need to reduce 20v power supply to 12v

Thread Starter

led_lights

Joined Jan 18, 2018
26
power supply.jpg need to reduce 20v power supply to 12v . 20v is a laptop power supply need to run a 12v touchcreen moniter
can this be done with a fixed resistor? how much resistance?
 

wayneh

Joined Sep 9, 2010
17,154
can this be done with a fixed resistor?
Short answer: No.

A resistor 'works' (but is inefficient) only when the load has a constant and known current demand, like a lightbulb. That does not describe a monitor.

A linear voltage regulator such as L7812 is a "smart" resistor that solves the varying-current problem, but does not solve the inefficiency problem. Depending on what you're doing (the load of the monitor), you may not care about the power loss due to the regulator.

The state-of-the-art solution is a DC-DC buck converter that can accomplish the voltage change with efficiency over 90%. You can buy these pre-made on eBay for very low cost, so I'd look into that option if it was me.
 

Tonyr1084

Joined Sep 24, 2015
6,452
Easiest solution I can come up with for you is to go to an Xfinity store and ask if you can grab one of their old power supply modules. I've gotten many from them and they're typically 12 volts around 4 amps. I've done some modifications to one. I wanted a 13.8 volt output. So I had to dig into the supply and find the voltage reference circuit. Had to cut a trace and add some variable resistance so I could attain the 13.8 volts under load I wanted.

So if you want to turn a 20 V PS into a 12 V PS then you're going to have to open the unit and find the circuit associated with regulating the output and change the reference point. That's the only way you're going to change the output of your 20 into a 12 volt output. It's a lot easier to just go to Xfinity and ask for one of their supplies. Believe it or not, they get a lot of equipment back. Routers, modems and receivers. Nearly all of them operate on 12 volts. They don't reuse the supplies. If you're handsome like me and get some older woman at the counter you could easily talk her into giving you 3 or 4 of them. I've gotten as many as 2 at a time but have never had reason to grab more than that.

Also, if you happen to have an old satellite receiver that has an external power supply like the one for your computer then you can just as easily use that. Likely you can build a connector for it to plug into. From there you only need to wire up the new connector however you want to. If ever you want to use that supply for the receiver at some future date - no problem. Nothing has been modified.

Look around for old printers. Many are given away on Craig's list (KSL(dot)com) where I live. I have 18 volt supplies and 32 volt supplies from printers as well as 12 volts. Got some 9 volt DC supplies from old cordless phones. LOTS of 5 volt supplies from cell phones. Couple 24 VAC supplies from old water softeners and sprinkler systems. There's LOTS of places to find what you need without having to build.

But if build is what you want to do then you're going to have to start by opening the supply. If you can't identify the reference circuit then post a picture. Good pictures get good results. Someone may ask you to zoom in on a specific part. Then it's possible someone will tell you what to change or modify. But that all depends on your willingness to open the box. - - - The one I opened? I used black tape to close it back down. Then used hot melt glue to seal around the edges. Then removed the tape and sealed the remaining parts.
 

WBahn

Joined Mar 31, 2012
26,398
You will very likely find that the cheapest way is to simply get a 12 V adaptor that can provide at least 3.5 A. Amazon has 12 V / 6 A supplies for under $11.

You might also see how much a replacement supply for that particular monitor is, that way you don't even have to worry about getting the connections to mate properly.
 

MisterBill2

Joined Jan 23, 2018
9,797
Short answer: No.

A resistor 'works' (but is inefficient) only when the load has a constant and known current demand, like a lightbulb. That does not describe a monitor.

A linear voltage regulator such as L7812 is a "smart" resistor that solves the varying-current problem, but does not solve the inefficiency problem. Depending on what you're doing (the load of the monitor), you may not care about the power loss due to the regulator.

The state-of-the-art solution is a DC-DC buck converter that can accomplish the voltage change with efficiency over 90%. You can buy these pre-made on eBay for very low cost, so I'd look into that option if it was me.
Wayneh is totally right! Also, for a monitor it will be a lot more convenient to have the correct supply, instead of a bunch of parts. Just be sure that you get a 12 volt supply with the correct polarity on the DC connector, because some applications use center negative.
 

