Introduction: Variable ATX Bench Powersupply - FabLab Edition

Picture of Variable ATX Bench Powersupply - FabLab Edition

A good power supply is very useful when working with electronics, but it can be pricey. However, you could have one laying around in the form of an ATX PSU (Power Supply Unit). They are the most commonly used power supplies for computers. In this instructable I'll show how I converted my ATX PSU in a variable benchtop power supply. I have got some ideas from many similar projects:

I combined ideas of several of those projects I found, to give my PSU the most options as possible.

*As I mainly use breadboards for my projects, I added female headers to my powersupply. I also made crocodile clamps that fit in those headers. For appliances that draw more current, I added banana jack terminals. And for powering my arduino a USB port was added.
*For the powering of some DC motors I wanted a variable powersupply, so I added a variable output with an LM317 (a voltage regulator, ), controlled by a 10-turn potentiometer.

*Lastly I added fuses as safety measure. An ATX PSU should have a short-circuit protection built in, but this way we are completely sure we don't destroy it.

Wanting to add as many features as possible, I also made a case for it. I chose for plexiglass, as it is nice to see the cable management inside (I am a cable management freak). The plexiglass was laser-cut at FabLabXL, a FabLab ( in Brussels, Belgium.

As for heat management and safety I don't recommend installing everything in the PSU itself.

As we are dealing with a high voltage power supply, I recommend only attempting this if you have a basic knowledge of electricity, and its dangers. Inside the PSU there are some BIG capacitors, that - when charged - will kill you upon touch; even if the PSU is off! Always let them discharge for a few days before opening the PSU. As stated before this is why I don't recommend mounting everything inside the PSU, but making a seperate enclosure. I am not responsible for injuries or deaths.

Step 1: Basic Information + Parts

Picture of Basic Information + Parts

The ATX PSU has a lot of wires coming out of the back. They can be recognized by their color ( ).

  • Black: ground (GND)
  • Orange (+ brown): 3.3V
  • Red (+ pink): 5V
  • Yellow: 12V
  • Blue: -12V
  • Purple: 5V Standby.
  • The brown and pink wires are sensing wires (they verify if the output is 3.3V or 5V respectively).
  • The green should be connected to GND (black) to turn on the power supply.
  • The grey wire is "power good", and gives +5V if all wires are connected properly (i.e. green connected to black, borwn sensing wire to orange wires and pink sensing wire to red wires).

To give a stable output, the PSU needs a minimum load on its highest current wire, in my case the 5V line (see the label on the PSU). So a 10 Ohm 10W power resistor is added between 1 red and 1 black wire.

1) Basic parts

  • 10 Ohm 10W power resistor
  • 2x 220 Ohm resistor
  • Red & Green LED
  • Switch
  • Banana jack terminals (x6)
  • Female headers 1x4 (x7) (you can cut them from longer strips)
  • USB port
  • Screw terminals (optional)
  • Cable connectors (optional)
  • Fuse holder (x6)
  • Fuses (I choose 5A for the normal ones, 1.5A for the variable)
  • Crocodile clips and header pins (optional)

2) Variable circuit parts

  • LM317 (voltage regulator)
  • 1uF capacitor
  • 100nF capacitor
  • 1.2k Ohm resistor
  • 10k Ohm 10Turn potentiometer
  • 1N4007 (or similar) diode
  • Heatsink for LM317 (CAN NOT be mounted against the case!)
  • Screw terminals (x3)

3) Case

  • Plexiglass 4mm (about 0.5m x 0.5m)
  • Threaded rods M4 18cm (x6)
  • Washer M4 (x24)
  • Nut M4 (x24)
  • Handle
  • ATX screws (x4)

Step 2: Assemble the Basics

Picture of Assemble the Basics

To make use of the fixed output voltages of the PSU (3.3V, 5v, 12v and -12V) we just need to connect some wires. Ofcourse we will add a switch, some indicator LED's and connectors to make it really useable.
Always make sure to use heat shrink tubing and tape where possible, we don't want short circuits.

