I'm very impressed by your freehand sine function drawings.

FULL BRIDGE RECTIFIER

Extremely useful and clear description of a switch mode power supply. I like how even though you didn't get your expected conclusion with this video, you continued it anyway and shared it with the world. Thank you.

Time traveling from 5 years in the future... thanks for this. It's instructive. I liked the primer on switching power supplies. I only had experience with old-school versions. And thanks for sharing your "failures" as well as your successes. I think we learn more from failed projects than successes. I am in the middle of building my ATX bench power supply. If I ever need a variable supply, I know to just buy one! Best wishes and continued success!

This is EXACTLY what I was looking for. Excellent freehand drawings and great detective work looking at all the different IC's and messing with the potentiometers. This is something I'd been wondering about and wanting to do. Now that I see what the limits are, I'm glad you sacrificed your power supplies to show us. This definitely wins my subscription and thumbs up!

Great GreatScott! Thank you! Small notation: please never left unconnected side pin of variable resistor (potentiometer). 04:45 and 05:55 Short it to central pin always! It will keep your load (sometimes very expensive) safe of short voltage spikes out of range.

You take great pride in your Penmanship. This shows every time you put pen to paper. The same is said with the quality of your projects. Every detail, every component, every finishing touch. This speaks volumes and it's something that so many have lost. Everything we do is like signing your name, this is the lesson I try to teach my son. I am glad to see you trying to show so many others the value of this very important lesson.

Finally I found, best of the best channel for my electronic reference. This is just like I'm in electronic class. Thanks

Yeah Scott this is what we wanna see

I once had a friend coming to me with his old, potentially broken, computer PSU. We decided we'll have fun with it by modifying the thing to make a lab psu. It was a really cheap looking PSU, the components inside are poor too, but we just kinda go with it. We turn that on and it didn't show any living sign, then I saw a switch which I thought was the 110V/220V fuse thing, then I switch that to the other side. Suddenly, a huge capacitor blows up followed by some diode and ultimately the whole circuit was in fire. We panicked and unplug the power, throw the firey thing outside, and a black-blue smoke is coming from what seems to be a bigger capacitor. That day, we learned our lesson to not mess with any shitty old electronics.

I've been looking for EXACTLY a video this clear and concise for making a variable PSU for like a year and a half. Thank you so much.

The BEST VIDEO on youtube.. Thank you so much for breaking the Power Supply down into Blocks with the expected Wave Form and Voltage. Please DONT STOP this method of explaining Electronics as You are a TEACHER really.. Thank you for taking the time to help us un-informed dummies. May the Force be with you.........

The decision to abandon the experiment on the realization it requires disabling all the built in redundant safety features is not only "the right way to do it", but also quite praiseworthy! Great vid

Unexpeced, but very good conclusion. Thanks for another great video.

I am awed and humbled every time I watch your videos. Not to mention your beautiful circuit diagrams and handwriting.

Thank you for trying to mod an ATX power supply for variable voltages. I have wondered how easy it would be to do so and now know thanks to you. Now it is time for me to look for a proper lab power supply. Take care and keep the videos coming.

Your handwriting is so perfect Edit: THANKS FOR THE 208 LIKES

Thanks for sharing, I was wondering when the warning about beeing killed by the power supply might come and was surprised you mention it at the end and not at the beginning of your video ^^

Thank you so much. That was fascinating. I have always wondered about the guts of a SMPSU. I knew the theory of course, but never had the need to build one.

I enjoy watching the videos as a guidepost of how much I've learned about electronics. Back in the day I'd have been pretty clueless from the start, but thanks to GreatScott! and others this time I got about halfway through before i had to pause/think and learn. Next time even further.

Expensive is a relative term my friend.

The first power supply that you were modifying, that style works great. The high wattage ones with the UC384x chips are usually 2-switch forward converters and regulate very well. When you're modifying the feedback circuits in these, you must not modify the circuit around the UC384x chip. Since the feedback is with an opto-coupler, the voltage division and comparison is done on the secondary side with a TL431 programmable shunt (or sometimes but rarely a zener) and that sends a simple on/off signal through the opto-coupler back to the UC384x chip. You only need to modify the divider going into the TL431. The TL431 is usually a TO92 package and looks like a small transistor.

