How to turn an ATX computer power supply into a lab power supply

How to turn an ATX computer power supply into a lab power supply
How to turn an ATX computer power supply into a lab power supply
Anonim

You can get ATX power supplies for computers at prices ranging from 25 to 35 euros. A real laboratory power supply will cost you at least 90 euros, but you can also convert the ATX power supply from a computer you no longer need into a DC voltage source of 3.3; 5 and 12 volts which will be sufficient in most cases if you only have relatively simple tests to do. Of course, your device will not have the characteristics of a sophisticated laboratory power supply, but you will be able to use it under strong currents and benefit from effective protection against short circuits.

Steps

Part 1 of 2: prepare the power supply box

Convert a Computer ATX Power Supply to a Lab Power Supply Step 1

Step 1. Obtain an ATX power supply box

You can buy one from your computer hardware dealer or online. You can also remove it from an obsolete computer by simply extracting it from the device case. Some older ATX power supplies also deliver a voltage of −5 volts. If you want to buy yourself an ATX power box online, visit the following sites:

  • the CDiscount site;
  • the Amazon site;
  • or www.atxpowersupplies.com.
Convert a Computer ATX Power Supply to a Lab Power Supply Step 2

Step 2. Turn off the power and disconnect its 220 volt cable

Not all power supplies come with a power switch, but if there is one, you'll find it on the back of the case. Also take the precaution of do not connect to earth, this presents a risk of electric shock. Indeed, if leakage currents were to discharge to earth, they would only have one path to take to do so, and that will be your body. This can be particularly unpleasant if not dangerous for you.

Convert a Computer ATX Power Supply to a Lab Power Supply Step 3

Step 3. Unmount the power supply from the computer

First loosen the screws that hold it in the case, then disconnect the cables that connect it to the motherboard and other system components. Then remove the box from its housing.

Convert a Computer ATX Power Supply to a Lab Power Supply Step 4

Step 4. Allow the power supply to discharge

Let it discharge its high voltage filter capacitors for a few days. Some people suggest connecting a 10 ohm resistor between the ends of one of the black wires and one of the red colored ones on the secondary. You should know that this measure will not have the desired effect because in this way you will only discharge the capacitors of the output voltages of the box which do not represent any danger. The high voltage filter capacitors of the power supply primary which are charged to a potential of the order of 300 volts, therefore lethal, will not be able to discharge by this means and will remain very dangerous if you accidentally touch their pins or the components connected to them.

Do not use a power supply that you think is damaged. If it is, the protective systems are probably no longer working. Most recent power supplies are fitted with a protection circuit whose role consists of slowly discharging the capacitors of the primary part of the power supply if it is subjected for a short time to a surge peak at its input. If these surge surges are prolonged, the protection circuits may no longer function and the power supply will not turn off. If your power supply is of an older model and has been permanently connected to a voltage higher than that of its manual voltage selector, the circuits of its primary stages will probably have been destroyed and it will no longer work. You can see this when the mains voltage input fuse is destroyed

Convert a Computer ATX Power Supply to a Lab Power Supply Step 5

Step 5. Obtain the necessary components and tools

You will need the following items to make your diet:

  • six power connection sockets for 4mm diameter banana type plugs with screw contact body, try to find as many colors as you have output voltages;
  • a resistor with a value of 10 ohms and at least 10 watts of power;
  • two light emitting diodes (LEDs); one red and one green;
  • two current limiting resistors of 330 ohms and a power of 0.25 watts;
  • a small single pole SPST toggle switch for chassis;
  • a drill and a metal bit set;
  • a soldering iron with a power of 40 to 60 watts;
  • wire cutters;
  • heat-shrink tubing of various diameters and a heat gun or hair dryer.
Convert a Computer ATX Power Supply to a Lab Power Supply Step 6

Step 6. Open the power supply case

Remove the screws joining the sheet metal cover to the lower part of the box on which the printed circuit is fixed.

