How to Spot a Psychopath

I've been asked variations of this question often enough that I thought I'd give it its very own blog post. I hope it's suitably grateful.

I have a Canon Powershot A95, a wonderful little digicam - smart enough that I can take decent pictures with it, cheap enough that I can afford it, small enough I can carry it everywhere. Moreover, I just found out I can actually remote-control it with gphoto2. Huge vistas of time-lapse photography opened up before me. Only, it's hard to take a 7-hour time-lapse on one set of AAs.

Canon sells the "ACK600", an AC power adapter rated at 4.3 VDC and 1.5 A; but they sell it for $50.

On the other hand, just around the corner is a junk shop with a big cardboard box full of wall warts that have become separated from their widgets. Last time I went the guy sold me 5 for $4. Am I safe (assuming I get the polarity right) feeding my camera from a wall-wart rated at 4.3 VDC and 1.5 A? Or is the ACK600 especially well-regulated or something? Is the ACK600 likely to be supplying 4.3 VDC exactly, or something higher? Am I going to damage my camera if I feed it 4.3 VDC from my lab power supply?

Anne

I would not be surprised if none of the plugpacks in the junk shop's box are suitable for powering your camera. But it shouldn't be difficult to power the camera from some other plugpack.

(Note: This is another of my famous "all care, no responsibility" answers. No smoking wrecks which once were cameras will be replaced by the management.)

The great danger of old plugpacks is that they may be unregulated. A regulated power supply will (to a first approximation) always output its rated voltage(s). This means it's fine to plug a big "12V 10A regulated" power supply into a little "12V 75mA" device; a regulated power supply may behave oddly if it's extremely lightly loaded, but that's very seldom a problem.

An unregulated plugpack, on the other hand, will only output its rated voltage when it's fully loaded. If it's completely unloaded, it'll deliver precisely root-two (1.414...) times its rated voltage.

If an old heavyweight linear plugpack (as opposed to the modern lightweight switchmode type, which are almost always regulated and usually accept a range of input mains voltages) isn't specifically labelled "REGULATED", you should assume that it isn't.

(The other trap waiting for you in the old-plugpack box, apart from breaking out the multimeter to make sure which output wire is the positive, is power supplies that have no positive output wire. Some plugpacks output alternating current, instead of the direct current that most small devices expect. An AC power supply for an old modem or something may have voltage and current specs that look fine, but if you plug it into a DC-expecting device and that device doesn't have rather robust reverse-polarity protection, a small noise and a funny smell will soon occur.)

It's reasonable to assume that most gadgets will accept their rated input voltage plus or minus ten per cent, but lightly loaded unregulated power supplies can easily be outputting more than 1.3 times their rated voltage. That can blow stuff up.

It's good that input a bit above or below the rated input voltage for a given device is generally fine, because the official power supplies for some devices have weird voltages. Yours is one such; "4.3VDC" is an oddball voltage that you're almost certain to be unable to find an a box o' power supplies. Your camera will probably run perfectly well from a 4.5V regulated plugpack, though, as long as that plugpack can deliver enough current. 1.5 amps is a bit on the high side (and the camera won't need it most of the time - only charging the flash is likely to push it above an amp), but current ratings that high are easy enough to find in off-brand switchmode plugpacks from electronics stores these days.

And yes, a bench power supply set to the same voltage and with enough current capacity will also power your camera just fine. This sort of setup, with an inline ammeter (don't trust the current meter on an inexpensive bench supply for more than approximate readings), can make it easy to figure out exactly what the acceptable voltage range for a given device is.

The camera may, for instance, still work but with obviously slower zoom speed at 3.7V, and it may not draw any more current (or actually draw less) at 4.8V than at 4.3V. In that case, you know you're pretty safe with the higher voltage. If a device starts drawing more current as you raise its input voltage, it's a good idea to stop raising the input voltage.

Even without such investigations, I'd be very surprised if you couldn't also rig up an old-style external battery pack wired to an appropriate DC input plug. Three alkaline D cells in a holder (for 4.5V nominal, and something in the order of 12 amp-hours of capacity even at a constant half-amp drain!) would probably be fine, and give you impressive run time. I'm also about 90% sure that four 1.2V NiMH cells in series (for 4.8V nominal) would be fine, even though they'd be rather more than 1.2V per cell when freshly charged.

Getting back to old plugpacks: The final problem with them is that they can be dangerous.

Plugpacks have been built down to a price for many years, and it's common to find a gadget that's passed lots of electrical safety tests being powered by a very dodgy plugpack indeed. It's not easy for a gadget importer to change the power supplies in devices that take mains input, but it's very simple to get a device safety-certified with a top-quality plugpack, then increase profits by putting ten-cent assembled-by-peasants plugpacks in with the actual retail gadgets.

A sensible design for a plugpack should include some kind of overcurrent and overheating protection. Some have simple fuses, but those are difficult to replace if they pop; a self-resetting overcurrent and thermal circuit breaker is much better.

Those sorts of things cost the manufacturers, oh, maybe even a dollar each, though. So it's perfectly normal to find linear plugpacks, old and new, that have no emergency cut-out features at all.

I remember one line of cheap multi-voltage one-amp plugpacks of which I bought a few on Super Clearance Special, years ago.

I ended up having to carry two of those plugpacks out into the garden, dangling them gingerly by the cable as the smoking interior made noises reminiscent of an angry rat.

The vast majority of plugpacks, even ancient ones with appallingly poor efficiency, will behave themselves perfectly well pretty much forever. If they die, they die harmlessly, with an open-circuit transformer or something.

But it's still not a bad idea, whenever you plug in an AC adapter, to think about what might happen if that plugpack decided to catch fire while you were out of the house.

If it's plugged into a powerboard that's sitting on your wool carpet: Not so bad.

If it's between a pile of old clothes and a box of tissues: Consider a change.