Hi Scott. A few years ago I have done the same modification as you did, and I consider the result a full success. Although the voltage range is limited, it varies enough to make the bench PSU useful for my purposes. However, I understand why you disencourage your readers to do the same. The risks of dealing with 240vac is immense and you don't want to be responsible for people risking their lives, specially when forgetting to discharge high voltage capacitors is a common error even among professionals. You are right and I agree with your stance, BUT for those who actually have the skills and knowledge and responsibility to safely do this modification, I think it's totally worth it. It's a nice project, easy to do, fun and rewarding to see it working in the end.

Nice video, glad you posted it. Very good breakdown of the mechanics behind the workings of the power supply. I had made an ATX bench power supply and also wanted a variable output. I made a box that plugs into the +12 and -12 (to get 24V) which is a variable DC-DC converter. I used the MC34063 as the basis for the converter. It's a little clunky having an external converter, but it's very useful and I use it all the time.

Hey Jeremy, The problem is that your playing with a chip that uses 2 comparators. These are way more complicated. However that KA7500 chip is a replica/update of the old TL494/TL594. You can redesign the circuit around that IC to create a voltage and current feedback loop and therefore a variable voltage/current supply, (good luck building that in a 1 week project though ;) I've been looking into this circuit, but it gets super complicated IMO to make it variable over a wide range. The FET duty cycle and core saturation math gets a bit blurry for me. All the examples I've found attest to the potential operating range of the circuit, but they don't really explain the details of how to design specifically for an variable output. Where and when to compromise specs is the challenge. The best I can tell you build for about a 90% duty cycle at your maximum power output, while trying to maximize the timing frequency at that output, but I'm not 100% clear on that one. I've found some interesting circuits to try and will probably do a video at some point. There is a way to use a power PNP BJT with a capacitor on the base to reduce the ripple effects across a wide bandwidth. Bill Herd over on HackaDay's YT channel hinted ages ago he was going to create a variable SMPS that used a linear design at the output. I've half built a project on that idea, but that is about 3-4 down my list. If you see this post and want to contact feel free Jeremy. I'm using YT to try to connect to more of a community. YT also keeps me committed to completing my projects despite being partially disabled and stuck laying around most of my day. My channel is still not monetized and I'm not trying to copy anyone. I'm just here to keep it interesting ;) -Jake.

this feeling when you dont understand a thing, but you feel so inteligent while watching it xD

I use a few of these for lab power supplies. They run from about 5v to 25v. I tend to remove most of the stuff on the secondary side of the board, and just leave the 12v winding and circuit. The ATX uses the spare fet and tiny transformer for sleep mode to maintain 12v to the KA7500 ( TL494 ). If we then lift pins 15 and 16 above ground, we can sense the current via a very low R resistor to ground from center tap, and use that to use the [pin 15/16 op amp to control current, while using the pins 1 and 2 for voltage control. Then we have a current controlled ( up to 12 amps or more) and voltage controlled 9 up to 25v or so) power supply, that will power very low impedance loads ( motors etc). This requires all of the monitoring stuff to be wiped from the board, or at least disconnected from the pins 1,2,3,4,15,16. So we use the 5v reference from the 7500 to go to both of the 7500 op amps, then we can simply use a divider for the voltage to the first op amp, another divider from the CTap of the tranny to ground to the second op amp, some feedback from pin 3 to both, and we have a totally stable power supply, that is completely controlled. It is actually simpler than it sounds really.

I tried this same project about a year ago and became fed up. So I just ended up just buying one of the DIY kits off of Aliexpress/Banggood for around $9.00 US. Love your videos GreatScott! been a fan for 2 or 3 years now.

Sen bir dahisin! Helal olsun sana dostum..

@5:00 HiVolt surgical gloves. Hooray

This was awesome! I'm fairly handy with a soldering iron and have a crap ton of old PSUs laying around. At first I was thinking of trying this. After seeing someone who knows a lot more than me fail at it, I think I might just buy one. Thanks for saving me from ruining my own power supplies!

OMG this is the first time in my life I was able to completely understand a GreatScott video!

2:15- RIP output rectification,we will miss you.

@GreatScott! can you make a Video building a single phase Inverter (VFD) using an arduino?

I really like the inclusion of failures in your videos. They're a learning experience a lot of other youtubers completely gloss over.

Yea I just made a custom housing for my ATX power supply and used a buck converter with a voltage/ammeter display. Works great. Gets me 1.33v -11.8v. And of course kept the constant 3.3v, 5v, and 12v banana jacks.

Maybe a video walking your subscribers through the different stages of caps, how many Joules of energy it contains, how much can kill a person, and proper PPE/protocols for dealing with these caps?

