Reducing voltage of laptop power supply

I have a Toshiba 3.4A, 19V laptop power supply that I want to reduce to 12V.
Is it as simple as finding a 19V zener and changing it to a 12v zener?
                Thanks, Mikek

On Friday, April 14, 2023 at 9:59:07 AM UTC-4, Lamont Cranston wrote:
> I have a Toshiba 3.4A, 19V laptop power supply that I want to reduce to 12V. 
> Is it as simple as finding a 19V zener and changing it to a 12v zener? 
> Thanks, Mikek

No. That question is beyond unbelievable.

On Friday, April 14, 2023 at 9:04:12 AM UTC-5, Fred Bloggs wrote:
> On Friday, April 14, 2023 at 9:59:07 AM UTC-4, Lamont Cranston wrote: 
> > I have a Toshiba 3.4A, 19V laptop power supply that I want to reduce to 12V. 
> > Is it as simple as finding a 19V zener and changing it to a 12v zener? 
> > Thanks, Mikek
> No. That question is beyond unbelievable.

 Well not really unbelievable.
  Is there reference against which the output voltage is compared to in a switching power supply? 

                          Mikek

On Friday, April 14, 2023 at 10:07:39 AM UTC-4, Lamont Cranston wrote:
> On Friday, April 14, 2023 at 9:04:12 AM UTC-5, Fred Bloggs wrote: 
> > On Friday, April 14, 2023 at 9:59:07 AM UTC-4, Lamont Cranston wrote: 
> > > I have a Toshiba 3.4A, 19V laptop power supply that I want to reduce to 12V. 
> > > Is it as simple as finding a 19V zener and changing it to a 12v zener? 
> > > Thanks, Mikek 
> > No. That question is beyond unbelievable.
> Well not really unbelievable. 
> Is there reference against which the output voltage is compared to in a switching power supply? 

Yeah- the controller chip usually has a reference in it, and the output voltage is scaled up from that via a resistor voltage divider. You want to change that voltage divider. But there could be hazards, your feeding more output into the error amplifier feedback loop and it may become unstable, and some of the other performance characteristics may degrade. You may lose various kinds of protections against destruction too. 12/19 is a pretty big reduction, it may not be enough to operate the externals to the regulator, things like biases. Save yourself the headache and buy a 12V supply.

> 
> Mikek

On Saturday, April 15, 2023 at 12:07:39 AM UTC+10, Lamont Cranston wrote:
> On Friday, April 14, 2023 at 9:04:12 AM UTC-5, Fred Bloggs wrote: 
> > On Friday, April 14, 2023 at 9:59:07 AM UTC-4, Lamont Cranston wrote: 
> > > I have a Toshiba 3.4A, 19V laptop power supply that I want to reduce to 12V. 
> > > Is it as simple as finding a 19V zener and changing it to a 12v zener? 
> > > Thanks, Mikek 
> > No. That question is beyond unbelievable.
>
> Well not really unbelievable. 
> Is there reference against which the output voltage is compared to in a switching power supply? 

There might be. Cheapskates might rely on the mains voltage being stable and predictable enough and effectively divide it down from that.

You can't tell until you've got inside the power supply and looked, but a cheap circuit is more likely to use a bandgap reference at about 1.2V and compare it with a divided down version of the output. If you could find the divider you might be able to bodge that. It's not likely to be a cost effective exercise.

Sticking a 6.8V zener in series with the 19V output might work, but it would need a 25W zener, on a heat sink, so that wouldn't be cost effective either.

Just buy a 12V switching power supply.

The Australian version of Newark lists this for about $US 20. It can  deliver up to 5A

https://au.element14.com/pro-power/pp10008/ac-adapter-ite-12v-5a/dp/3340927?st=12v%20power%20supply

-- 
Bill Sloman, Sydney

On 2023-04-14 10:24, Fred Bloggs wrote:
> On Friday, April 14, 2023 at 10:07:39 AM UTC-4, Lamont Cranston wrote:
>> On Friday, April 14, 2023 at 9:04:12 AM UTC-5, Fred Bloggs wrote:
>>> On Friday, April 14, 2023 at 9:59:07 AM UTC-4, Lamont Cranston wrote:
>>>> I have a Toshiba 3.4A, 19V laptop power supply that I want to reduce to 12V.
>>>> Is it as simple as finding a 19V zener and changing it to a 12v zener?
>>>> Thanks, Mikek
>>> No. That question is beyond unbelievable.
>> Well not really unbelievable.
>> Is there reference against which the output voltage is compared to in a switching power supply?
> 
> Yeah- the controller chip usually has a reference in it, and the output voltage is scaled up from that via a resistor voltage divider. You want to change that voltage divider. But there could be hazards, your feeding more output into the error amplifier feedback loop and it may become unstable, and some of the other performance characteristics may degrade. You may lose various kinds of protections against destruction too. 12/19 is a pretty big reduction, it may not be enough to operate the externals to the regulator, things like biases. Save yourself the headache and buy a 12V supply.
> 
>>
>> Mikek

I power all sorts of stuff off random laptop bricks, mostly the 
old-fashioned ones with the 5/2.5 mm coaxial connections.

