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Gannu_1 4th November 2017 01:28

DIY: Bench power supply using a computer's ATX SMPS
 
28 Attachment(s)
I wanted a source of 12V power at home that could output at least 4-5 A to do some testing with bulbs, HID ballasts, head units etc. before proceeding to install it in the car (helps in troubleshooting DOA components). While researching on the internet, I read about these old discarded computer power supplies (SMPS) being converted to bench power supplies which could output +12 V, +5 V and +3.3 V and it was a fairly simple process.

Fortunately, I had a spare 450W power supply stashed somewhere after I sold off my NAS system (Network Attached Storage). Not sure why this power supply remained unsold but now I am glad it didn’t all this while! I made one when I found some spare time during a weekend.

Attachment 1692454

This way, I wouldn’t have to go to the car in the basement parking and pry open the fuse box trim every time I intend to test some new component.

Parts required:
  1. Computer power supply/SMPS with the 230 V power cable. I used a Corsair 450W power supply which had an output of 34A on the +12V rail.

    Attachment 1692455

    Attachment 1692456

  2. Binding posts - 4 nos. of various colors for the +3.3V, +5V, +12V and GND. I got these from a local outlet that deals with electronic components. You can get them in various sizes too – I chose a larger size.

    Attachment 1692457

  3. Ring terminals - 4 nos. of medium size. The terminal would be inserted into the binding post thread so choose the size depending on the binding post.

    Attachment 1692458
    (Image source: Google Images)

Tools required:
  1. Soldering iron

  2. Solder spool

  3. Heat shrink sleeve - 8 mm dia, 300 mm length

  4. Wire cutter/stripper

  5. Cutting pliers

  6. Phillips head screw driver

  7. Cable ties – 4 inch, 12 nos.

  8. Center punch

  9. Drilling machine with a HSS drill bit (choose the size of the bit based on the binding post thread dia.)

  10. Screw driver/ratchet with hex. socket

  11. Crimping tool

  12. Heat gun

  13. Digital multimeter (DMM)

Warning:

Although the track side of the PCB is not accessed in this DIY, I should put up this warning for anyone who's planning to remove the PCB from the unit.

Power supplies have large filter capacitors that store electrical energy for a long duration even after the unit is turned off. This can give a nasty shock if you end up touching the terminals accidently without discharging them!

I would recommend using a 60W bulb to discharge them before attempting to work on the power supply – attach the bulb to a bulb holder, use the two wire ends to connect to the cap’s leads and the illuminating bulb would slowly fade away discharging the cap. Shorting the terminals isn’t recommended since it will result in a big spark and can damage the PCB’s tracks in the process.

Procedure:
  1. Before attempting to convert a discarded PSU to a bench power supply, test if the PSU is working. Plug the 230 V cable to the socket and turn it on. Using a bare wire or a paper clip, insert one end on pin #16 with the green colored cable (PS_ON#; highlighted below) and the other end to any of the pins with a black colored cable (GND) on the 24 pin connector:

    Attachment 1692480
    (Image source: Corsair)

    With a paper clip inserted in pin #15 and #16:
    Attachment 1692481
    (Image source: Google Images)

    The fan inside the PSU should spin if the unit is working.

  2. Unscrew the top panel of the power supply:

    Attachment 1692460

    If there’s a 120 mm fan on the top panel, disconnect the 2-pin connector from the PCB and remove the panel:

    Attachment 1692461

  3. Trim the output cables from the power supply using the pliers.

    Attachment 1692462

    Don’t throw away the cables – they can be reused for those odd jobs.

  4. Cut the cable ties inside the unit carefully and remove any sleeves:

    Attachment 1692463

  5. All power supplies use a standard color coding for the cables for different voltages:

    Yellow - +12 V
    Orange - +3.3 V
    Red - +5 V,
    Black - GND,
    Green – PS_ON#

    Snip the remaining unwanted cables from the PCB using the wire cutter and trim the length of the remaining cables to 80-100 mm (while closing the top cover towards the end, bending the bunch of cables can be a pain).

    For each binding post, 4-5 cables are more than sufficient. The rest of them can be snipped at the PCB. Bundle the wires of the same color and tie them using the cable ties:

    Attachment 1692464

  6. The green cable is a single cable which is used to turn on the DC side of the power supply. Solder it to the GND terminals or connect it to a black wire from the bunch. I've soldered it to the GND terminals on top of the PCB:

    Attachment 1692465

  7. Using the center punch, mark the center of the holes on the top cover where the binding posts are to be fixed:

    Attachment 1692466

    Take special care if there’s a fan mounted on the top panel. The gap between the edge of the fan and the power supply will be less leaving little space for the binding posts. Unscrew the fan after marking the center points.