  • Be careful not to touch the visible metal covers of the capacitors and any wires that may be connected to them. You could receive a fairly severe electric shock if you accidentally touch them.
  • Warning: You will inevitably lose the commercial warranty coverage that applies to your power supply if it is new.
  • We advise you not to carry out the following interventions only if you are qualified in electronics. Do not remove the printed circuit from its support unless you can not do otherwise. PCB tracks and solders can still present a significant risk of electric shock if you haven't given the capacitors enough time to discharge. If you have to change a component, take care to measure with a multimeter the voltage at the terminals of the primary filtering capacitors and possibly force the discharge using the tip of a screwdriver whose handle is insulated.; you will see a spark occur at the tip of the tool, but the discharge will be harmless. If you have to completely change the printed circuit, always take the precaution of replacing the plastic sheet that isolates it from its metal support.
  • Any currents exceeding 30 milliamps per volt are potentially fatal and will, at best, apply an electric shock to you that you will remember painfully. Before any intervention, check that the AC power cord is unplugged from the power supply and from its wall outlet.. Use a multimeter to remove any doubt.
Convert a Computer ATX Power Supply to a Lab Power Supply Step 7

Step 7. Separate the connectors from the output wires

The power supply output connectors are white or black nylon blocks crimped at the ends of the wires for easy connection to the computer's motherboard. Cut the output wires from your power supply a few inches from the connectors so you can reuse them in your later projects.

Convert a Computer ATX Power Supply to a Lab Power Supply Step 8

Step 8. Group the lead wires by color

Most PC power supplies follow a standard color code to identify the voltage and function of the wires connected to it. You may find threads in colors not listed, such as brown. Here is the color code of the power supply output wires:

  • red: +5 V output;
  • yellow: +12 V output;
  • blue: −12 V output;
  • orange: output +3, 3 V;
  • white: output −5 V; this wire only exists on old power supply units;
  • purple: +5 V standby power output; it is on this wire that we will connect the green LED indicating the power supply standby;
  • black: electrical ground output (0 V);
  • gray: output of the activation indicator, the red LED will be connected to this wire, indicating that the power supply is active;
  • green: activation command input, this wire activates the power supply when connected to 0 V.
Convert a Computer ATX Power Supply to a Lab Power Supply Step 9

Step 9. Identify the locations of your external components

Use an indelible marker to mark the locations of the components you are going to add to the box of your power supply. Choose one of the sides that does not have slots or a ventilation grid to position them. Indicate with your marker the locations of the LEDs, the output terminals and the corresponding voltages. Do the same for any other components you want to add, remembering to set the power on switch.

You can free up space in your box by placing its fan outside. Also plan to add other fans if you want to get high power from your power supply. If you do not have enough space inside the box, you can arrange the access and control elements on an external aluminum plate

Convert a Computer ATX Power Supply to a Lab Power Supply Step 10

Step 10. Make your holes in a free area of ​​the box

Use a metal drill with a diameter of 3.5 mm, to make your pilot holes. Then enlarge them using a conical cutter to reach the required diameter until you can fit the insulating barrel of your connection sockets. Do the same for the locations of the on and standby indication LEDs as well as for the switch if it has a cylindrical mounting barrel. When your holes are drilled, deburr them using a rattail type file.

  • Double check the locations of the holes you want to make so that none of the items you add can come into contact with an electronic component in the power supply.
  • Take all the necessary precautions during your sheet metal work so that no metallic debris gets lodged between the electronic components of the power supply or under its printed circuit. This could destroy the device.
Convert a Computer ATX Power Supply to a Lab Power Supply Step 11

Step 11. Make the necessary holes for the LEDs to pass through

Perforate the sheet metal using a drill bit the diameter of which corresponds to that of the component body.

Convert a Computer ATX Power Supply to a Lab Power Supply Step 12

Step 12. Make the cutout for the switch passage

If the assembly of your switch is rectangular, you can make this shape making juxtaposed holes with a metal drill of 3.5 mm, taking care not to overflow on the outline of the rectangle. When you have finished these holes, join them to each other with wire cutters then file the residual metal peaks until you reach the outline of the rectangle.

