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Old January 8th, 2014, 10:54 PM   #1 (permalink)
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Laptop 18VDC scavenge for pedal power

I acquired a typical 18-volt DC laptop supply that had its cable mangled. Opened it up and set it up for some test fiddling, and it puts out a nice 18-19VDC. I have a 0.1uf cap from B+ to ground before the last terminal strip and the regulator circuit I am building.

In my experimenting, I have rigged up a LM317T voltage regulator with a 5K linear pot as R2, two 680-ohm resistors in parallel (~330 ohm) for R1, one N4001 diode across the input and output , and a 10uf non polarized cap on the output, basically along the lines of the standard instructions (Radio Shack), plus some other web input.

I had it working well yesterday with only one 680-Ohm resistor, but getting a range of 1.4 to 10.5 volts. Trying to get a bit over the 12-volt mark at least. The calculator I used said 330-ohms would get my range closer to 18-20volts at the top, but it works for a moment with everything cool, and then gets VERY hot on the sink, and I get nothing on the output.

I am wondering if the range I am seeking is limited with R1 needing to be more around 560-ohm, thus I am overloading something. Or another diode? Or a different cap? Or a new hobby?


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The white goo is high-strength marine sealant. They didn't want anything moving around in there....

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Old January 9th, 2014, 05:08 AM   #2 (permalink)
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The heatsink is not able to dissipate enough power and it's causing the overload circuit in the LM317 to shut it off. How much current are you supplying and at what voltage?
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Old January 9th, 2014, 11:32 AM   #3 (permalink)
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The DC supply is 19.5v at 4.62A. It occurred to me in a dream that the current was an issue. A bigger (much bigger) heat sink maybe? It looks like it is heating up at the wire connection at the regulator input.
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Old January 9th, 2014, 12:29 PM   #4 (permalink)
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Current overload. It's more about what you're supplying (the load) than the rating of the laptop brick. Too much power is getting sucked to your ground.

It's a design problem. A bigger heatsink isn't the answer.

If you could draw out a schematic of exactly what you've built I could be more help.
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Old January 9th, 2014, 12:51 PM   #5 (permalink)
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Here is the basic approach. I left out D2, R1 is effectively 340-ohms.


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So I don't know if adding another N4007 will work, as D2 or elsewhere. D1 is a N4007, 1000v/1a, not a N4001.

Thanks!
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Old January 9th, 2014, 02:49 PM   #6 (permalink)
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I guess first thing is to check that your diodes are oriented correctly for polarity. Replace the ones you already used with fresh ones. And check for any shorts, of course.

You can also add a cap in parallel with the pot like:
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Old January 11th, 2014, 11:48 PM   #7 (permalink)
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I redid the circuit board following the example you provided, less the extra rectification diodes and the 1000uf cap,as I believe the power source already has provided something appropriate. It is working fine in voltage test mode, with a range from 1.25 to 19.4v. Runs much cooler too. Some pics:

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Now to find a suitable enclosure.
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Old January 12th, 2014, 09:35 PM   #8 (permalink)
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Running it on a chain with 4 or 5 pedals at 9 to 10.5 v seems fine, no noise whatsoever. However , the heat sink still gets pretty warm to the touch. I am wondering if additional diodes in series, or a diode with higher current rating might make a difference. Works very steadily though.
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Old January 13th, 2014, 11:44 PM   #9 (permalink)
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I added some additional heat sink and that seemed to help. LED voltage display too, 6$ off EBay with free shipping from Georgia ( the State), not China.

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Old January 15th, 2014, 11:20 AM   #10 (permalink)
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Just ordered one of these:
http://www.elecrow.com/power-supply-...96s-p-655.html

It's switch-mode, but I've had good success with the $2.50 ones that use the same chip - no oscillator noise.
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Old January 15th, 2014, 05:13 PM   #11 (permalink)
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As you may know Fatknuckle,D1 and D2 are not there for rectification,they are there to protect the regulator when you switch off the power supply and the output capacitor(s) discharges.You should be fine though,as long as the output voltage stays beneath 25 volt and the capacitor isnīt too big.
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