  8. Drill the holes at the marks using the drilling machine:

    Attachment 1692467

    Attachment 1692468

    Keep a bunch of newspapers below the panel when you do the drilling.

    The ungainly mess you see above is because I didn’t have the HSS bit of the right size for the binding posts and ended up with smaller holes than what was required. I had to enlarge the holes manually and grind the sharp edges using the dremel which removed the paint. Don’t do that mistake! :p If you are unsure of the size of the HSS bits required, get 3-4 bits of incremental sizes. They are cheap! So to set this right, I had to source some matte black spray paint cans to paint the ground surfaces to prevent it from getting rusted:

    Attachment 1692469

    Attachment 1692470

    Attachment 1692471

    Attachment 1692472

    A snap of the dining table (also serves as the workbench) during the process:
    Attachment 1692482

  9. Screw the binding posts to the top panel using the hex. socket attachment:

    Attachment 1692473

    Attachment 1692474

    I mounted the posts in this order: GND, +3.3 V, +5 V, +12 V.

  10. Using the wire stripper, remove the insulation of the cables of each bundle and wrap them:

    Attachment 1692475

    Solder the joint, insert a heatshrink sleeve into the bunch and crimp the ring terminal firmly to the soldered joint:

    Attachment 1692476

    Repeat the steps for the other 3 bunches. Finally, use the heat gun to shrink the sleeves over the crimped joint.

  11. Insert the lugs into the binding posts and tighten the second nut:

    Attachment 1692477

    Before attempting to close the top panel of the power supply, connect the power cable and do a quick test using the DMM to check if the unit is pushing the voltages through the 3 posts.

  12. Screw the fan to the top panel, connect the 2-pin connector to the socket on the PCB, insert the panel back to the unit (this may require some effort due to the bunch of cables on the binding posts) and screw it. We are good to go!

Results:

Testing with a DMM:

Attachment 1692478

https://www.youtube.com/watch?v=aq08JBH4630

With an H8 55W bulb:

https://www.youtube.com/watch?v=Dnx4w6COedU

Testing the 12V to USB converter for the dash-cam charger:
Attachment 1692483

Thanks for reading! Let me know if you have any queries.

Eddy 5th November 2017 20:16

re: DIY: Bench power supply using a computer's ATX SMPS
 
Thread moved from the Assembly Line. Thanks for sharing.

sudeepg 5th November 2017 21:39

re: DIY: Bench power supply using a computer's ATX SMPS
 
Quote:

Originally Posted by Gannu_1 (Post 4299278)

For each binding post, 4-5 cables are more than sufficient. The rest of them can be snipped at the PCB. Bundle the wires of the same color and tie them using the cable ties:

Great DIY Ganesh! For someone who does a lot of DIY, this is a valuable tool and makes a lot of sense. I didn't understand why you need 4-5 cables instead of just one. Each cable should output either 3, 5 or 12 volts, isn't it?

reignofchaos 5th November 2017 21:48

re: DIY: Bench power supply using a computer's ATX SMPS
 
Just make sure the power supply can take it without the fan. It wasn't designed to run without a fan so the entire setup might be a fire hazard.

Of course if you use a large enough supply with decent components (which that corsair vs supply isn't), this should not be an issue!

Thad E Ginathom 6th November 2017 01:44

re: DIY: Bench power supply using a computer's ATX SMPS
 
Very nice idea, Ganesh :)

Quote:

Originally Posted by sudeepg (Post 4300146)
Each cable should output either 3, 5 or 12 volts, isn't it?

Each cable could take the voltage, but could it takes the amps?

sudeepg 6th November 2017 09:49

Re: DIY: Bench power supply using a computer's ATX SMPS
 
Quote:

Originally Posted by Thad E Ginathom (Post 4300209)
Very nice idea, Ganesh :)

Each cable could take the voltage, but could it takes the amps?

Indeed! It does look like to have 34A on the 12V rail it needs a higher gauge wire. Thank you!

R2D2 6th November 2017 12:00

Re: DIY: Bench power supply using a computer's ATX SMPS
 
If it were a single output wire then he'd have to upgrade the gauge to a 8 or even 4 AWG.

However, since he's taken the factory fitted output wires from different 'rails' (as they are called in PC PSU terminology), bunched them together and crimped them to a ring terminal, the 'bunch' should be ok to take the load.

However I'd have used a larger ring terminal for 34A. However, that may create a fitment problem.