Hi GreatScott, I am your big fan. I like all your video especially the modifications of rasperry pic to game boy, since this is the video makes me start to watching all your videos, keep going to create more video with more interesting things. I have an ideas for you, can you make a AC timer switch with connection to Android phone so it can programme the timer through the smartphone. 😊😊

I have been using old XBox 360 power supplies, as well as Dell dock supplies like their D846D, for some time like this - as adjustable voltage/lab bench supplies, and they work great. They are adjustable over a fairly wide range (with some protection disabling) given what they were built for (12 and 19 volts respectively). This is a great way to get a custom voltage with high current at a cheap price. Great video, as usual.

I created a lead-acid battery charger using a PC power supply with the infamous TL494. I didn't remove the divider resistors though. I simply added a parallel variable resistor to the pulldown resistors to get higher voltage on the 12V rail. Replaced the capacitors to match higher voltages on all rails. you can find the mod on the internet though. Good work Scott.

I'll just add my .02USD here that with videos like this a warning at the start would be a good idea because man did I cringe when you pulled that PSU board out with your fingers that close to those big caps. That said I always look forward to your videos because you make them so even a dummy like me can understand them.

According to your block diagram, it looks like a FLYBACK converter right?

2:49 aaahhh...so that is how they maintain stable voltage with different current draw, Thank you GreatScott, this step by step explanation with flow chart help me a lot to understand about power supply

the mark of puting the "Great" in Scott, admitting when it wont work, then sharing your failure, and then trying again. Again, I like the variety and creativity of this video. Cheers.

4:00 - And by measuring this voltage, while the power supply is turned on, I'm waiting for the point where you accidentally shorted out something with the probe and got a huge spark and a faceful of vaporized electrolyte

There's a great 38v dc-dc variable buck boost converter module with led display available on eBay that is easily added to an old power supply that makes a great variable shop supply with adjustable current limiter.

This is exactly what I was thinking of doing. I do not have the skills as good as yours so i was looking and I came to the very solution that you came up with. It's just not worth the time. Thanks for putting this out there to quell anyone else thinking of trying to do this.

This is a really awesome project!! For such an involved modification, this made a lot of sense. Thanks for the excellent explanation.

I am so proud that you did not recommend this, because I made myself a DIY ATX powersupply just yesterday!!! I would be mad if this actually worked. one question, why didn't you put fuses in your DIY ATX power supply? wouldn't shorts be dangerous or kill the powersupply?

Buy a cheap chinese LED supply -- the kind with the perforated metal case. The feedback supply is much simpler, and the 48V ones can be adjusted from 0 to ~63V. It's a bit noisy, but an LC filter cleans it up nicely. All you need to do is make sure the r values are low enough to ensure proper bias current.

Hi sir , your explanation about the circuits are more complicated to me during past videos you made . Any way during few days i try to learn from your video tutorials and experiments by pausing and rewinding it. I am so excited to comment your video first time . i am picking up your way of explanations and ya i am learning . Any way this is a really freaky awesome video which i understood what is switch mode power supply . i am an electrical and electronics engineering student , i am just completed my one year now , 3 more years to complete my course. i think your videos pave way to success in my engineering life . any way i will catch you next time in the comment box thank you so much for your video tutorials.

I was thinking about that idea few days ago after i watched your ATX lab supply video and here it is. Thanks

Another great video Love ur videos

Top Tip: Lick all the terminals while switched on to deep clean the ends.

the quality of your videos is outstanding. keep it up

i really enjoy your projects and the way you explain. helps me in working on my own projects and/or is also just interesting. please dont stop doing those :) cheers!

7:35 nice advice man😅🤣..Well if it wasn't expensive i wouldn't have ever watched your video...and you wudnt have 7.44 mins of my watchtime...😄

I used to be a fan..... Now I'm an air conditioner...

I just subscribed to your channel. Don't know why I have not done so earlier. Your explanations are some of the best on UKposts. Well done on showing why NOT to do this to a ATX supply.

Inspired by this video I just did a crude power supply with +5V +12V +3V, -5V, -12V and an adjustable voltage from 1.2 to 11V using a buck converter inside the power supply, as I had room for that. The converter uses a LM2596 and can handle up to 3A, good enough for my experiments. Thanks for the idea!