Part 2 of 2: interconnect all the components

Convert a Computer ATX Power Supply to a Lab Power Supply Step 13

Step 1. Fit the connection sockets

Position the connection sockets in the corresponding holes, assemble them and tighten the nuts that hold them, without forgetting to fit the tightening distribution washers. Then check their insulation from the sheet metal of the box.

Convert a Computer ATX Power Supply to a Lab Power Supply Step 14

Step 2. Connect the 10 ohm load resistor

Connect one end to a red wire coming out of the power supply and the other to one of the black wires. It operates as a permanent load necessary to keep the power supply running. This resistance must be placed against one of the metal walls of the case to effectively dissipate the heat it will produce. You can also use, if you have the possibility, an aluminum heatsink of a fairly large size that you can place outside the case. Be careful that none of the lead wires come in contact with the body of the component or its heat sink.

  • You can also use a switch containing a 12 volt bulb that will act as the load resistor needed to keep the power supply running.
  • If you're not afraid of soldering, you can replace the 10 watt load resistor with one of the cooling fans that was originally inside the case. Pay attention to the polarities, however, and connect it by matching the red and black colors of its wires to those of the power supply output.
Convert a Computer ATX Power Supply to a Lab Power Supply Step 15

Step 3. Connect the switch

Connect one of its two pins to the green wire of the standby output control and the other to one of the black ground cables.

  • Some power supplies require the gray and green wires to be interconnected in order to operate.
  • If you do not want to use an additional switch to activate the power supply, all you have to do is permanently connect the green wire to one of the ground wires. The device will then be controlled by the switch placed at the back of the box if there is one. In this case, you will not need the green standby indicator diode and you will just have to ignore the gray wire whose end you can insulate after cutting it.
Convert a Computer ATX Power Supply to a Lab Power Supply Step 16

Step 4. Connect the red light emitting diode

This LED will be the power supply activation indicator. Slip a small section of heat shrink tubing over the gray wire that you will solder to the longer pin or anode of the red diode.

Convert a Computer ATX Power Supply to a Lab Power Supply Step 17

Step 5. Solder a 330 ohm resistor to the red diode

Solder one of the legs of the 330 ohm resistor in series to the shorter pin or cathode of the red LED. Position the latter in the hole where it should go and use a little hot glue to hold it in place. You can now solder one of the black wires on the tab left free from the resistor, having taken care to slide over it a section of heat-shrinkable tubing long enough to cover the component and its two welds after the first tests.

Convert a Computer ATX Power Supply to a Lab Power Supply Step 18

Step 6. Connect the green light emitting diode

This LED is the power standby indicator. Slip a small section of heat shrink tubing over the purple wire that you will solder to the longer pin or anode of the diode.

The role of the +5 V standby or +5 VSB line is to keep certain stages of a computer's motherboard powered on, allowing it to detect a wake-up order from signals such as network activity (WOL) or an impulse from the activation button (PWR-ON). The current flow supplied by this standby power supply is of the order of 500 mA to 1 A, even if all the other output lines are deactivated, and this as long as the device remains connected to the mains voltage. This current is more than sufficient to supply a light emitting diode indicating that the power supply has been put on standby

Convert a Computer ATX Power Supply to a Lab Power Supply Step 19

Step 7. Solder a 330 ohm resistor to the green diode

Solder one end of the resistor to the shorter leg or cathode of the LED. Position the latter in the hole where it should go and use a little hot glue to hold it in place. You can now solder one of the black wires on the tab left free from the resistor, having taken care to slide over it a section of heat-shrinkable tubing long enough to cover the component and its two welds after the first tests.

Convert a Computer ATX Power Supply to a Lab Power Supply Step 20

Step 8. Connect the white wire from the −5 V line to its terminal

This negative voltage line is only used on older models of ATX power supplies. Solder, if it exists, the white wire on the socket which corresponds to it. Take care to cover your weld with a piece of heat shrink tubing or insulating tape.