EDIT: Most PSU fans have their duty cycle controlled by a thermistor. I'd retain the fan as far as possible or make sure the heatsinks have some other method of cooling either active or passive.

ROG_AK 6th November 2017 16:40

Re: DIY: Bench power supply using a computer's ATX SMPS
 
Great mod! The power supply will be fine. His planned usage will not push it to it's limits and it can easily be used in this passive mode without too much of a risk.

Sutripta 6th November 2017 20:55

Re: DIY: Bench power supply using a computer's ATX SMPS
 
Does not matter in this case, but one should be aware that in a multitap transformer setup, only one of the outputs is monitored/ controlled. Normally the 5V/ 3.3V.

Regards
Sutripta

DRC 7th November 2017 11:41

Re: DIY: Bench power supply using a computer's ATX SMPS
 
Super cool Gannu!!

Though many wireheads (just like petrol heads :-) ) use the ATX supply for any DC power needs, not many would take effort to create a proper set up like this one.
And, Its a joy reading your posts.

As already mentioned by someone, this kind of SMPS monitors only 5V for regulation. So, if you are primarily using 12V, it may not be as closely regulated and trip at times. One of the work around for this is to connect a phantom load to 5V.

NiInJa 7th November 2017 17:28

Re: DIY: Bench power supply using a computer's ATX SMPS
 
Excellent DIY

I am just wondering if it can also charge a car battery without cooking the circuit once it reaches a full charge.

I havent been into electricals and electronics since my engineering days but let me know what you think.

Regards,
NJ

R2D2 8th November 2017 20:46

Re: DIY: Bench power supply using a computer's ATX SMPS
 
Quote:

Originally Posted by NiInJa (Post 4301339)
I am just wondering if it can also charge a car battery without cooking the circuit once it reaches a full charge.

Batteries need charging at 13.2 (float) to 14.4 VDC (bulk). The output from a this SMPS is 12 VDC which is too low to fully charge your battery to 12.66 VDC which is the voltage at full state of charge. So, in a nutshell, this SMPS won't work for battery lead acid battery charging.

Gannu_1 12th November 2017 12:57

Re: DIY: Bench power supply using a computer's ATX SMPS
 
1 Attachment(s)
Quote:

Originally Posted by sudeepg (Post 4300146)
I didn't understand why you need 4-5 cables instead of just one.

When you connect a load that draws a lot of power (e.g. a 100W bulb on 12V), a single cable may not be sufficient. Either a bunch of cables as in this mod or replace it with a single lower gauge cable (larger cross sectional area).

Quote:

Originally Posted by reignofchaos (Post 4300153)
Just make sure the power supply can take it without the fan. It wasn't designed to run without a fan so the entire setup might be a fire hazard.

Quote:

Originally Posted by R2D2 (Post 4300383)
I'd retain the fan as far as possible or make sure the heatsinks have some other method of cooling either active or passive.

I haven't removed the fan from the unit or disconnected it! It is pretty much there and spins when the power supply is turned on. You can see it in the last pic.

Quote:

Originally Posted by DRC (Post 4301094)
One of the work around for this is to connect a phantom load to 5V.

Initially, I did connect a 10 ohm 10W ceramic resistor to the 5V rail as a constant load but removed it later since the unit was working fine without this load.

Attachment 1695108

Quote:

Originally Posted by NiInJa (Post 4301339)
I am just wondering if it can also charge a car battery without cooking the circuit once it reaches a full charge.

The Wikihow article suggests it can but has a caveat:

Quote:

You can use your power supply 12V output as a car battery charger! Be careful, though: if your battery is too discharged, the power supply short circuit protection will trigger. In that case, it's better to put a 10 Ohm, 10/20 Watts resistor in series with the 12 V output, in order to not overload the power supply. Once the battery is near 12V charge(you can use a tester to verify that), you can remove the resistor, in order to charge the remaining of the battery.

R2D2 12th November 2017 15:35

Re: DIY: Bench power supply using a computer's ATX SMPS
 
Quote:

Originally Posted by Gannu_1 (Post 4304046)
I haven't removed the fan from the unit or disconnected it! It is pretty much there and spins when the power supply is turned on. You can see it in the last pic.

Oh, then that's fine as it takes care of the cooling.

Quote:

The article suggests it can but has a caveat:
Well, it can charge the battery from fully discharged to 12V (the max voltage of the SMPS) but leaving the battery in that state would lead to it being severely under charged.

A LA battery with a no-load voltage of 12V is only @ 25% SOC and the plates would sulphate very quickly Batteryfaq OCV & SOC table See table 4.4.2. Most low maintenance flooded car batteries are of Sb/Ca chemistry.


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