* The first step is to cut off all connectors and bundle the wires of the same color. I recommend opening the case to blow out all the dust, but as mentioned before: be careful! Try not to touch the components and certainly don't touch the leads of the big capacitors!
* Take the switch and attach the green wire to one leg, and a black wire to the other leg. I used cable connectors, but you could just solder it.
* Now solder the resistors to the anode (long leg) of the LED's and add heath shrink tubing. Next, solder both cathodes (short legs) to a black wire. Solder the purple wire to the resistor coming from the red LED anode and the grey wire to the resistor coming from the green LED anode. (Look at the schematic)
* We also need to add a dummy load: a 10 Ohm 10 W power resistor. Simply solder it between a red and black wire coming from the PSU and make sure to insulate the connections. Attach it to a heatsink (or the PSU itself), because it gets hot!

* For the power wires, take a look at the schematic for a detailed overview. The basic idea is (“>” = 1 wire):

  • GND >>>> Banana >> Header
  • 3.3V >>> Fuse (5A) holder >> Banana > Header (2 orange wires and the brown sensing wire)
  • 5V >>> Fuse (5A) holder >> Banana > Header (2 red wires and the pink sensing wire)
  • 12V >> Fuse (5A) holder >> Banana > Header
  • -12V > Fuse (1.5A) holder > Banana > Header
  • Variable > Fuse (1.5A) holder > Banana > Header
  • 5V > Fuse (5A) holder > USB port positive side

* Add cable connectors to the 3.3V, 5V, 12V and -12V wires coming from the PSU and attach them to one side of the fuse holder. Again, you could just solder them together. Do the same with a short wire that will be connected to the variable circuit in the next step. 4 black wires can directly be connected to a banana terminal.
* Now we need wires that connect our fuses to the banana terminals. Take wires of about 5 cm and add cable connectors to both ends of the wire. You could also simply solder them together.
You can cut off pieces from the PSU wires, they should be long enough.
* Next we will prepare the header pins. Take a strip of perfboard (prototyping board) and place the headers on regular intervals. Take 2 header strips for ground. Now solder them to the perfboard. The perfboard makes it possible to easily mount them inside a case, and to ensure an even spacing.
* Now solder the banana terminals to the headers with a wire of about 5cm.
* Solder the usb port on some perfboard and add a screw terminal. Connect a black wire from the PSU to one side. Connect the other side to the fuse. Make sure to check your connection here before plugging in a USB device! Compare it (with a multimeter) to a USB wall adapter you have laying around.

I used 2 wires per color, which should give me enough current. If you want more current, use more powerful fuses and more wires. It is very important to attach the brown wire (3.3V sensing) to the orange wires (3.3V) you use! Also connect the pink wire (5V sensing) to the red wires (5V).

I also made some crocodile clips that fit in the headers. Simply connect a male header pin to the crocodile clip using a length of wire.

Now your PSU should be working. The only thing left to do is add the variable output and make a case for it!

Step 3: Make the Variable Circuit

Picture of Make the Variable Circuit

For the variable output, we need some characteristics of the LM317 voltage regulator first. The output voltage is defined by this formula: Vout = 1.25*(1+(R2/R1)). The resistors are indicated on the schematic.
I used a 10 turn 10k Ohm potentiometer I had lying around, and adapted the other resistor to it. I really recommend a 10 turn potentiometer for an accurate setting of the output. Because we can only go as high as 12V (it will be a bit lower in reality), Vout should be 12V. Because R2 is 10k Ohm, we know: R1 = 10000/((12/1.25)-1) = 1162 Ohm. As the LM317 will not be able to give the full 12V I rounded it up to 1.2k Ohm, which will give us a theoretical maximum output of 11.7V. The lowest output will be 1.25V.
Now we can start building the circuit!

* Solder the small circuit (see schematic) together on a perfboard and replace the LM317 connections with screw terminals. Also add screw terminals for input 12V, variable output and ground wires. Make sure the polarity of the diode and electrolytic capacitors are right.
* Cover the bottom of the perfboard with hot glue and glue on another piece of perfboard.
* Solder a wire to each of the LM317 legs, and don't forget heat shrink tubing.
* Connect the wires coming from the LM317 to the screw terminals, do the same for input (+12V wire from PSU) and ground (PSU wire).
* The only terminal left - output - should be connected to the fuse for the variable output. Use the wire we made in the previous step. You could leave out the screw terminals and solder it directly.
* Now attach the LM317 to a separate heatsink! The exposed metal on the LM317 is not ground, but is Vout, so we need to make sure the heatsink doesn't come in contact with any wire that is not Vout. The enclosure of the PSU should be grounded, so that means we can not use it as a heatsink for the LM317.

