[mono]

Having gotten a couple hours sleep since first playing with my adapter, I reconsider that the circuit I previously proposed and constructed may not be suitable "as-is" for battery charging, for two primary reason:

1) It makes the assumption that the cells used have enough internal resistance that the current delivered is less than transformer is capable of. If your cells cause enough current to be drawn the voltage will drop below the 9V + X (regulator vDrop) and the circuit won't be stable. It will charge batteries but in a test it caused a slight ~ 0.025V AC ripple. A resistor in series on the output and an adjustment to the regulator's output may resolve this.

2) Upon reaching the timer's shutoff point, you'd then have batteries continually at higher potential, voltage on the output of the regulator. An inline diode would fix this, or a more elaborate circuit, but as is, it's less than ideal... though LM1085 supposedly has some protection in this regard it still isn't a good design for this reason.

However, I may just leave mine as-is and try it... if anything goes POOF or BOOM I'll report back.

Edit: Went through one complete charge cycle with some manually, almost fully drained pack of 6 x 1800mAH AA NiMH cells. They appear fully charged (though I've not tested resultant capacity as I didn't even know what their capacity would've been through alternate charge methods, only their rated spec). I'd wondered if the 'sink on the regulator was necessary, especially since it required cutting out a small rectangular hole in the wart's casing. For higher voltage or powering a small amp it may not be but for this charging test it certainly seemed necessary as the 'sink got pretty warm.

Above as #2 I'd mentioned battery potential above regulator. Indeed after the "charger's" timer had shut it off there was ~ 40mA of current backflowing into it from the batteries. No POOF or BOOM though, regulator withstood the backflow of current and whole thing still works. I may take this circuit out of the adapter, tweak it and put it into one of several other wall-warts I have that can sustain > 300mA @ 9V, as the peak current for charging this AA pack was a little under 300mA (below 9V though), don't recall exact figure but maybe 260mA. 260 puts it close to the VA 2.2W capability I'd speculated about previously too, so it seems 2.2W is a good conservative estimate for this transformer. In summary circuit works but might be better suited as cheap headamp supply, still sub-optimal for battery charging as originally configured.

 

[psilosome]

I don't want to have to unplug the charger (from AC power) to reset the timer, so I set up a little experimemt to try and sort out the behavior of the thing. I just hooked up a 3 k resistor across the terminals, which causes the led on the charger to light up. It has stayed lit now for over 24 hours, so apparently the timer is not shutting things down. Weird...

[edit: about five days now, I'm stopping the experiment]

I'm guessing that (1) the current draw (about 7 mA by my measure and calculations) is too low to get the timer going, or (2) I have a defective charger

Maybe I will try a lower value resistor, or alternately can someone please confirm that the timer resets when the DC barrel plug is disconnected?

 

[craftech]

Am I correct in assuming (now that I have four of these things) that if I want to use one in place of two 9 volt batteries as a power source for a headphone amp they will work as is? They each measure around 21.9 volts and from what I am gathering from this thread they work as a wall wart with no mods.
John

 

[psilosome]

I think they could work as simple wall warts, with some caveats: (1) the mysterious timer which is supposed to shut things off (see my post earlier today and mono's report of current backflow); (2) they're not rated to put out much current, max 120 mA or thereabouts, and I recall reports here of PIMETAs requiring about 200 mA; (3) I'm not sure how clean the power is, so without some downstream cleaning-up there is a chance that noise and ripple could adversely effect the sound of your amp.

I'm disinclined to crack one open if not necessary, but once done it probably would not be hard to circumvent the timer.

 

[mono]

As-is, these are not regulated. That makes them unsuitable for directly powering an amp. Some amps with buffered power (think A47, not CMOY) will sound moderately OK, but not good, with such a dirty source.

Pimeta doesn't need more than 120mA, but because these are unregulated VA type chargers, what you'd have is the voltage level at ~ 21V unloaded, but with a load that linearly drops down the voltage based on current usage. In other words, this is another reason to use some post-wart regulation circuit.

What I had pictured and built into mine is sufficient for directly powering a Pimeta, I did do that. For cheap and easy amp use though, you should choose a 7815 linear regulator. It'll give 15V out, which is sufficiently lower than the 18V rating for the wart, and high enough for the amp to drive most cans... if yours are above 200Ohm I would seek more voltage, a different wall-wart transformer.

So the 7815 circuit can be very simple, I'd put a couple small caps between 1) Input and ground 2) Output and ground. Input and ground cap needs be ~ 25V rating, output cap could be 16V. That about $1.25 worth of parts, 7 solder joints and 1/2 cubic inch of space.

As for eliminating the timer circuit, you could simply desolder the transformer leads from the whole PCB and remove the PCB. By removing it you have AC output from the transfomer so you need a simple diode bridge (4 4001 or better diodes, pennies a piece) rectifier or equivalent integrated single-"chip" type.