Could you make an electric bike? I think this would be interesting. Like when you also want him to do this. (Sorry for my bad english but Iam German xD) ------------------------------- Könntest du ein E-Bike machen? Ich glaube das es ziemlich interessant sein würde xD. Like wenn ihr es auch sehen wollt.

make an electric bikee

Nice, I like the way you put up your failures as well as your successes, good bit of forensic electronics

What an awesome video! Helped me piece togheter a lot of stuff from college to understand how PC power supply works! Thanks a lot!

I like your diagrams; they are very helpful (and very neat!). Thanks.

For low current applications, we can use +12V and - 12V which together gives 24V. But the smps ground becomes a Hot terminal. To vary that voltage, a seperate circuit has to be incorporated.

I got the ATX 24Pin Power Breakout Board from aliexpress a year ago. Those are pretty neat.

I cant even draw a straight line with ruler. Your handwrite is amazing

Very good explanation of the switching power supply. The hack will be good for convert the PS rather to an exactly 12 PS than a substitute for the bench supply. Also a bit over 12 volt could be useful as car or caravan battery charger.

I looked at doing this but before getting as far as you did ad actually waving a soldering iron at it. I came to the conclusion that as there are both inductive and capacitive elements that would need to be calculated to give correct outputs and that they are not easily varied as you change outputs, that it was more likely better to just go a buy bench power supply.

Thanks for talking some sense into my stubborn head. I was positive I could make an adjustable power supply, but if Great Scott the Hacker says don't bother, guess I won't.

Awesome hack! By the way, as you must have seen, the fan also turns off at low voltages since it is also connected to the 12-volt output, this can cause the switching transistors to heat up and fry.

Thank you for your video GreatScott! I too think that this is a little dangerous! Prodding around inside a live power supply. I have created my own modular bench power supply using an LTC3780, ammeter display, 3D printing casing... looks smart! Inspired by one of your earlier videos :) so thank you.

Thanks for another good video with clear explanations and useful suggestions. I think that I would still use the computer PSU, but as you suggested at the start either use a boost or a buck converter for variable voltage. Most my circuits are less than 12V., so a buck converter should be sufficient. (I still have some malfunctioning PSU's laying around. Perhaps I should try to revive those when I can find some time off...)

Thank you for your outstanding information. Its not what I wanted to hear, but what I needed to hear. And your conclusion make perfect sense.

Great explanation and clear drawing skills. Thanks for sharing

Hey Scott, I did something like that yesterday, but i ended up rewounding the core to give me 3.3, 5, 12, 18v ground referenced and two 18v floating for a DRSSTC full bridge driver. Had to use few schottky, one more ferrite, few electrolytics, decreased the switching frequency from 65 to 40kHz as my new core was N27 type, increased soft start and tricked the smps that everything is just all right.

Hey GreatScott!! Nice video. Good to see how you are so good at application of the theory. Nice.....

Thank you for doing this investigation! I've got a 1kW Mackintosh power supply which I am attempting to convert into a bench supply with both a buck and a boost converter. Upon seeing your title I thought there might've been a simpler way! unfortunately not but atleast that warrants the purchase of a 600W Step up converter :D

Nice! I built the first version you showed, i'm sad this hack isn't reliable :( Anyway, thanks for the shared knowledge

No mention of the dangers of messing with the mains side of power supplies, or about discharging the input caps... Well, maybe we'll get rid of some people :) Glad you came to your senses.

wow that looked so simple... My approach to large circuits like that is to immediately salvage everything off it and not even consider using it. Well done

Actually, this is the video I was looking for, I tried it once before and I failed . but you made it clear , thanks that was really helpful.

Good work To protect the ATX again short circuit. It will be better to connect the Green wire (power on) to Relay contact with black wire. And to energize the relay with the 12 v or 5 v accorditly.

wow, awesome tutorial as always. I've got a long way to go to understand electronics anything like you do!

love your channel dude. you are awesome! love the way you explain everything, even your accent!! teaching myself electronics to maybe start a channel in my language.

i have the exact same ATX power supply, and i have been modding it for an adjustable output. an LM317T adjustable voltage regulator comes in handy after de soldering all those excess wires that take up half the power supply. if you look at it, the wires are grouped by color, and its then you can determine the function as well as its output voltage.