The heatsink is important, as the LM317 can get pretty hot: P (power) = (Vin-Vout).I. This means that with an output of 1.25V and with a current draw of 1.5A it will give about 16 Watts of power. The reason I extended the leads of the LM317, is to keep it a further away from the rest of the circuit. This way, the heat that it produces, will not influence the resistance of the resistor R1, and the output will be stable.
An LM317 can output a maximum current of about 1.5A, so put a 1.5A fuse here.

Step 4: Make and Assemble the Case

Picture of Make and Assemble the Case

For the case, I used plexiglass of 4mm, which I cut with a lasercutter from a FabLab. If you don't have access to a lasercutter, you could always use a saw (preferably a jigsaw) and a drill. Wood could be an alternative for the plexiglass. Because I used many components, it's not a good idea to cram everything inside the enclosure of the PSU. It will be nearly impossible to make all the holes, and air will not be able to circulate and cool the PSU.

* Start by designing everything on paper to make sure everything will fit. It is much easier to correct mistakes in cardboard than it is in wood or plexiglass. The design is up to you, but I added my design in svg format.
* Don't make the case too small, we want enough space for the wires. I made mine 10cm longer than the PSU.
* Once you have a design you like, draw it on the computer and export it as dxf files for the lasercutter. If you don't plan on using a lasercutter, you can skip this step.
* Cut your pieces using a lasercutter (or jigsaw).
* Now drill the holes for the threaded rods (M4) and for the handle. Put thread in the holes for the rods (optional) using a tap ( Remember to drill a smaller hole when using a tap.
* Cut 6 threaded rods of 18cm. Add washers and nuts. They will connect the both sides of the case.

Screw everything in place and add some hot glue if necessary (LED's, USB and switch).
Secure the PSU to the back panel with ATX screws (they were used to connect the PSU to the PC case).
The potentiometer is installed by removing the turn knob from the body and replacing it, once through the hole.
Also make sure the heatsink from the LM317 is not touching the PSU enclosure! I attached it to the plexiglass case using some velcro. I installed the small circuit board in the same way.
The big advantage of using cable connectors is the easy of connecting everything together in the case.
Finally, tape the unused wires together or cut them of. Make sure to add tape to the ends to avoid short circuits

Step 5: Test Your PSU & Enjoy!

Picture of Test Your PSU & Enjoy!

We're done! The only thing left to do is to test the PSU. If everything works fine, you can securely tie the nuts.
Now you can enjoy your homemade variable benchtop power supply unit!


caesar2164 made it! (author)2017-07-18

Thanks for the instructable!
I took your idea and the one of using a motherboard extension cord from:

to make this:
(it's actually done with the variable voltage circuit now as well...)

andresharambour made it! (author)2017-06-26

Thank you for this awesome instructable! I did mine inspired in yours and added a little LED voltmeter to show the variable output. The rest is very similar because I loved the acrylic case idea.


THERJ (author)2017-06-11

Thanks for the detailed DIY. I am wondering if it is crucial to use fuses as I have seen several DIY where they have not been used. what kind were used in this DIY?.

Also, I am having some trouble with the variable circuit, Can the pot be directly soldered onto the board?

What is the reasoning behind adding glue to the bottom of perfboard and gluing another one on the bottom?

what is the Pot connected to on the variable circuit Board?

You state that the LM317 heat sink should not be mounted against the case. Does this mean it cannot be inside the PSU and has to be ouside the case?

Thanks in advance!

ThomasVDD (author)THERJ2017-06-13

The fuses are extra protection, they are simple glass ones.

Pot can be on the board, but it is easier to mount if it's connected via a wire. It's the variable resistor symbol in the schematic. It should indeed be outside the case, or isolated with some thermal pads.

The glue and extra pcb are for mounting.

Reloadedonce (author)2017-04-07

Hello, this is definitely a great project and so I'm busy building my own :).

Though, I'm making the variable output so that my Vout max would be plus minus 37V.

Therefore I took for R2 a 10kOhms potentiometer and for R1 360 Ohms.

So far, I don't have a resistor of 360 Ohms so I used several resistors to achieve this value. 3 x 100 Ohms in series with 2 x 30 Ohms.