One good approach for light-duty use is to buy an eBay buck module rated 
at about three times your expected maximum current.  (Chinese amps are 
on the small size.)

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Fri, 14 Apr 2023 06:59:03 -0700 (PDT), Lamont Cranston
<amdx62@gmail.com> wrote:

>I have a Toshiba 3.4A, 19V laptop power supply that I want to reduce to 12V.
>Is it as simple as finding a 19V zener and changing it to a 12v zener?
>                Thanks, Mikek

Unlikely. There are too many possibilities, and most are not going to
be hackable.

An external regulator would work. Details depend.

Or just buy a 12 volt power supply.

On Friday, April 14, 2023 at 10:07:39 AM UTC-4, Lamont Cranston wrote:
> On Friday, April 14, 2023 at 9:04:12 AM UTC-5, Fred Bloggs wrote: 
> > On Friday, April 14, 2023 at 9:59:07 AM UTC-4, Lamont Cranston wrote: 
> > > I have a Toshiba 3.4A, 19V laptop power supply that I want to reduce to 12V. 
> > > Is it as simple as finding a 19V zener and changing it to a 12v zener? 
> > > Thanks, Mikek 
> > No. That question is beyond unbelievable.
> Well not really unbelievable. 
> Is there reference against which the output voltage is compared to in a switching power supply? 

The whole of the power supply is designed for providing 19V.  If you try to change that to 12V, you would need to go through the design process again, to assure it will work correctly.  

To answer your question, there is typically a resistor divider that feeds a portion of the output voltage to the controller chip.  This is compared to an internal reference (or it could be a separate reference, but that's not important).  To change the output voltage, you change one or both of those resistors.  If the circuit has capacitors in parallel with the resistors, make sure you get the data sheet for the controller part and adjust the capacitors if required. 

Why not just buy a 12V supply (there's plenty of them around)?  If you buy off eBay or Alibaba, be sure to derate the power output by 2 or 3 or 4x.  

-- 

  Rick C.

  - Get 1,000 miles of free Supercharging
  - Tesla referral code - https://ts.la/richard11209

In article <9d266bbf-dadc-cac9-cb6f-5eb86f79bf17@electrooptical.net>, 
pcdhSpamMeSenseless@electrooptical.net says...
> 
> I power all sorts of stuff off random laptop bricks, mostly the 
> old-fashioned ones with the 5/2.5 mm coaxial connections.
> 
> One good approach for light-duty use is to buy an eBay buck module rated 
> at about three times your expected maximum current.  (Chinese amps are 
> on the small size.)
> 
> 
> 

That is what I do. There are buch and a lot of boost modules, some are 
buck/boost.  You can even get them with a volt meter and amp meter for 
very little.

On Fri, 14 Apr 2023 08:41:38 -0700 (PDT), Ricky
<gnuarm.deletethisbit@gmail.com> wrote:

>On Friday, April 14, 2023 at 10:07:39?AM UTC-4, Lamont Cranston wrote:
>> On Friday, April 14, 2023 at 9:04:12?AM UTC-5, Fred Bloggs wrote: 
>> > On Friday, April 14, 2023 at 9:59:07?AM UTC-4, Lamont Cranston wrote: 
>> > > I have a Toshiba 3.4A, 19V laptop power supply that I want to reduce to 12V. 
>> > > Is it as simple as finding a 19V zener and changing it to a 12v zener? 
>> > > Thanks, Mikek 
>> > No. That question is beyond unbelievable.
>> Well not really unbelievable. 
>> Is there reference against which the output voltage is compared to in a switching power supply? 
>
>The whole of the power supply is designed for providing 19V.  If you try to change that to 12V, you would need to go through the design process again, to assure it will work correctly.  
>
>To answer your question, there is typically a resistor divider that feeds a portion of the output voltage to the controller chip.  This is compared to an internal reference (or it could be a separate reference, but that's not important).  To change the output voltage, you change one or both of those resistors.  If the circuit has capacitors in parallel with the resistors, make sure you get the data sheet for the controller part and adjust the capacitors if required. 
>
>Why not just buy a 12V supply (there's plenty of them around)?  If you buy off eBay or Alibaba, be sure to derate the power output by 2 or 3 or 4x.  

I highly doubt that a 19V laptop is going to run at 12V.  

You MIGHT get lucky ?

I would bet it won't work.

boB