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Some years ago, one of the beer brewing megacompanies ran a series of ads based on 'exceptional' people, including the folks who wipe up sweat in NBA games.  One of their targets was the guy who was going to invent a "battery-powered battery charger."  Of course, I already had such a device in my pocket, a reserve battery pack for charging my cellphone.

This little box's disadvantages are that it charges slowly and that it requires a regulated 12V DC to charge.

That makes charging it in the car a problem.  The car has 12V, but it's 'somewhere between 10.8v and 14.8v'.  A regulated voltage is generally regulated to either 10% or sometimes to 5%.

Worse, the available voltage swings from below the desired target to above, requiring a more complex regulator circuit.  If I had a lot of time, I might look at a switching supply with a flyback design.

But I don't have lots of time.  I've had this project in back of the back burner for a long time, and I want it for my trip on Tuesday.  So I went with another option.  When the car is running, the available voltage should run from 13.2 to 13.8 volts.  That's enough to use an LDO regulator.

What's an LDO regulator?

Hmm.  Well, to start, one of the widely available and cheap integrated circuits is the three-terminal regulator.  It comes in variants with more than three terminals (pdf) and in variants whose output voltage can be adjusted.  But in the most basic form, it's simplicity itself.  You buy one for the voltage you need (you DO need a bog-standard voltage, don't you?) and the polarity (only two: positive and negative).  You hook up the 'common' terminal to your 'ground' and the 'input' to your input, and the regulated voltage appears on the output ... except it's never THAT simple.

These devices need low-esr capacitors on input, output, or both in order to work properly, and that usually means tantalum capacitors.  Okay, not hard.  And they are designed for limited currents.  Fine, I only need half an amp.  One-amp regulators are widely available.

But ... the bog-standard three-terminal regulator requires an input at least two volts above the output.  I can guarantee only a bit over one.  Oops.

Fortunately, there are 'low dropout' (LDO) regulators available.  They're just a bit harder to find.  Not much harder, but you won't find them at Radio Snack.  Fine.  I ordered one a while back, and when I couldn't find it I ordered a bunch more, and paid for second-day shipping.  Mouser promises that if the order makes it through the warehouse by 20:00 Central, they'll ship that day.  I put the order in at about 15:30 Central Time.

The order didn't reach the warehouse in time.  At about 03:00 (yes, AM!) I got an email from Mouser saying that they were sorry it was delayed, and they were shipping overnight at their expense.  They took a loss on this for a small customer--kudos to them.

Of course, that was about an hour after I found the one I had misfiled.  But it never hurts to find spares.

Refer to the PDF above.  It's a very nice device for automotive use, including protection against voltage spike that can occur when a load drops off the system, and against voltage inversions.  That also protects the output capacitor, but not the input capacitor.  I put protection on using the power mosfet trick, which will drop about another 1/10 of a volt with the device in question (see the Saturation Characteristics graph).

What it won't do is protect the input capacitor from overvoltage.  I mislaid my small stock of tantalum caps and decided to settle on Radio Snack parts.  (Made by LTE, the Budweiser of electronic parts.)  And they only have 16v tantalums.  I would have chosen 20v, at least.  (From 20v to 25v, in 1uf, 10uf, and 100uf, Mouser lists 802 products, most of them stocked, from 37 cents to nine dollars.  That's just radial, leaded, not counting axial and surface-mount.)

But 16v will do.

I wired the parts together, along with a couple of LEDs (one on each side of the box--did I mention that I drilled the box, cut a circuit board, epoxied the wired-up LEDs in place ...) that will be daylight-visible on about 4/3 of a milliamp.  (At night, the human eye gets a speckle fringe around them.)  I had the output-side connector made up on a length of wire.  (Did I mention the strain-reliefs that I drilled the holes for?)  I had the input-side 'cigarette-lighter' plug with its cut-to-length wire.

I tinned the wire ends.

Then I buzzed the wires and connectors out to make sure that what was connected should be, and what wasn't shouldn't.

And I got a short circuit in the plug.  Only when I reversed the meter, I didn't.  (Tip: Many meters reverse their polarities on continuity and resistance tests so that you don't get stray currents through semiconductor junctions.  It doesn't work as well with the wider range of semiconductor devices available today, and can fail abysmally around junction FETs, whose gate electrodes turn into very delicate diodes when their polarity is reversed.)

So either something was Very Strange or the plug contained some semiconductors.  Since I got the connections I should, it seemed very unlikely the plug had a complex device like a regulator.  (If you've gotten this far, Amy, you may be thinking Differential Diagnosis.)  But a diode to dump current on reversed input?

I carefully took the plug apart.  Turns out it was held together by its spec sticker!  Well, there is indeed a diode (hidden inside opaque heat-shrink tubing), which will blow the fuse in the plug if the reverse voltage lasts long or is very high.  Which makes the polarity protection that I put on superfluous, though better behaved.

I decided to leave the diode--maybe a poor idea--and put a legend on the plug with a paint marker.  Now that it's dried (and the epoxy smell has finally dissipated from my apartment) I'll clamp the plug shell halves together with some transparent heat-shrink, and finish assembling the box.

Then I have to test it.  Ideally, I would use a very wide range voltage supply and a variable-current load.  I have the variable supplies, but one of these days I'll have to build the load device.  (The main problem is handling heat dissipation.  Mosfets would be ideal for the semiconductor part, but they are prone to thermal runaway.  Ideally you arrange things so most of the power is dissipated in resistors rather than in the semiconductors.  More things to think about!)

AND I have to get another little demo device fixed (a battery short blew out at least one part, and I plan to replace them all).  And get laundry done tomorrow.  And adjust my sleep schedule for an early-morning drive to Raleigh.

And I have about eight things to review, courtesy of Oliver Suddin. 

Now pet the shaggy dog and get back to work!