If you're patient enough I'll be doing something similar to all I described above to one of mine, except that I want one with 12V regulated output, the only different being that I'll choose a 7812 regulator rather than a 7815. Hmmmm, come to think of it I don't even know if I have a 7812 here and don't need enought parts to put in an order anywhere yet so I may improvise, but I'll whip up "something" similar and post it in more detail, simple to follow.

 

[Syzygies]

Ok, I have over a dozen of these, so I destroyed one for the common good, to get some decent scans of it. I had to wait until I was back on the right coast to scan them, my Epson 1200 has much better depth of field than my cheapie Canon scanner.

Each solder-side scan has been flipped to match the corresponding component-side scan, so one can superimpose traces, like looking at an Eagle file:

The missing components on the first board are a 100uF 35V cap, and two 2.2uF 50V caps. The missing component on the second board is a switch. Earlier pictures in this thread document how the boards are connected to each other.

The case is not easy to get apart, I'd have to destroy a bunch of them to get good at it. I'd think the right approach is like opening an oyster (at which I'm far better), admit you're going to make a screwdriver scar somewhere, get in there and wedge the case apart.

That said, I'm hoping that these pictures will satisfy people's curiousity without opening their own. This is a better look than you'll get without destroying one yourself.

I'm treating these as black boxes. I trust them only to charge batteries of sufficient capacity and voltage to not need an intervening circuit, or through trickle charge circuits I've made. My amp power jacks are NC switches wired to disconnect the amp when a plug is inserted.

I admire the experiments others are carrying out here, but for me life is to short to go into one of these again. I'll still buy every one that crosses my path; at $3 they're too good a deal to pass up.

 

[mono]

I got around to moddin' another one...

2nd one wasn't as easy to get open, had to use bench vise and it looked worse afterwards. Luckily with a file and some very fine steel wool it turned out ok though I forgot to take pics of it.

This time I'd wanted to put a linear LM7812 (12V) or LM7815 (15V) into one on a dedicated circuit board w/o timer... but I didn't have either of those so I settled for an LM7805 with a raised ground pin. This wart was meant solely to power a low-powered headamp and as such didn't need any heatsink on the regulator as configured for 16.5V (nor would 15V for a CMOY, YMMV for something else). I had it powering a near-CMOY fine and the regulator didn't even get warm @ 7V drop from input 23.5V (see schematic). Note that the Vdrop may be lower than 7V too, as 23.5V was no-load reading, I could've shot for closer to 20V output except that this would greatly limit the current the transformer could supply. IMO, 18V regulated output is about the peak one could target and expect to work for higher-current amps (up to ~ 200mA) but @ 200mA output I'd expect you'll need a heatsink, some vent holes in the plastic wart casing.

[Edit] I must be mathematically challenged, above I wrote "up to ~ 200mA..." which is clearly wrong. I now estimate this transformer at closer to 2.3W output. WITHOUT any testing I'd ballpark the current capabilities of it's regulated output at around 135mA @ 15V. You might note that is only 2W, but I accounted for ~ 2V drop from regulator (and I'm not even sure it can be calculated like this). At some point I may put a variable load on mine (@ 16.5V) and will then have a more accurate figure but suffice to say it's not capable of 200mA @ 18V output, especially after regulated! [/Edit]

Transformer leads used were the yellow and black. Brown lead is a short tap, about a volt lower input. I just folded over the brown lead and slipped heatshrink over it to isolate it.

Schematic is just an example of what "I" did with spare parts in a junk bin, you might do better if you wanted to spend $ on it (or have a better junk bin) but I am pleased with my results... linear power doesn't get any cheaper nor much smaller than this wart.

There are also some pics showing the quick-n-dirty circuit board, serves it's purpose and might be quicker to follow though I'm sure if you wanted to spend more time you could better it, or you might be more gifted at layout, I just sorta start stuffin' parts and then "there it is", very little pre-planning on something this small & cheap.









More pics

 

[psilosome]

Nice work mono! I'm going to try something like this with my next parts order.

 

[Syzygies]

Yes, nice work! That's the right attititude, tossing their boards in favor of a new, clean design for one's purpose. Now if someone could explain how to easily get these open!

 

[mono]

I think the key to getting them open is to use multiple passes at compressing the shell, maybe 6 or more. I put the clamp or vise on opposite sides of the seam (ON the seam itself) and tightened it but only enough to hear the seam barely start to crack apart, then i loosen and slide it down some and/or, then rotate it 90'in the jaws and repeat. Probably easiest mistake to make is trying to open it with too much force in one spot or too few passes at moving it in the vise/clamp/etc. I didn't end up needing a screwdriver slid into the crack for this 2nd one

Since there's no way around mangling the thin raised lip portion of the seam I've just been filing that off later till it's flush with the rest. I cleaned up the inner mating channels more than I needed to, too, as I had been just refitting them together to see if it was done but when it came time to glue it back together (used PVC cement but even super-glue might work) I held it with a C-Clamp and that would've probably gotten it together ok regardless of how clean the seam was (though maybe it needed less cement with a clean seam).