Convert SMPS (Laptop PSU) to Variable Voltage and Current Source - Page 1
Forum section: electronics / projects

author: jaipursky
author_rank: Newbie
timestamp: December 16, 2015, 08:40:24 pm
content: I want to Modify a Lenovo Laptop PSU (19 V 6.5 Amp) to make it variable voltage and current supply. I do have follow questions and appreciate your guidance.We know that SMPS voltage regulation is done using a voltage divider. This voltage divider feeds a comparator which compares output voltage to its reference voltage and decides if the mains switching needs to continue.1. Should I modify the reference voltage using potentiometer or modify the Voltage Divider itself. which one is better design to provider wider voltage range. I need 0.8 - 35 V2. Most Modern chargers have three wires (Ground, VCC and ID [Smart Pin]). It seems like the ID pin is used to adjust the Current required by the battery. Can I use this PIN to adjust the output current.3. What is the possible max voltage range (low to high) one can get from such PSU supply.On the PCB I can see two Optocouplers. One chip is a dual package and other one is single.Thanks you!

author: uski
author_rank: Frequent Contributor
timestamp: December 16, 2015, 09:02:49 pm
content: Hi,A few things that pop off my mind :- The ID pin is often a 1-wire bus connected to an EEPROM that allows the laptop to read the information about the current the adapter is able to provide, and stop battery charging if the adapter is too weak. I have never seen any direct control from the laptop, except in the Apple chargers. But my experience is limited.- You say the power supply is currently 19V and you want up to 35V output. It is very likely that it is not possible, some capacitors in the PSU might be limited to 25V. Also, the transformers/inductors in the power supply may not allow for operation at the voltages you mention. It is impossible to know the range you can achieve without looking very closely at the design of your specific PSU.Generally speaking, I am against advising beginners to hack a homebrew PSU. I know many people do it, I've done it too, but it's not great. Lab PSUs are generally cheap and provide essential features like proper current limitation and other safety features that will not be available in a hacked PSU. Also, laptop PSUs might provide safety features which are appropriate for a laptop but which will prevent proper use as a lab PSU. Generally it should work, but when debugging a design, you want to be sure your PSU is OK... if you're wondering if the problem comes from your design or your PSU, it's not fun at all.uski

author: ajb
author_rank: Super Contributor
timestamp: December 16, 2015, 09:08:36 pm
content: The ID pin is typically read by the laptop's power management system to identify the adapter and then the laptop adapts its power scheme to limit the amount of current it draws to what the adapter is capable of. The adapter will have no provision for adjusting its output voltage or current limit (unless its a 'universal' multi voltage type, but then typically the user has to set that).How you would go about doing what you propose depends a lot on the topology of the converter and the particular components selected in the initial design. Suffice to say, though, that it will be far easier to build an adjustable CV/CC supply as an add-on to the laptop adapter than to bodge the adapter itself into a CV/CC supply. You'd have to add a feedback path for CC mode, and adjusting either CV or CC set point and crossover behavior without sacrificing the stability of the converter will be tricky. The output voltage range will be limited, since the output voltage has to also power your control electronics, and the output will likely be even uglier than it is already without substantial filtering.

author: madires
author_rank: Super Contributor
timestamp: December 16, 2015, 09:09:26 pm
content: A laptop PSU woudn't make a good lab PSU. You could use it to power a classic linear PSU circuit, but that doesn't give you 35V. I'd recommend to get an inexpensive or used lab PSU.

author: Gyro
author_rank: Super Contributor
timestamp: December 16, 2015, 10:04:35 pm
content: Given that the Laptop PSU is already a SMPS, then you could do worse than hooking it up to one of the cheap ebay B3603 buck converter modules to make a quick and basic variable voltage / current PSU. It's not going to get you above your Laptop PSU voltage though, it will probably get to about 18V.The main forum thread is:https://www.eevblog.com/forum/reviews/b3603-dcdc-buck-converter-mini-review-and-how-the-set-key-could-be-fatal/There are several other threads if you do a forum search, including some asking similar questions for breadboarding PSUs.There's a helpful Youtube video too:Edit: the only 'practical' way to get up to 35V would be to put a Boost converter before the buck converter and you really don't want to go there!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 16, 2015, 10:34:07 pm
content: Typically, the design can only be varied over a small range, perhaps 15-20V.Tim

author: jaipursky
author_rank: Newbie
timestamp: December 31, 2015, 01:37:25 am
content: Thank you for great valuable comments.Ok, I see the challenge with increasing the voltage and limitation because of 25 v Capacitors in the output section.I changed my mind, so another idea is to modify the PSU only to reduce the voltage and not worry about limiting current.Once I am able to modify the PSU to produce variable voltage (0.8 - 19v max) the plan is to use TWO Identical PSUs (19v each) and plug them in series, This will give me 1 - 38 volts easily.I have many supplies lying around with no use. Buying a Variable Bench supply is easy but I am more interested in using what I have, the idea is to learn by experimenting. I am ready to put in efforts into it with a little help.Two relevant parts used in my supply are. I am planning to connect the PSU secondary to an external variable power supply and see the variable voltage on respective input pins.1. AS358M - Low Power Dual Operational Amplifiers2. Cosmo K1010 Phototransistor Photocoupler (typical PTH mount)Attaching a document here with more details. Looking for your comments.

author: richcj10
author_rank: Unknown
timestamp: December 31, 2015, 02:03:16 am
content: You could do a buck/bost topology. Efficiency isn't a problem here because it is powered from mains. A good LT part will do the voltage control. The cc part is a little harder but not impossible to overcome.Sent from my SM-N910T using Tapatalk

author: Kleinstein
author_rank: Super Contributor
timestamp: December 31, 2015, 10:37:04 am
content: I would not modify the switched mode regulator as suggested in the doc file - thats rather difficult, especially if you don't have a detailed plan of the supply. With note book supplies it hard to get there anyway.A switched mode regulator is efficient and can thus be small because little waste heat is produced, but switched regulator are also rather slow in regulation and usually can only tolerate a limited range of loads. So to much capacitance or an wrong inductve load at the output can make the regulator oscillate.So I would strongly suggest having a linear regulator at the output, despite producing quite some heat. This is also easier to build. LInear regulation to gether with some filtering can also reduce the noise from the switched mode regulator.

author: madires
author_rank: Super Contributor
timestamp: December 31, 2015, 12:36:34 pm
content: Quote from: jaipursky on December 31, 2015, 01:37:25 am... after verifying that the outputs are floating.