Nearby, Randall had some questions about power supplies. Maybe it's worth a thread - others here have more and deeper expertise than I have, but I'll start out with the picture as I understand it:

• For most purposes, we want a constant-voltage power supply. There are situations - like welding maybe - where constant current might be interesting, but that's even further from my expertise.
  
  
• As a complete beginner, just use any 5V PSU which has adequate power output. An ATX PSU for a PC is a popular and inexpensive choice, a bench PSU with digital readout and current limit is a high quality choice, and a little board with a regulator on it is probably fine for many purposes. For small projects, a USB cable gives 5V.
  
  
• But for more detail, read on.
  
  
• There is no such thing as a constant-voltage power supply! But within limits, you can get close. Any given power supply will be able to keep the voltage up, up to a certain amount of current. If the circuit being powered demands more current than this, the voltage will go down, maybe become unstable, maybe cut out or maybe something will melt or catch fire.
  
  
• So a power supply has a maximum current rating. But it isn't a constant current supply - it's not trying to push amps, it's trying to hold up voltage. A decent bench supply will have a variable, selectable, current at which it will trip, which is handy if you don't want to damage your circuit when it misbehaves, or damage your PSU when you short out its outputs.
  
  
• Even within the current limit, a power supply will not be completely constant voltage. There will be some noise or some ripple on the line. A higher quality PSU will produce cleaner power.
  
  
• To a first approximation, your circuit will take the power supply, let's say at 5V, and by means of wires will provide 5V power to every chip and component.
  
  
• But the world isn't quite like that - your power wires will not be zero resistance or zero inductance. Therefore the voltage delivered to each component will generally be less than 5V and will generally also fluctuate up and down.
  
  
• So it helps to use short fat wires. To be numerate about it, you'd have a noise budget and you'd calculate how much current each component uses and how it fluctuates and therefore how much voltage drop and voltage fluctuation you'll get, from the resistance and inductance respectively.
  
  
• It also helps to add capacitance: usually a medium to large electrolytic on each board, and a small to medium bypass cap next to each component. A component with greater peak current demands will benefit from more bypass capacitance nearby. For example, a DRAM might drive just one output wire, but a 6502 drives 16 address wires and 8 data wires.

What is the capacitance for. You place it across the power and ground pins of the component, right? What exactly does it do, and why doesn't it short the power?

Capacitors do not conduct direct current, hence they don't short out the power supply. Capacitors will pass alternating current, which gives them the ability to suppress switching noise in digital circuits.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

The larger capacitances are known as Bulk Capacitors, and they help to smooth overall voltage of the system by being a current reservoir - it normally takes a short while for a power supply IC to respond to an increase in current draw, during which time the voltage would drop if there wasn't a capacitor to draw the extra current from.

The smaller capacitances are known as Bypass or Decoupling capacitors, which you put near individual components, and as BDD said these help to filter out high frequency switching noise from the power inputs to the ICs. They can also act as current resevoirs for the individual ICs, but only for a very small transient - not a larger sustained change in power draw. It is important to suppress switching noise on the power pins because an IC (normally) measures its inputs relative to Ground and Vdd, so if your ground or vdd suddenly "bounces" up or down for a short while, the IC might get confused and momentarily read the input as a value it is not. Similarly the outputs are literally powered from the power pins, so they would bounce around too if not for decoupling caps, so the switching noise gets amplified through the IC ready to confuse the next IC.

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Want to design a PCB for your project? I strongly recommend KiCad. Its free, its multiplatform, and its easy to learn!
Also, I maintain KiCad libraries of Retro Computing and Arduino components you might find useful.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

You already managed to make it about a billion times clearer than any of my university lecturers

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Want to design a PCB for your project? I strongly recommend KiCad. Its free, its multiplatform, and its easy to learn!
Also, I maintain KiCad libraries of Retro Computing and Arduino components you might find useful.

I just came across this site which has an animated illustration of the idea of putting bypass caps close to the chip they are associated with:
Circuit Integrity - Bypassing

See also the linked 'decoupling' page - no animations there, but plenty of explanation.
Circuit Integrity - Decoupling

Edit: replaced with archive links: original now dead.

Sometimes power supplies from home appliances can be useful.
I dismantled an old DVD player and got a power supply on a small separate board, which I successfully apply for my home-made devices. +12v stability was insufficient. I added a few turns to the winding and an low-power LDO-regulator. Works fine.

Edit: spelling

I like that! How many extra windings? How easy to do this?

If the bobbin is completely filled with a wire, there may not be enough room for extra turns. But, if there is a gap between the core and the winding, then everything is as easy as falling off a log. Perhaps, three to five turns will be enough. Because of the low laboriousness, it is easy to find out experimentally, for a specific case.

Yep!


A number of years ago, an external SCSI tape drive came my way. I didn't have any use for the drive mechanism (a DDS2) but the nifty switch-mode power supply in the enclosure has come in handy.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Sorry if this is OT.

But one thing I recommend with any new design is at a bare-minimum have some type of power regulation. Like a 7805 or something at least.

It's completely my fault...but I've fried at least 2-3 chips this year for different kits I've built. All because I just plugged a 9v or even (gasp) 12v wall-wart into a kit I built only to realize I forgot it didn't have any voltage regulation. So I sit there like an idiot wondering why it doesn't work and then I notice some chips are hot enough to roast a turkey.

So I guess what I'm saying is...always plan for idiots like me who plug 12v into 3.3V parts...lol

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Cat; the other white meat.

You haven't lived until you install a 500 volt electrolytic in backwards and then bring the (tube) amp out of standby. I did that—once—and never forgot about it.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Ah yes, the re-invention of the "confetti generator".

Mike B.

Ha! Glad to know I'm not the only one that isn't perfect.

Speaking of high-voltage and power supplies, I worked for a company one time and we had a little server room that I built with some racks, etc.

Anyway, for different reasons we had the main UPS (that was pretty large) sitting off the floor and onto a shelf about crotch level (wait...it gets better...).

So one day the power company is working outside and they drop a 15KV line onto our building's 1KV line...I don't remember the exact numbers but it's something like that...a main street line being dropped onto our building's much smaller line.

Naturally, they fry a bunch of our equipment. Computers, copiers, modems, etc. Basically everything the owners were too cheap to put surge protectors on. Not that it would have helped with that much voltage.

Of course this knocked out our computers and shut them down. So the idiot that I am decided to turn the UPS back on (remember, it's at crotch level)...

I heard a BANG! And sparks and smoke started spraying out of it....the whole front of my pants took the brunt of the sparks!

Luckily, I was able to move out of the way quickly with no damage....but you should have seen me dancing in that small server room! HAHA.

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Cat; the other white meat.