Those are some pricey caps.

$35 -- cheapest in-stock at Mouser. Same part at Digikey $33

Well, I'll keep plugging at it, but I'm going to work on higher voltage and more ripple.

Again, thanks for the info.

[ETA:]

Okay, I'm getting a better mental handle on it now -- thanks. I'll have to read a bit more on the ripple, but I do see the point. Interesting to note that TI recommends an input bypass of 0.1uF ceramic. Due to the discrepancy of capacitance from the values you're recommending, should I assume that the 'bypass' is serving a completely different function here? (I'm aware of bypass as used for, e.g. VCC in to an IC.)

Much better transformer there for my purposes too. My search-fu at Digikey is limited by my lack of knowing better what I'm looking for. This is looking more do-able now.

I assume there aren't any particular gotchas for the bridge IC, as long as I pick one that'll handle the current. This one or any similar.

ETA: Finding more web sites repeating the same ripple calculation. Well, okay, I'll take it on faith. 24 mF gives me .7V ripple, if I'm not dropping a decimal point or something. And that just seems way off. .7 = 2/(120*.024)

[power]

The biggest concern I have is with the power dissipation in the regulator.    I'm pretty sure you can overheat the thing (depending on the heatsink) at less than 3 Amps when there's more than a couple of volts dropped across the regulator.

Which is why I want to not oversize the transformer, in terms of output voltage. I will have heatsinking, but no point in pushing that limit, or wasting power.

It's been a long time since I've calcuated ripple.   The voltage regulator will knock-out the ripple, as long as the ripple doesn't exceed the point where the voltage regulator drops-out.   So, I usually just use a capacitor that's 1000uF or more, one that's available, and one that will fit physically.   (A higher voltage into the regulator will tend to minimize ripple problems, but more it's more power/heat in the regulator.)

I'll keep digging on the ripple question.

I'm thinking 15W 12V secondary, but I could be way off. I'm figuring ~2V drop on the regulator, and 1.7 for  the bridge, giving 9.7V, and then just adding some headroom.

12VAC is plenty for a 6VDC supply.    I think you're forgetting that the peak AC voltage is about 1.4 times the RMS.   At 12VAC your capacitor will charge-up to about 17V minus the diode drops.   (I'll usually use a 12V transformer for a 12V power supply.)  And at no-load, the transformer will probably put-out more than 12VAC.    If you can find a 9V transformer, that should be enough.

No, I'm not forgetting about it. I honestly don't know how to treat it. Hence, asking for info. I understand the notion that as each peak occurs in the rectified voltage, the cap charges up a bit more. And so ... eventually, it would reach peak V. Except that there's a constant discharge as well. We can, as thought expermiment, imagine a load across the capacitor producing an RC time constant less than 1/60th of a second. And maybe, in a practical circuit such as I'm contemplating, this can be simply discounted.

Peak on 6VAC is ~8.5 -- not enough. Peak on 9VAC is ~12.73, so I'd be dropping 5V across the regulator, if the VDC does indeed approach the peak. Going back to my initial back-of-the-napkin 9.7V * 1.414 = 6.86, that get me to this brutishly expensive transformer (for my purposes, anyway). I'm selecting for current output, not VA rating. Moving to 10V out, the price improves considerably, but then I'd be dropping 6V across the regulator.

If it really comes to a $46 transformer to do this well, I might as well buy a new Pila charger -- they're ~$45. Yeah, I've looked at Digikey for a 6V wall-wart supply, which can be had for about $17. Just thought it might be a nice little project to do one evening.

http://www.zen22142.zen.co.uk/Design/dcpsu.htm

Thanks for the suggestions, but I have a hard time trusting a site providing an equation which produces clearly nonsensical results. The author give mV = mA/2fC for calculating the ripple voltage, and states capacitance is in Farads. So, I can use V and A to suit my application, since mA is on the top of the fraction, so I'm just multiplying both sides by 1000. And, just for fun, lets use a 1 mF filter capacitor.

2/2*60*.000001 = 16,666.66......

I'll grant that 1mF would be a dump cap to use, but I don't believe there's any way to get a kilovolt ripple from a 12VAC transformer output. He provides a shortcut for 50hz, wherein capacitance is in microfarads, and using that as the unit gives more believable results, but even so, one could still then use a very high value capacitor, and still get clearly bogus numbers.

So, anyway, I'm still looking for how to spec out the proper transformer.

Hi Folks.

Yeah, I'm still around. Just have a lot of distractions these days. Anyways ...

The 6V 2A switching supply for my Pila IBC charger went bonkers on me. I use this thing every day. I have some backup in place, but need to get it replaced. So, of course, I'm thinking of building one, instead of buying. Yeah, likely not cost-effective. But I'll feel better about the reliability of a plain linear supply, vs. a random wall wart switcher I find on the web.

