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January 17, 2017 - Power/ground bus wiring

A project log for The Cardboard Computer - IO is my name

My goal is a 4-bit CPU using recycled cardboard substrate and Diode Transistor Logic. This is an educational platform for me.

dr-cockroachDr. Cockroach 01/17/2017 at 14:489 Comments

Taking a day to wire up a master power switch and power / ground bus module. With this I will route power and ground connections to each panel. The module is nothing more than the switch and more brass fasteners at the top of the I/O panel.

Discussions

Dr. Cockroach wrote 01/18/2017 at 02:18 point

I'll have to root around but I think I have 100n's in my stock, thanks :-) My ham radio projects were so simple that decoupling was not a real issue. That is till now :-)

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K.C. Lee wrote 01/17/2017 at 17:32 point

There are ways of using large caps and still follow the specs. You'll need a hot swap or current limit circuit.  Few actually bother following the rules however.

I have designed this for a USB charger as  I needed a large bulk cap for switch mode regulators.  :)

R3 limits the charging current to C3.  As the voltage drops between the input and cap is below 0.7V, M1 is latched on shorting cap to the input rail. This produces the large 240mA spike which is within 500mA that uC had arbitrate for.  I played with the R3 and C3 values so that it is greater than 100ms (per spec).  

Value of C3 can be reduced (usually with higher ESR) to limit the current to 100mA. 

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matseng wrote 01/18/2017 at 02:03 point

Nice solution. Saved for any possible future use ;-)

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Yann Guidon / YGDES wrote 01/18/2017 at 02:10 point

pretty :-)

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K.C. Lee wrote 01/18/2017 at 03:04 point

FYI: This has a link to the spec and talks a bit more about how the inrush is tested.

http://www.testusb.com/inrush_issue.htm

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Yann Guidon / YGDES wrote 01/17/2017 at 16:45 point

Don't forget decoupling ;-)

There is no such thing as too much decoupling :-D

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matseng wrote 01/17/2017 at 16:53 point

Can be on the USB power bus. :-)   But technically that is not decoupling but rather bulk storage capacitance that makes the inrush current go over the specs.

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Dr. Cockroach wrote 01/18/2017 at 01:32 point

In keeping it simple, guessing that .01 and 1mf will be used a lot :-) Wish I had a scope at times like this :-)

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Yann Guidon / YGDES wrote 01/18/2017 at 02:12 point

ceramic 100nF is most usual...

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