If you need a negative −5 V voltage, look for an ATX power supply that has a 20 pin output connector with a 4 point push-fit extension. A still good PC AT power supply will do the trick as well. A material that is too old could however bring you some surprises due to the aging of the electrolyte of its filter capacitors

Convert a Computer ATX Power Supply to a Lab Power Supply Step 21

Step 9. Connect the remaining red wires to the +5 V terminal

Strip all the red wires, group them together with a self-locking collar and twist their stripped ends together so that the whole thing only forms a large bare conductor. Tin the resulting section and solder it to the socket corresponding to the voltage +5 V. Take care to slip a piece of heat-shrink tubing of a sufficient diameter to cover the group of wires and the solder on the terminal of the socket.

If you only have three red wires, you may need to connect another cable to them, which will sometimes be pink in color

Convert a Computer ATX Power Supply to a Lab Power Supply Step 22

Step 10. Connect the yellow wires to the socket corresponding to +12 V

Strip all the yellow wires, group them together with a self-locking collar and twist their stripped ends together so that the whole thing only forms a large bare conductor. Tin the resulting section and solder it to the socket corresponding to the voltage +12 V. Take care to place a piece of heat-shrink tubing of sufficient diameter to cover the group of wires and the solder on the terminal.

Convert a Computer ATX Power Supply to a Lab Power Supply Step 23

Step 11. Connect the orange wires to the corresponding socket for 3.3 V

Strip all the orange-colored wires, group them with a self-locking collar and twist their stripped ends together so that the whole thing forms only a large bare conductor. Tin the resulting section and solder it to the socket corresponding to the voltage +3, 3 V. Take care to place a piece of heat-shrink tubing of sufficient diameter to cover the group of wires and the solder on the terminal.

  • Note that some power supplies may have a gray or brown colored wire whose role is to indicate whether the output voltages are correct. If you look at the circuit board, you might see the PWR-OK silkscreen print on the circuit board, where this wire comes out. Some power supplies also have a fairly thin orange or red wire that is coupled to the same colored cables at the connector on the motherboard. If this is the case, take care to leave it grouped with the orange or red wires as appropriate. Its role is to measure one of the output voltages, 3, 3 V or 5 V to ensure optimal regulation of the output voltages. If you don't connect it, your power supply will not work properly. If in doubt, check the lowest positive voltage supplied by your power supply, 3, 3 V or 5 V.
  • Some of the more recent power supplies are equipped with output voltage measurement leads. These should be left connected to the output wires to which they were connected before you disassembled the case. If the 3.3V output only has one or two orange colored wires, you should have a brown wire connected to one of them.
Convert a Computer ATX Power Supply to a Lab Power Supply Step 24

Step 12. Connect the available black wires to the ground socket

Strip all the black wires, group them together with a self-locking collar and twist their stripped ends together so that the whole thing only forms a large bare conductor. Tin the resulting section and solder it to the socket corresponding to the mass (0 V). Take care to place a piece of heat-shrink tubing of sufficient diameter to cover the group of wires and the solder on the terminal.

  • Check that your wires are not badly connected and that their soldering is correct by pulling lightly on them then cover the stripped parts with a piece of insulating tape to avoid any risk of short circuit.
  • Soldering a large bundle of eight or nine wires to an output socket is a very tedious task. To avoid this, the most suitable solution will be to keep three of these wires and cut those that will not be used at a short distance from the printed circuit and insulate the end. The three wires that you will connect to the ground terminal should be more than enough. Do the same for all the groups of wires bringing the other output voltages to their terminals if necessary.
Convert a Computer ATX Power Supply to a Lab Power Supply Step 25

Step 13. Test your diet

Connect the mains power cable to the back of your device and to a wall outlet. Turn on your power supply by activating the main switch on the back of the box if it exists and check that the green standby LED lights up then activate the power switch that you have added. You should see the power-on indication LED turn on. Using a multimeter that you have taken care to switch to the DC Volts position, measure each of the voltages present on the output sockets in relation to that of ground, being careful not to cause a short circuit. If the measured voltages are correct, your power supply is working like a charm.

Convert a Computer ATX Power Supply to a Lab Power Supply Step 26

Step 14. Reassemble the case

When you are sure that everything is working correctly, reassemble the box, taking care to put everything in place so that the wires cannot touch any internal component. Repeat your tests after reassembly.