I still like the down-up boost converter attached to the 12V power side and an external potentiometer !!! I agree your writing is hypnotizing ! Lol

Hello great Scott.... Even if we connected the ATX green (stand by) wire to black (ground) wire it would still not work as there is load in the computer power supply...... How did you get around and about that .... Mine is not working and I have cornfimed the color codes for the wire are ok

Holy Cow . . . . oops!, I mean 'Great Scott! . . . .what a fabulous video/tutorial. Your presentation, explanation/documentation style is really world class, and leading the viewer though the exploration/education process only to have such an unexpected but wonderfully safety conscious conclusion is a fun thing and I admire you for it. Thanks much.

Hey GreatScott, nice attempt on trying to convert PC ATX PSU to variable DC output. I liked the video even though the results weren't that positive. Nice advice on getting a proper bench PSU. Since watching your videos, I've become interested in acquiring an oscilloscope/logic analyzer, so I was wondering if you could do a video of different Oscilloscopes, their variations in functionality and price range, and your favorite type of oscilloscopes. I think it'll be educational for new hobbyists and DIYers. Thank you for your great content!

Thanks for your video. There should be a high frequency diode fefore the capacitor. This needs to be included in your explanation.

To GreatScott I do enjoy watching your videos.

Hi, GreatScott !! Do, you please make a video on "How to make a 2 to 10 variable amp 12volt lab bench power supply" I cannot understand the topology that how to the amps can vary. So do you please clarify this section !!

I don't saying anything... Just WOW... 🤩😍 I was searching that kind of ideas... But now get it...

i did the same but i bought a variable regulator with led screen to show amps and volts and placed it on top with a couple other circuits

So Creative ! I have not seen anyone done such a mod on a power supply!

variable zener diode TL431 fixes this problem apparently, I haven't done it myself yet but I found instructions on a website, you use it to set a trigger level for the optocoupler, the feedback mechanism switches the pwm on and off based on whether the optocoupler is on or off ( when output is above a certain level defined by a zener diode ), thats how it works in some designs at least.

I once made this modification to a KA7500/ tl494 based atx supply successfully. Not only did I modify it for variable voltage but also variable current output. Worked well for lead acid/ lithium rechargeable charging. The atx I used had a protection chip with three voltage inputs 3.3, 5 and 12 volt. If the voltages varied by more than a few percent it would trigger a protective shutdown. I lifted the three input voltage pins of this chip and created a shunt regulated divider from resistors and colored led's. This gave the protection chip the voltages it wanted to see with the added benefit of still protecting the supply from output voltages of more than 16.5 volts on the 12 volt rail. Some load/ voltage ranges caused audible switching noise so I don't know if it would survive long term in these ranges or not. I killed it though by running it off of a generator while I was also using a welder. I had a bad connection in the main power wire to the welder and I think the arcing on the ac power line fried it. As far as hooking things up to the output of the supply, it did very well. Protection chip still did it's job in the higher voltage ranges it was modified to.

Good Job trying and even better Job with the warning at the end!!It's real easy to forget how powerful these power supplies are and how one mis-hap could be dangerous. Watching your first video on this, along with the Video on the viable power supply. I created a combined unit.(Picture on my Google page). One side, I have the power Supply with a Volt meter. It uses 12V, 5V, 3.3V, and -12V rails. Than next to the voltmeter, you plug from the outside, any combination of 24V, 17V, 15.3V, 12V, 8.3V, 7V, 5V, 3.3V and 1.1V. Than on the other half of the unit(w/Volt/Ampmeter), I have a 5AMP Buck Converter, powered by a 24V 5 amp (LED) Brick power supply. It is CC/CV adjustable 24V-1.5V. Its very compact and runs Great! I also used Plexiglas, so you can see the internals. All Ideas for this project came from Great Scott!! Of Course, I also have a couple of units, made apart...LOL EDIT-On The variable side of my DIY power supply, below the power switch is 2 banana female plugs(hot/cold). When the LED power supply is Not plugged in, I can direct power from my Power Supply side(with Banana cables). But Unfortunately the most I could use on that side was 24 V 1 amp. Bucked down, the amp draw got bigger, but not enough for the higher powered devices. Reason I also purchased the LED brick. I hardly ever use the LED brick, the Power supply side normally has the power needed. Lots of Great Ideas come for this channel! New to DIY,😊 Quick Q----I noticed you mentioned that the power on the other side was upwards of 325 Volts DC and I am sure a TON Of Amps....Been looking to create a Battery Tab welder, and exploring all options before I decide on the way I will go. How many amps do you think you get on the 325 V side before it will force shut off the unit? Do you think its possible to use one of these, except a MOT. Just an Idea???