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Friday, October 16, 2009

DC-DC Converters

It's been mentioned on the Endless Sphere forums that some AC power supplies might work from 48-50VDC and up, and be able to power various low voltage things (like LED lights, bike computers, etc.) from a bike's traction pack.

I may have found a way to power them from even lower voltage traction packs (like mine, at 24-36VDC) pretty easily, via a cheap (free, actually) source of DC-DC converters, in old celphone chargers and other portable-device AC adapters.

I started going thru my collection of ac adapters, including a couple acquired from Freecycle today, and found a couple of Samsung celphone chargers (TAD137VSE) that output "5.0V @ 0.7A" for "100-240VAC @ 0.7A" which also output the regulated supply at as low as 23VDC input. I'm sure the current draw would be higher at the lower voltage, but I didn't test it at it's full output, only with a 1Kohm load on it (~5mA), which drew pretty much nothing on the source side according to the Sorenson's ammeter.

I tested by hooking the AC input of the charger to the DC output of my Sorenson 0-60VDC adjustable linear bench power supply, the 1Kohm load to the + and - output terminals of the charger, and the DMM on VDC across the resistor. Adjusting the Sorenson's output from 0V upward resulted in output from the charger around 3.5-4VDC starting at about 18VDC input from the Sorenson, swinging up to 5.14VDC output at a 23VDC input, stable at that voltage all the way to the 62VDC max output of the Sorenson.

It is stable at 5.14V with that load or no load; might regulate better to 5.0V at a higher load. One diode in series with it should drop it enough to not worry about for the LED boards on the bike lights.

It's around 3.5W output capability at 5V, about 3.1W with the diode dropping almost half a watt across it at full load. So depending on what the LEDs draw, one of these might easily supply that LED board. Two paralleled would work if one won't, and they're quite small and light, even if left in the original little wallwart cases. If used like that, the long lower-voltage wire from it could be run to the light, rather than the higher full-pack voltage, making it safer if a wire were ever shorted somehow.

Alternately, they could be built into the lights' housings, and just run full pack voltage there (with a fuse or micro-breaker at the pack end).

I have a bunch of others not yet tested that range from 4.5V to 6.0V outputs, and from 100mA to over 1A output capabilities.

Some of the ones tested wont' start until they reach 50V or more. The Samsungs were the first ones I found that work at such a low voltage.


  1. I was having a hard time finding a regulator for the led voltage meter I'm using on my scooter.... Thanks! You just gave me the answer I needed.... go find a cheap car type cell phone power charger.

  2. Which you can probably get for free off your local Freecycle group. :-) I see celphone accessories of all kinds on the groups here, at least once a week and usually more often.

  3. FWIW a similar Samsung charger (TAD177JSE) also outputs 5 volts but at slightly higher current (1 amp). I've tested one and it delivered 5.01 volts with no load at 24.8 volts input from a pair of 12 volt SLA batteries.


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