Now comes the problem, one of those resistors 100 Ohm starts to heat enormously because smoke comes 2 seconds after applying power (37VDC).

After replacing the resistor by a new one, it also burns out.

Is there possibly an explanation for this?

Thanks for reading.

ThomasVDD (author)Reloadedonce2017-04-08

There certainly is an explanation, electronics is not magic (although it sometimes seems like it) ;)

First, in the case you omit the lm317: the power dissipation in the resistors is too high. You can calculate it: P = UI = U^2 / R.
So let's say your potentiometer is at 0 ohms, that means you have a 360 ohms resistor between 37V and ground. So P = 37^2 / 360 = 3.8 Watt (!)
Normal resistors only allow a power dissipation of about 0.25 Watt, so it will heat up and burn out.

In the case you do connect the lm317, this shouldn't happen. Since 0 ohms on your potentiometer will give you 1.25V output, you power dissipation will only be 0.004 Watt. That means you made a mistake in your wiring. I would suggest to rebuild it, and take another look at the pinout of the part.

I hope it helps, good luck!

Reloadedonce (author)ThomasVDD2017-04-17

Hi, I didn't make any mistake in wiring but though I checked my print with a magnifying glass and found there was a connection made between the adjust pin of the LM317 and my GND on the circuit board. This was made because of very little rests of solvent and thin. Even after cleaning this remained. I'm glad there was no danger for my components and myself of course :-). Anyway thanks for answering my reply, indeed, 0,25Watt resistors shouldn't be used for what I needed. I created a plus minus 240 Ohms resistor by putting 2 x 470 ohm 0,5Watt parallel ;-).

Additional information: same result even when taking out the LM317.

Rhasta (author)2017-04-06

Hi, and thanks for posting this.

My PSU seems to have two 12V rails and they're both 18A. The 5V is 15A and the 3.3V is a whooping 24A. (screenshot)

1) Should I put the dummy load on the 3.3V.

2) Can I connect the two 12V rails together so they can share a load higher than 18A?

2a) If I can merge the two 12V, do I have to put the dummy load on the 12V instead?

3) If I put the dummy load on the 3.3V, should I make it lower than 10ohms?

4) As someone suggested, can I put a useful load instead of a dummy one, like a fan, even though it would use much less power?

Thanks. Love this site.

ThomasVDD (author)Rhasta2017-04-08

That's a beefy powersupply :P

1) Yes, put it on the 3.3V line

2) Not sure, it depends on how the powersupply is internally wired. I would keep them seperate and put an extra banana jack. Also, 18 A is already very high for the wiring, so it's probably not a good idea to actually draw that current :P

3) You could, but 10 Ohms is probably fine

4) Sure, as long as there is a significant load it's fine ^^

Good luck!

Rhasta (author)ThomasVDD2017-04-09

3.3V rail with a 10 Ohms for the dummy load it is then.

As it turns out I'm going to need 16V and 24V for some motors so I have decided to use the 2nd 12V rail for a buck/boost converter and skip the LM317 part.

Thanks again.

pmulkins (author)2017-02-14


Great project, thought I would give it a go, but still learning. Was a little confused about this: " I used a 10 turn 10k Ohm potentiometer I had lying around, and adapted the other resistor to it." when reviewing the schematic and the images, it looks like you only used two of the three connections, which one did you omit?

I have:


Any help would be greatly appreciated!


ThomasVDD (author)pmulkins2017-02-19

Potentiometers always have 3 pins, and depending on how you want to use them you can use 2 or 3 of them.

The value that is written on the potmeter (10k here) is the resistance you will measure between pin 1 and 3 on the image below. Pin 2 is called the wiper and will "glide" along the resistor. When measuring the resistance between 1 and 2, you will see that it changes between 0 Ohm and 10k Ohm. The same can be done by using pins 2 and 3. You have now made a variable resistor, which is exactly what we need.

When using all 3 pins, you form a voltage divider, look on wikipedia for some more information.

Good luck!

knahrvorn (author)2017-01-25

Really nice project, and awesome looking case! Don't mind if I build something heavily inspired by this :-) Only thing I miss is a small display showing variable voltage.

One question, though: Which wire gauge (AWG) would you recommend for this?

