Is this scheme reasonable?

Sure, that's reasonable. Rechargeable batteries may be a better solution. For a cap to deliver 1 A for 1 second with a 1 volt droop requires 1 Farad, not microFarads, 1 FARAD!

If the cap only supplies the current required *above* the 750 mA provided by the wall wart, it might be able to be a bit smaller, but that depends on the actual current required.

I'm thinking rechargeable batteries with a built in charger. How long do the AAs currently last? Six Dollar tree AA cells cost $1.50. Rechargeables are several times that much. Since it will be recharged often, you might get by with three AA or even AAA lithium ion cells or just a single rechargeable 9 volt battery.

This is kind of a repeat of what Rick said, from a different point of view:

For the amount of energy stored, caps are BIG. And a capacitor's voltage falls as it delivers energy -- by definition. If it doesn't, it's not a capacitor.

Rechargeable batteries are much smaller than caps -- and it's what the thing's designed for anyway. The easiest thing to do, which should be reliable, is to populate the battery tray with rechargable NiMH cells, then put a charger on it that runs at a trickle rate (i.e., 50mA for every 1000mAh of capacity). That _should_ let the batteries last a good long time, but whether it'll stay charged is kind of up in the air (or maybe it'll work fin normally but not on Thanksgiving or Christmas).

Make the charger from a wall-wart and a resistor.

The only other way I can think of to make this work "better" is with a smart charger -- and then you're messing with some pretty fancy electronics to keep the batteries happy. So -- probably not a good idea.

Finally -- have you considered an old fashioned, works just fine, _mechanical_ covered trash can?

A conventional cap, no. Maybe a super expensive supercap.

I wouldn't want an electronic trash can. All that cheap, badly programmed microprocessor/LCD appliance junk will break in a couple of years, and be annoying meanwhile.

Thanks for the quick responses everyone. It looks like rechargeable cells are really the best way to go. The regular cells last about 3 months. I hate the garbage can. I hate throwing batteries in the recycle bin. I hate the garbage can. It was advertised as motion sensitive. It is not. It senses reflected IR, not motion. So if the damn sensor is covered with IR reflective material the lid stays open. Did you know that some clear plastic sheet reflects IR? Did I say I hate the garbage can? Unfortunately for now it is the best solution. Though a foot operated model is in the future right now it just won't work. Thanks Again, Eric

I think you have this pretty much backwards. A "smart" battery charger is the only real way to go if you want the batteries to last. The "trickle" charger may keep the batteries up given that most people don't use such a trash can for long periods, but once rechargeables are charged, they don't want more current unless they are lead acid. I didn't look it up, so do you know that NiMH will take a trickle charge without damage? I don't think I've heard of that.

Proper charging electronics doesn't need to be hard to design or use.

A couple of supercaps in series might work fine. For instance, SCMT22C505MRBA0 AVX CAPACITOR 5F 20% 5V 0.065ohms $5.23000 in onesies from Digikey.

Two of those in series, with a TL431 across each one (set for 4.8V or so) to equalize the voltage, plus a charger with about a 50 mA current limit ought to work, I should think.

Cheers

Phil Hobbs

I have some 2.7 volt 50 farad caps that cost $1.29/ea. 4 in series is

I've been tinkering with them intending to run a strobe light I attach to a buoy so I can find it with my kayak in the dark. I've used alkaline (3) AA cells to power it for 6 months (flashes 12 LEDs, 6 hours after dark until sun-up, that's when I kayak), I also tried a store bought solar garden light with one 50 farad cap that works pretty well but not as bright as the strobe which is visible from 2 miles on a good night, and 1/2 mile most nights.

I know Eric from these groups, and I'm assuming that he doesn't want to run out and design a PCB just for a trash can.

Eric!! "Accidentally" start a greasy paper towel on fire and toss it in the trash. Then instead of putting the lid down, freak out and put the can on the porch where it won't burn down your house.

Then get a decent freaking garbage can!!!

You seem to be unable to think outside the box many times. I can't imagine building a PCB for a project like this. I'd use an expression involving cats, but it is prohibited in this house.

Eric how much current voltage is it? I've got little Phihong wall warts that do 12V @1.25 A.

George H.

We have a nice kitchen trash can that opens when you step on a little pedal thing.

Would someone there have a problem with the mechanical version? Wheelchair or something?

For a serious engineering project, throw away the electronics and drive the lid motor yourself.

That's pretty good. At 1 amp, they will droop 0.4 volts/second, which should get the thing open.

Or 5 volt zeners. Or maybe they will just equalize on their own.

On 5/11/2017 3:49 PM, George Herold wrote: ...

I've got some here. performance does not match the label,

Is there data on leakage or self-discharge of these beasts? The datasheet just says "low leakage current" and this could just mean anything...

Pere

The caps I'm using give the leakage at .16 ma.

You can use both cap and battery/wart.

Common enough trick we used servicing 8 tracks back in the day. The solenoids they used for ejecting the tape were designed to suck power from the car's 12V system fused at 30 amps... Bench power supplies couldn't always make it work so we wired in a 30,000 uf /25V cap across the output to the supply and it would work every time.

A trick model rail road enthusiasts and others use to conserve power on solenoids, relays and actuators, is to wire a resistor in series with the coil to lower the average current then wire a cap across the resistor to deliver the high current to get it moving.

Well, they wouldn't open the bin more than once or twice per charge, so I'd just leave it plugged in.

Cheers

Phil Hobbs