Friday, January 8, 2021

How to use ATX computer bench power supply to run power tools like 12 volt drill

If you are fed up of charging your 12 volt power tools batteries - perhaps its time for a change and the good news is there is a very cheap alternative to turn your cordless drill to corded version. This also can be done for grass trimmer or other tools that runs on 12 volt lithium batteries.

In this project we gonna use the 500 Watt ATX computer bench power supply or PSU and it has an input power of 180 volts ~ 240 volts AC with 8 Amp of current and 12 volt output 22 Amp of current. That's a lot of power more than enough to runs the 12 volt power tools from all range of brand with the like of Makita, Bosch, Hitachi, Dewalt, Milwaukee, Worx, AEG. You could get this cheap especially the one from China cost me only $10 for such a powerful PSU.

How to turn on the ATX power supply without motherboard? 

There is a trick and usually it is written on the label of the psu which indicates PS-ON (power supply on). Commonly its the green wire for most ATX power supply and you have to connect it to the ground wire usually black color. Once the PS-ON (geen wire) connect with the ground (Black wire) you can turn on the power supply and it will run normally. On the computer motherboards they already created electric path that connects between the two. See below

Now you have the power supply turns on - the next step is to open the power tools battery casing and cut off all the battery connections to the circuit board of the battery but don't remove the battery from the casing as it is useful to hold all the components as it is inside the battery casing. Then solder thick wire on the positive and negative terminal of the battery circuit board and drill out a hole for the wire to extend outside the battery casing.

After the battery wiring is done you can start to test the power supply - it might works sometimes but it will automatically shut down at times and you need to keep on unplug and plug in to turn on the power supply again.

Why the ATX power supply automatically shut off when use on high current power tools drills or grass trimmer?

It's because the ATX psu has a circuit protection unit that automatically turns off when it draws more voltage or current than what is set. The protection covered over current and over voltage to protect all its component from power surge that could damage the circuits. However to use power tools such as drill or grass trimmer we need that surge current to run especially during the initial starts when it needs more amp to kick the motor to spin. 

How to remove the over current / over voltage protection from the ATX power supply?

I will discuss this specifically for KA7500, SDC7500, TL494 based IC because they are the same thing from different manufacturers. There are other ICs that might have different pins configuration you have to find out from the datasheet. 

Before proceeds any further please take note of the safety precautions to ensure prevent damages and physical injuries to yourself. 

  1. Do not attempt this modification if you are unsure
  2. Do not proceed if  you don't have the required tools otherwise you might hurts yourself
  3. Use safety protection equipment i.e. eye-wear, hand gloves to avoid injuries.

Tools you need:

  1. Philips screwdrivers - to open the psu casing
  2. Extra long wires - so you can power the tools at long distance from the power source
  3. Wire connectors - you need a good wire connectors because you are using this psu for high powered tools where it will get heated up very quickly the connectors will melt if you use cheap and weak connectors
  4. Wire strippers - optional (for fast and easy wire stripping)
  5. Wire cutter
  6. Solder + solder coil + flux (optional) + solder stand
  7. Multimeter (with continuity measure) - this is the most important device you need
  8. Sharp tools to remove electrical path on the circuit board

Precautionary measures:

  1.  After you have open the PSU unit - use the continuity test of your multimeter to check all the connection nearby that you will solder especially pin 4 and ground. Take note of which connection are connected and disconnected. This is to prevent accidental joint later that will cause the electricity to stray out of control - it's very dangerous and your multimeter will make sure you are 100% correct.

The modification process on ATX power supply circuit protection

To disable the circuit protection very simple what we need is to join the pin 4 (DT) to the ground connection and then cut the original circuit protection that joint to pin 4. To join pin 4 to the ground use 3.6 kOhm resistor see the video. Remember the ground pin as shown in this picture is on pin 7 but its located on crowded places nearby other components. 


Ground pin are actually located on many places you could use continuity test to trace where are the other ground pin by make reference to the pin 7. Take one of your multimeter probe place it on the pin 7 and then use the other probe to find where it is connected - when it beeps meaning that is direct connection aka same ground pin. Find the one which is space far apart to one another that make it easier to solder later on. In my case i found pin 7 directly connected to pin 9 and 10 so i will join the pin 4 with 3.6 kOhm resistor to the pin 9 and 10 instead of 7 to make it easier to solder.

Soldering the resistor on pin 4 is tough but you have no choice to do it and that's where you need to multimeter continuity test. Once you have soldered the resistor on pin 4 use the continnuity test to ensure no accidental direct connection to other components nearby. This help you to be 100% sure you are do it right without any mistakes. 

Once you have joined the pin 4 to 9 and 10 then cut the original electrical path that join pin 4 to other circuit on the board. Use sharp object to scrap the electrical path and make sure don't damage the others nearby. 

Another thing to take note is the resistor try to cut the legs as enough as possible because you don't want the extra length of the legs later accidentally cause short circuit with components nearby. Once it is done then you are good to go. However before reassemble everything back - try to test continuity to make sure what you have soldered done perfectly and no accidental stray connection occur. 

If everything is good it's ready to close the casing and test the PSU on your power drills or grass trimmer. It should work and no more auto shut down - you can also feel the gush of power to the drills.

ATX power supply cannot turn on without motherboard | SMPS power supply | Switching power supply | turn cordless drill to corded | bench power supply for cordless drill

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