PSU from laptop power bricks - Page 1
Forum section: electronics / beginners

author: psvedman
author_rank: Newbie
timestamp: June 07, 2018, 06:20:54 pm
content: Hello.I have built myself a simple LM317 adjustable psu that can give me about 1 amp at 12 volts with a wallwart supplying the power. It doesnt have constant current as I felt that wasnt neccessary. Now I need something better so I scored a couple 19V 5A laptop power bricks at a thrift shop for a couple dollars each. I got a bigger 19V 8A brick as well. I would prefer not to mess with AC, and decent transformers with multiple taps cost a lot anyway.I have been googling around looking at PSU designs trying to find some simple and reliable way to use these bricks.My desire is for a PSU that can give me both a fixed +5V, a fixed +12V, a variable 0-19V output, and +/- 12V. It doesnt have to be high current, but 2-3A would be nice to have. Constant current on the variable ouput would be nice. Also, I would like to be able to draw from all five ports at once if I have to.I have very limited lab space so a compact unit is neccessary, it would be difficult to find space for several different PSU:sMy idea is as follows:Using one brick in conjuction with a decent boost converter to get 25-ish volts that can be split down to a +12 and a -12V supply for op-amp experiments and the like. I would do that with a virtual ground circuit like the ones you find here :https://tangentsoft.net/elec/vgrounds.html. I have tried a few of these in a breadboard and they seem to work fine.A second brick in the same case would have three other DC-DC circuits in parallell, a 7805-based +5V , a 7812-based +12V and a variable supply with CC and CV adjustable between 0 and 19-ish volts DC.I worry about two things :1. Would it be possible to tie a virtual ground from brick 1 to the ground on brick 2 without major issues ? I am uncertain about how virtual ground compares to "proper" ground.2. Will parallelling three different power rails on a single SMPS supply output create imbalances in the PSU brick?Grateful for any pointers you guys and gals can give me./Peter

author: cellularmitosis
author_rank: Unknown
timestamp: June 07, 2018, 06:37:07 pm
content: The key will be finding out whether the laptop brick output negative terminal is tied to ground, or is floating.You can use a multimeter in continuity mode, connect one lead to the third / ground prong of the wall plug (while it is unplugged from the wall), connect the other to the negative lead on the barrel jack output. If there is continuity, it is definitely earth-reference output (i.e. NOT floating).However, I'm not 100% sure if the inverse is true (perhaps there is a way it could be earth-referenced but have high-impedance to ground while turned off?). If you want to be 100% certain that the outputs are floating relative to each other, you could stack two outputs using a resistor between them. If they are floating, you should measure 38V end-to-end stacked. If they are not floating, you just shorted out one of the supplies through the resistor, and the resistor will get hot. A 2.2k across 19V is 8.6mA, which is 164mW in the resistor, which will make a 1/4 resistor hot to the touch without destroying it.Just to be clear, the connection is: brick 1 positive output --> resistor --> brick 2 negative output. measure from brick 1 negative output to brick 2 positive output.

author: cellularmitosis
author_rank: Unknown
timestamp: June 07, 2018, 06:43:24 pm
content: Btw, I've also been sketching out some ideas over the past few days for a single-supply lab power supply with variable current limit, after I was introduced to the LM723, which got the gears turning in my head. This has lead to quite a rabbit hole of googling through eevblog lab supply threads!A few years ago I made a board for a simple LM317-based supply which has 6 fixed-current limits, rather than true variable current limit:https://github.com/pepaslabs/DualLM317BenchSupply

author: psvedman
author_rank: Newbie
timestamp: June 07, 2018, 08:29:32 pm
content: Thanks, I will test the bricks this weekend. I believe they will probably turn out to be floating, but we will see.That LM 317 project of yours looks pretty neat, nice pcb and all. Mine is similar except for the lack of a cc selector. I built a couple different LM317 cc passthrough dongles I can put between the output and the load, a bit of a kludge but has worked decently enough so far.

author: viperidae
author_rank: Frequent Contributor
timestamp: June 08, 2018, 07:32:21 am
content: Watch out connecting the a virtual ground to another PSU. They are usually isolated, but there is also a noise suppression capacitor between the primary and secondary inside the brick. That might cause a lot of noise.

author: Zero999
author_rank: Super Contributor
timestamp: June 08, 2018, 08:29:22 am
content: Quote from: cellularmitosis on June 07, 2018, 06:43:24 pmNote that connecting the current limiter like that results in an additional voltage drop. 20V out, with 24V in, at full load is very optimistic. In reality, the maximum output voltage, at full load will be something like 17V to 18V.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: June 09, 2018, 01:58:04 am
content: got 3 old ibm laptop psu, 19V 2A; 2 not working, in the end power connector soldering became bad ...the output very noisy around 200-300 mV; so it was a lot common choke and converted as regular choke and ceramic capacitors for filtering, the result was good, at least scope on 1mv shows nothing; +15 max output .was a prototype - but seems working.so thinking to make 4 independent outputs ( 4 laptop psu)but a lot need to take into consideration:1. produced EM noise ; the unit was covered by copper shield, guess if i dismantling it, had removed copper; then all 4 will I need insert in a shield case ...2. a significant LC filter for each channel. bought a set (https://www.aliexpress.com/item/-/32841058417.html) use all for a single channel ...3. I'm not sure the best way for filtering on main side. leave as is + one common filter; or de-solder existing from all 4 ; and construct one common , some heavy duty for all four ....4. then need a a really custom case , heat sinks , and so and so ...at least i have 4 voltmeters with a good precision ,https://www.aliexpress.com/item/-/32838869757.html

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: June 09, 2018, 02:33:39 am
content: Here's what I've done for a similar reason:I use three ~19VDC laptop power supplies - all at least 65W and all isolated.One provides the single-rail supply needed to power a couple of RF power amps.The other two I use to provide +5VDC, +15VDC, -15VDC.Each of the two supplies is down-converted to +15VDC using $12 DC-DC down-converters (from Amazon). The two outputs are connected in series. The output from the first is also fed into another DC-DC down-converter to provide the +5VDC.I haven't bothered cutting open the supplies. I've shortened the cables and they sit cozily inside my instrument case. I have spliced together the three power cables and a short cable connects to a terminal block.The power supplies were spare and the three down-converters less than $40. I'm very happy with this solution.You can see the power supplies and the down-converters in the last image of this message:https://www.eevblog.com/forum/chat/whats-your-work-benchlab-look-like-post-some-pictures-of-your-lab/msg1574593/#msg1574593

author: GigaJoe
author_rank: Frequent Contributor
timestamp: June 09, 2018, 02:54:19 am
content: the devils in the details ...I had to play with up converter +40V - but converter itself produce a great noise (something ove 1V p-p). My attempt to build a precision one with as low noise as possible.

author: psvedman
author_rank: Newbie
timestamp: June 11, 2018, 10:57:08 am
content: Ok, so I checked on my bricks, they are both floating as they have no ground prongs on their mains plugs, they are all EURO-style connectors and marked as double isolated. (the square with a square inside)So, if I connect two of them up to get 19x2 = 38VDC that should work about the same as with batteries, wouldnt they? The bricks are identical in specs, same manufacturer and so on so the draw on them should be equal and as they both float grounding shouldnt be an issue, should it?

author: Cliff Matthews
author_rank: Unknown
timestamp: June 11, 2018, 02:26:25 pm
content: Another option (for some) might be the isolatedHP 32v/16v adapterslots of variants up to ~1.5a found on inkjet ptrs.