luck..


much later edit: i don't know why I read 2A in the pictures, it's 3 / 8  or 0.375A  (375mA) fuse, slow blow... so that would make it a fuse before the transformer.  Like I explained, the actual value is not critical, you can probably do with 250mA .. 500mA .... 375mA is kinda conservative for the transformer chosen.

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14 hours ago, Nerdium said:

I am building a SAP-1 Hobby computer, set to run on Intel 8086 instruction set.

Well that's not true at all.

Generally, the thing that determines the "type" of processor is the instruction set that it runs. A SAP-1 processor runs the SAP-1 instruction set, and an 8086 processor runs the 8086 instruction set... The ISA determines the common type. Below that you have distinctions for binary compatible and non binary compatible, and then possibly other architectural differences, such as differing pipelining/cache policies, or how many cycles given instructions take to complete.


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