Janis59

Joined Aug 21, 2017
1,356
However nicer choice may be order at ebay for 40 cents the SMPS labelled YB11L03 DC-DC LM2596S power supply/battery charger. That is 1x2 inch pcb designed for 7...35 V input and 2-30 V output up to 5 Amps adjustable for CC or CV regimes
 

MisterBill2

Joined Jan 23, 2018
9,797
However nicer choice may be order at ebay for 40 cents the SMPS labelled YB11L03 DC-DC LM2596S power supply/battery charger. That is 1x2 inch pcb designed for 7...35 V input and 2-30 V output up to 5 Amps adjustable for CC or CV regimes
That can work well, but only if you get the information on how to set up the board. AND, of course, you will still need a case for the circuit board because the board package is mains powered.
 

Janis59

Joined Aug 21, 2017
1,356
RE:""because the board package is mains powered.""
Ouch, then the safety is on the first place. By the way, look into datasheet of HV2405E. Thats realizing just what is need there, with circuit, two resistors and 4 capacitors. Adjustable between 5 and 18 Volts. Circuit I find only at AliBaba (10 cents).
 

Veracohr

Joined Jan 3, 2011
761
Fow what it’s worth, most Goodwill stores I’ve been in have large amounts of random power supplies. Even cheaper than Amazon.
 

Thread Starter

led_lights

Joined Jan 18, 2018
26
Short answer: No.

A resistor 'works' (but is inefficient) only when the load has a constant and known current demand, like a lightbulb. That does not describe a monitor.

A linear voltage regulator such as L7812 is a "smart" resistor that solves the varying-current problem, but does not solve the inefficiency problem. Depending on what you're doing (the load of the monitor), you may not care about the power loss due to the regulator.

The state-of-the-art solution is a DC-DC buck converter that can accomplish the voltage change with efficiency over 90%. You can buy these pre-made on eBay for very low cost, so I'd look into that option if it was me.
 

Thread Starter

led_lights

Joined Jan 18, 2018
26
sorry i been busy and havent checked on this since shortly after posting it. im getting ready to make an ebay purchase though so its a toss up to spend 15.00 for a replacement cord or seeing if i can find the buck converter. thank you for helping sorry i didnt reply faster
 

MisterBill2

Joined Jan 23, 2018
9,797
Digging into a packaged computer power supply to try and find the voltage setting resistors will be a large effort with little chance of success. Those devices are tightly packaged with no information available. All of the suggestions for finding a 12 volt supply are preferred.
 

an69 me

Joined Apr 3, 2018
3
i found an end that would plug into the screen power input so i soldered it onto a direct tv power supply. however i looked into the buck converter and ordered one on ebay for 3.99 just to have as it will go nicely with this 30v power pack i have laying around to power pretty much any device 5-30v....new problem though..installed the latest drivers-and older ones- but all i can do is scroll up and down a page, move the cursor and drag windows. it will not let me click any buttons, tabs, ect. was hoping to use this for photoshop which is why i purchased it. any help would be appreciated....thanks in advance!
 

wayneh

Joined Sep 9, 2010
17,154
i found an end that would plug into the screen power input so i soldered it onto a direct tv power supply. however i looked into the buck converter and ordered one on ebay for 3.99 just to have as it will go nicely with this 30v power pack i have laying around to power pretty much any device 5-30v....new problem though..installed the latest drivers-and older ones- but all i can do is scroll up and down a page, move the cursor and drag windows. it will not let me click any buttons, tabs, ect. was hoping to use this for photoshop which is why i purchased it. any help would be appreciated....thanks in advance!
Are you saying your keyboard is not working? You need to find the right kext for it or try a different keyboard. I had some success making Hackintoshes but it's been a long time now.
 

mcgyvr

Joined Oct 15, 2009
5,394
i found an end that would plug into the screen power input so i soldered it onto a direct tv power supply. however i looked into the buck converter and ordered one on ebay for 3.99 just to have as it will go nicely with this 30v power pack i have laying around to power pretty much any device 5-30v....new problem though..installed the latest drivers-and older ones- but all i can do is scroll up and down a page, move the cursor and drag windows. it will not let me click any buttons, tabs, ect. was hoping to use this for photoshop which is why i purchased it. any help would be appreciated....thanks in advance!
First I'd suggest starting your own thread..
Second there is absolutely no way we can give any help based on the massive lack of information in your post..
All we know is you have done "something" with some power supply for some reason we have no idea about and seem to have "some" computer problem that may be related to your mouse..
 
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