Beats An Extension Cord

What does your benchtop power supply have that [Pete Marchetto]’s does not? Answer: an extension cord draped across the floor. How often have you said to yourself, “I just need to energize this doodad for a couple seconds,” then you start daisy chaining every battery in the junk drawer to reach the necessary voltage? It is not uncommon to see battery packs with a single voltage output, but [Pete] could not find an adjustable one, so he built his own and put it on Tindie.

Presumably, the internals are not going to surprise anyone: an 18650 battery, charging circuit, a voltage converter, display, adjustment knob, and a dedicated USB charging port. The complexity is not what intrigues us, it is the fact that we do not see more of them and still wind up taping nine-volt batteries together. [Editor’s note: we use one made from an old laptop battery.]

This should not replace your benchtop power supply, it does not have the bells and whistles, like current regulation, but a mobile source of arbitrary voltage does most of the job most of the time. And it’s what this build hasn’t got (a cord) that makes it most useful.

18 thoughts on “Beats An Extension Cord

  1. I’m skeptical of the construction/safety of this unit just based on the fact that he is using ultrafire “5000mah” (LOL) cells. Maybe he should open source his design so we can see can give some constructive feedback instead of using this article as a glorified ad for tindie and his “product”.

    That being said this is a good idea and I’ve got some cells lying around and have been looking for a reason to design a BMS.

  2. Couldn’t this be made a lot cheaper?
    -Mobile phone charger: $15 (includes 18650 battery and charge circuit, 5V 2A output)
    -Buck/Boost converter with LCD and potentiometer for adjusting output ($5 on amazon)
    -Maybe throw in a $5 enclosure

    Seems to me like this could be made in five mins from 2 amazon components thrown in an enclosure for less than half the price…

    1. this comment intrigued me because i’m slowly coming around to being the kind of guy who buys these kinds of modules instead of building my own (which i’m not good at) or giving up (which i’m good at).

      i might have searched wrong, but i found two products on amazon, a $16.99 boost/buck converter with LCD, and a $6.95 boost converter with LCD. both of these have the problem that the pots they come with are tiny screwdriver-driven pcb-mounted trim pots. and they are both more than $5. and the cheaper one is boost-only.

      i think this is a good illustration of a frequently-commented-on phenomenon here. when people spec out how much something will cost, they tend to round down the price of a product that doesn’t quite meet their specs, and assume that’s the price point they’ll hit in volume.

      no, the $7 one doesn’t meet expectations. $7 doesn’t automatically become $5 in volume just because you know volume is cheaper. and the art of putting a knob on an enclosure when all you’ve got to start with is a trim pot is not a dismissable nothing. $60 might not be a compelling price (i’m actually 99% satisfied with the orderly AC cords on my bench), but the art of beating that price point isn’t as easy as this.

      1. Regarding screwdriver-operated pots: this is where just the easiest of hacking skills is helpful: as long as the wires are kept short (an inch or two), there is zero problem with unsoldering this (or just cutting it out with diagonal cutters) and wiring in a panel-mounted pot.

  3. Rofl that “datasheet” on the tindie listing is a joke along with the price for what is very likely off the shelf parts bodged together. I’d much rather diy something similar than shell out for a ready made one.

  4. After years of cobbling together a battery and regulator when needed I finally built something similar to this a couple of months ago, and I’m amazed how often I reach for it instead of one of the several other power supplies kicking around here.

    It uses a $4 boost-buck module from here: , and one of the usual cheap voltage+current LED meters.

    Input connector is just a standard barrel. Output is a pair of bananas. Controls are an input power switch, output enable switch, and the voltage knob. There’s no current limiting.
    It all fits in a Hammond 1590B aluminum case, 11 x 6 x 3 cm (which is also the heatsink).

    It uses any 5-26V external supply: car lighter cord, laptop power brick, 12V PowerPole source (ubiquitous around here), a laptop battery, a “battery booster” 12V brick, or Sony NP-F550 7.2V batteries (also ubiquitous).

    What’s amazing is how much power the little box can put out: up to 2.5A, 20W or 32V.

    The only gotchas are the input current is limited to 1.5A (so you can only get 20W out when you feed it more than 18V), and the current metering is on the low side, so your load can’t share a ground with other equipment plugged into the same supply — an issue when operating in the car (or at Field Day…). It still works fine, but the current display is incorrect.

    1. Combine a DPS5005 buck with a MeanWell SMPS and you get a pretty good almost lab quality adjustable PSU for almost nothing. I also can disconnect the switch mode and stick my 6s drone battery in there for portable use.

  5. “This should not replace your benchtop power supply” Why not? Just as there are boost regulators out there for practically nothing, for just a little more you can have one with adjustable current limiting as well. I thought the point here was showing how easy it is to throw something together from off-the-shelf parts and modules.

  6. Maybe not so v bad. Way small for me, maybe. I rarely work w nanites, and anything, almost, I power up, I want to run for a long time. I hate having to recharge my ph every day. They won’t last through 8 tough hours nor 24 so-so hours. Portable supply? Car battery, invertor, variac and diode or various battery chargers, (so you have 2 or 3 dif outputs at once,) and any number of diodes to drop 0.7Vdc for each one added. Throw in a solar panel to charge it. Love generators and extension cords, too! It’s ALL good!!! Carry on !

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