Not sure the LM350 regulator is the best choice, but I'm sure it'll work, and it has plenty of headroom. (I note that the Pila charger itself calls for 6VDC 2-3A, so I'm not sure why they supplied a 2A supply.) I can choose caps from the datasheet, I think, and toss in a fuse and some protection diodes, but below is the basic circuit.

Question is, how do I size the transformer. I've killed about an hour looking for a decent tutorial on the web, and come up empty. I'm thinking 15W 12V secondary, but I could be way off. I'm figuring ~2V drop on the regulator, and 1.7 for  the bridge, giving 9.7V, and then just adding some headroom. But some of what I've read indicates I need to account for ripple voltage, which, now I think on it, probably influences the specs for the filter cap coming after the bridge too.

Pointers to good tutorials, references, etc. appreciated.

I thought this was pretty well done. Embedded video at the link.

Digital Audio Explained

It's by Monty, the brains behind the Vorbis codec, among other things. (Ogg is actually the container file.)

... and some DVD/CD drives. I noticed some people use them for the stepper motors inside?

And the lasers.

Try http://batteryuniversity.com/

[base]

Even the "truth" is a fraud since "electrons" actually flow from ground to Vcc, yet we all think of it in the opposite fashion when start talking about current flowing from positive to negative.  Shouldn't they burn all that malarky and start teaching people the truth?  

Actually in the military electronics training they taught us using 'electron flow' (negative to positive) but did explain that classic EE training used 'current flow' theory (positive to negative). In the big picture either way can be used as long as you are consistent.

This, combined with some other issues, makes transistors give me fits. That and the lack of a concentrated, formal education (I mean concentrated as opposed to 'learn as I go when I'm fiddling with some idea', not as in accelerated night classes). At some point, quite early in life, I developed my own visualization of electron current flow. So I have to think harder about positive flow, though I've gotten better at it with practice. But the arrow is always pointing the wrong way for me.

The #1 thing, though, which made my initial grasp of transistors maddening was the terminology. Nothing I read explained that the terms 'emitter' and 'collector' are with reference to the base, not the circuit. So even simple circuit drawings gave me fits, because I was trying to interpret the device (transistor) with respect to the circuit, just because made sense using English words as I understood them. (And the arrow thing didn't help either.) At some point, somebody here pointed to an article which cleared that up, by explaining that the terminology was inherited from tubes, where those terms referred to the role of nodes (?) in a tube, relative to the plate. If I'd learned tubes first, I'd have an easier time. But Grob, and other books, go into semiconductors before going into tubes, if they go into tubes at all.

This thread was helpful: http://arduino.cc/forum/index.php/topic,105053.0.html -- just work with the common collector voltage regulator for starters.

1.) How much current can the CR2032 give?  I have looked and not only do I see different numbers.. I usually only see it for mAh...  not mA?

You know that amp-hours is a measure of the energy available, for example, if a battery can supply 1mA for 10 hours, that's 10mAH. Also if it can supply 2mA for 5 hours, same thing.

The specification you're indirectly looking for is the C-rate, usually listed in the spec table as just "C". But then you'll find max. discharge current listed as a multiple, e.g. 3c. Or, if you're lucky, you'll find spec sheet like this one, which lists the actual current, but note the "2.5c" part. Another example is a LiMn battery which has a discharge rate of 8C, which is quite a bit of current -- I note this page doesn't specifically list C for this battery; I'm just familiar with it. (Max charge current is probably very close to, or equal to "C" for this cell.)

I think you'll just have to dig up real spec sheets on the batteries you're investigating. And I think you're going to need bigger ones.

Can we subscribe to this twitter channel?

Well, using the screen name from the screen shot, the RSS feed for it would be:
http://api.twitter.com/1/statuses/user_timeline.rss?screen_name=romaslina

How about twisting two wires together, and taping them, instead of using wire-nuts?

its not uncommon for me to walk into work and see one of the engineers had setup a test ... 12 volts 50 amps running though 22 gauge wire connected by twisting the ends and a little bit of electrical tape covering it

Well, as a temporary thing ... sure. In my last house, I discovered an outlet wired this way, using lamp cord. I'd been running a table saw from it.

[some]

I would imagine that taping co-workers to office chairs is misuse.

Then there's taping the cat.

Or taping postal workers?

I once took the cover off a co-worker's phone, and taped down the hook lever. He couldn't answer his phone. Looking back, I should've used USPS tape.

How about twisting two wires together, and taping them, instead of using wire-nuts?

Those are just some of the ways I'd mis-use sticky tape. YMMV ]

I was trying to find a spaghetti monster image on google to parallel the panel but can't find one that is comfortable to look at.

You're not denigrating His Noodly Appendage, are you? 

I'm not a Forth programmer. But I applaud your efforts, and your release!