ThomasVDD (author)knahrvorn2017-01-27

I used the wires from the PSU itself, they are 18 AWG. Feel free to copy the design, that's the reason I shared this project :)

cazino (author)2017-01-08

Hi Thomas, I am in the process of building your well explained design but I have a couple questions. a). I do not have a pink wire in my PSU...only one wire colors are grey, Green, violet, blue and Brown. What do I do in this case?. The other question is related to the -12v output. Do I need it, what is it used for? I mean having already a + 12 v output. Sorry if the question is too basic, I am just starting. Tks in advance

ThomasVDD (author)cazino2017-01-09


I'm glad you like the instructable :)
It is possible that you don't have a pink wire. In that case, just don't use it :p It's not an issue.
The -12V can be used if you want to power something with a so called symmetrical powersupply. Opamps often require +12V, GND and -12V. Connect it now, so you can use it if you need it in the future.

Welcome to the wonderful world of electronics ;)

imilesi (author)2016-12-09


hubi (author)2016-12-02

If you want more power at the regulated output, you could add a PNP transistor + Resistor like here

Mr_Larson (author)2015-06-01

Hi! I'd like to thank you for having shared your project. It gave me ideas to start mine, thanks to an unused ATX PSU. I have routed voltage on three outputs: 5V, fixed 12V and variable 12V. I've also added digital voltmeters (1 per output) and an amperemeter (for the variable output, but the AM does not work - I honestly don't know why...).
I also used a counter-connector to not degrade my ATX PSU, and take benefit from my 3D printer for the final casing buiding.
I've also bought a voltage converter on DealExtreme that uses the LM317: it is cheap, it works fine, but it heats so much! I have to find a way to cool it.

KévinG14 (author)Mr_Larson2016-04-17

Have you verified the wiring of your amp-meter? If it's plugged paralel just like your Voltmeter it sure won't work.

Mr_Larson (author)KévinG142016-04-25

Yes, I verified it, it is connected as it should normally be: in the current line (not in parallel) as it should contain a built-in shunt resistor. But I'm not so sure about it. And I have no real info on the product. So, it is of course a wiring issue, but which one...

signOnthe (author)Mr_Larson2016-11-04

This is old post but just want to give my advice about your AM.

1) Be sure its taking its minimum req voltage to work.

2) I'm almost pretty sure it has no shunt inside. It's look like Chinies stuff and never seen one of them with a shunt inside.

Mr_Larson (author)signOnthe2016-12-02

Hi! For the first question, I'm sure of it. For the second, I was wondering the same. In fact, this is a cheap AM found on the net, without a lot of explanations. I tried many different ways to cable it, and I think I burned it... I should try with a new one.

Thanks for the advices!

bdill (author)Mr_Larson2016-03-25

Just curious if you have the 3d blueprints for your case? thnx and both are cool builds...Im going to try to build one myself. :)

Mr_Larson (author)bdill2016-03-28

Hi, thanks! Yes, no problem to share. Could you MP me with your email adress? I'll send it to you.

FaizulI (author)Mr_Larson2015-10-05

If you don't connect the ammameter or ampere meter in series with the wire or output, it won't work.

ThomasVDD (author)Mr_Larson2015-08-21

Really nice build! Just keep in mind that you won't get the maximum current output, since you're not using all the wires.

Mr_Larson (author)ThomasVDD2015-08-21

Thanks a lot for the comment. But I don't understand the remark about the fact that I won't get the max current output (all the wires?).

ThomasVDD (author)Mr_Larson2015-08-21

If I'm not mistaken, you are only using the ATX connector that normally plugs into the motherboard. However, there are several more wires with MOLEX connectors. So you can't expect to draw e.g. 20A through 3 wires; while you could draw this current if you're using more wires. The current through the wire itself would simply be too high.

Mr_Larson (author)ThomasVDD2015-08-21

Understood, thanks. Exact, I only used the ATX. But I'm using this power supply for small electronic/electric application that don't require high current. I will reconsider it in case I need a max current. Thanks a lot again!

chandubandara (author)Mr_Larson2015-07-26

The sexiest one out of all I've ever seen! Thumbs up bro!

Mr_Larson (author)chandubandara2015-08-21

Thanks a lot!

shoe007 (author)Mr_Larson2015-07-27

That looks amazing!