  • By combining the outputs of your power supply, you can obtain various voltages:

    24 V between sockets +12 V and −12 V;

    17 V between sockets +5 V and −12 V;

    12 V between sockets +12 V and 0 V;

    10 V between sockets +5 V and −5 V;

    7 V between sockets +12 V and +5 V;

    5 V between sockets +5 V and 0 V.

    These voltages should be sufficient for most electrical tests you may need to perform. Pay attention to maximum flow rates when using your power supply; if the flow indicated for the −12 V terminal is 1 A and that allowed on the +12 V socket is 5 A, you will not be able to obtain an output current greater than 1 A at 24 V, because the line −12 V will not support it. You will be able to see the current limitations of each of the output voltages displayed on the nameplate of the box.

Advice

  • If you are not sure that the power supply is working properly, test the power supply in the computer it was in before retrieving it. Check to see if the computer turns on and its fan is spinning. Use a voltmeter to measure auxiliary voltages, such as those provided for disk drives. You should read there a voltage approaching 5 V between the black wire and the red one. A disassembled power supply from an old computer may appear to be in poor condition because it is not loaded on any of its outputs and its green activation wire is not connected to ground.
  • You can take advantage of the outlet hole of the box to place an automobile cigarette lighter socket to which you can connect devices operating in 12 V.
  • If the power supply is not working and no LEDs are on, check to see if its fan is spinning. If it works, you may have reversed the direction of connection of the indicator diodes, connecting the anode instead of its cathode and vice versa. In this case, open the box and reverse the green and gray wires connected to the LED, taking care to keep the 330 ohm limiting resistor on.
  • You can use the 12 V output of your power supply as a battery charger for your car. Be careful that the battery you want to charge is not completely empty, because the output current limiting circuit of your power supply will activate. If so, place a 10 ohm, 10 or 20 watt power resistor in series across the 12 V output of your power supply to avoid overloading it. Measure the voltage of the battery being charged and when it has reached ten volts, you can remove your resistance to continue the operation. This may allow you to restore a weakened battery due to the cold or if it is discharged because you left the traffic lights or the car stereo on the night before.
  • A power supply fan can be noisy; it was first designed to cool down a sometimes heavily loaded power supply quickly. You can stop it if you want to, but it's not recommended at all. You can work around the problem by cutting its 12 V power wire and soldering it to one of the 5 V output wires of your device. The fan will run more slowly and will be quieter. It will continue to cool the inside of the case, but much less efficiently than before. This solution will not be viable if you intend to draw high rates of current from your power supply. It is up to you to assess the situation and make your own decisions. You could also replace the original fan with a quieter model, which may also be more expensive.
  • To use equipment with high current requirements when starting, such as refrigerators for automobiles, you will need to connect a battery of sufficient capacity in parallel with the 12 V line in order to avoid triggering the safety circuits of your device. power when your refrigerator needs to restart a cooling cycle.

Warnings

  • Never touch the wires connected to the power supply primary capacitors. They are cylinders a few centimeters high wrapped in a thin plastic film, the metal of the upper part of which is sometimes exposed and often bears the symbol + Where TO. Solid electrolyte capacitors are a little shorter and have a larger diameter, but they are not covered with a plastic film. They hold a voltage charge like a battery would, but they discharge much faster. Even if you have allowed your power to discharge, you should avoid touching the circuit board except when necessary and only where you know you can. Use your multimeter's test leads and probes to unload the places you need to access.
  • Your new power supply is capable of delivering high power at its output, but it is only equipped with fairly basic overload protection. If you are not careful what you are doing, there may be times when you produce a spark at the low voltage outputs or even damage the equipment you are working on. It is for this reason that real laboratory power supplies are equipped with current limiting circuits adjustable from a few milliamps to several amps, regardless of the output voltage to be supplied.
  • To ensure the discharge of its low voltage filtering capacitors, plug your power supply into a wall outlet, turn on its power switch (or connect its green wire to ground). Now disconnect the power cable from the wall outlet and wait for the fan to stop.
  • Take care not to connect to earth when working on a power supply so as not to act as a conductor for possible leakage current.

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