Mr_Larson (author)shoe0072015-08-21


Pawel_Rafal (author)2016-10-02

hey there! great project! small offside question, what software did u use to draw those schematics ? (1st pictures in Step2 and Step3)

Pawel_Rafal (author)ThomasVDD2016-10-03

thx mate!

Go JunN (author)2016-05-23

Is the dummy load necessary? or can I use it as an another output?

ThomasVDD (author)Go JunN2016-05-24

It is necessary to get a stable output. I tested it for myself, feel free to do so as well ;)

Go JunN (author)ThomasVDD2016-05-24

The red is 5v right? and it is the one with more current than the other voltage output that's why putting a dummy to get a stable output is needed, does it mean I can still use it like to fan the LM137 or another USB port or anything that uses 5v am I right? BTW I love your project, very precise instructions and I salute the arrangements. One more question though, is it worth to increase the capacitance with larger capacitors and use powerful variable transistors relatively to the potentiometer, I know it is answerable only by myself but just ensuring it's worth since you are more experienced than I am.

ThomasVDD (author)Go JunN2016-05-25

Red is indeed +5 V. You could use the same red wire for other applications too, but I simply used the other wires. Mine had about 7 red wires, so it was not an issue. I guess you mean voltage regulator instead of variable transistor; using bigger capacitors is not necessary (these values are recommended by the data sheet). If you want more current, you could use a buck converter (switching regulator).

Thanks for the compliment :)

Go JunN (author)ThomasVDD2016-05-26

Thank you for the information :D I really appreciated it., One last very last question though, can I minimize the use of the fuses? or not? I'm really sorry if this question is very silly but an informative answer is very much appreciated :D

ThomasVDD (author)Go JunN2016-05-27

The powersupply should have short circuit protection built in, but I added the fuses just in case. My powersupply was far from new, so I added some extra security.

KhalCamargo (author)2016-01-11

Hey, awesome instructable! I'm planning on doing this!

I was thinking if instead of the LM217 I could use a LM2596? And, why not use the -12V output as ground to the variable part, resulting 24V max with the LM2596? I saw other instructables that did this.

ThomasVDD (author)KhalCamargo2016-01-12

The LM2596 is something completely different: it's a regulator to make a buck converter, not a linear regulator. It could be used, but you need more components to make it a buck converter (coil, diode, cap).

I didn't use the -12V line for 2 reasons:
1) The -12V rail is limited to 0.5A, not so much.
2) Using the -12V and labeling it as 0V can be dangerous. All other voltages (3.3V,5V and 12V) use the ground as 0V. So you may never use it as a 0-24V supply in combination with other voltages, because the 0V will not be the same for both! You could end up connecting -12V directly to ground, thus creating a shot circuit!

If you want higher voltages I recommend using a boost converter, it will supply more current and be safe to work with. Good luck! :)

KhalCamargo (author)KhalCamargo2016-01-11

LM317**** sorry

dthompso (author)2015-11-22

I'm working on building a power supply for an electrolysis project I need variable voltage from about 3.3v to about 9.5 also I need amps to go as high as 25 for the electrolysis, I plan to control 2 +12 volt solenoids (one at a time) using one of the yellow +12 volt wires and an atmega2560. I have a 500 Watt ATX that has two +12 volt rails both with around 18 amps. I like your design but I have questions (1) if the lm317 only controls 1.5 amps how do you get the higher amps? (2) Can I use a programmable variable controller instead of the lm317, in my research I think I've come across a programmable controller? (3) One other way I've researched is using PWM to program voltage changes do you know if a circuit like this will work with PWM.

Thanks in advance for your response.


ThomasVDD (author)dthompso2015-11-23

Hi, I'll try to answer some of your questions ;)
First of all, I don't know why you would use solenoids, as plates give a much bigger surface for the electrolysis process.
If you plan on drawing more than 5 amps, one wire will not be enough (if it's a small one), it will get too hot and melt.
The LM317 can deliver up to 1.5 amps, above that it will overheat and burn out. (You can make smoke signals with it though :P ) It's not suitable for such high currents.
I would use a high power buck/step-down converter, or use PWM. For PWM, use a high power mosfet and use an approriate heatsink.

Hope this helps, let me know if not ;)

About This Instructable




Bio: I study (civil) electronics engineering at the VUB in Belgium. I have a passion for making things, both useful and cool.
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