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.


Upgraded version of the ATX psu power supply

After using it for quite sometimes to power makita 12v UR100 grass trimmer - the ATX PSU power supply has been upgraded to a more versatile version. This time all the 3.3v, 5v, and 12v power are utilize so it has the capability to power multiple DC electronic devices such as the following;

1. Powertools (All 12v powertools from bosch, makita, dewalt, milwaukee, metabo, aeg, worx, ridgit, ryobi) such as grass trimmer, drills, rotary tools etc. Makita grass trimmer usually consume around 10 amp without load and 14A to 16A 

2. To power car 12v vacuum cleaner - usually this device consume around 7 amp of current or 84 watts.

3.  As Bench power supply to power DC motor ranging from 3.3v, 5v to 12v for electronic project

4. For charging gadget devices such as iPad, Samsung Galaxy phone, Galaxy tab, iPhone, Huawei, vivo, oppo, xioami. Usually standard charging for gadget devices around 400mA to 1000mA

This is really the perfect and cheapest built you can have the entire project probably cost less than $20 since the power supply itself only cost $10 and the rest of the components such as banana plugs, rotary switch, volt / amp meter, binding post probably cost less than $10 overall.

How to make ATX PSU as bench power supply.

Materials that you will need to make the power supply

1. Plastic bucket (optional) you could actually mill holes on the casing of the power supply just like most people did but only if you have the tools and time to do it. As for me I don't want to make more hassle doing that and reduce the work load by using a plastic bucket which you can mill with just a knife. Beside the inside casing of the PSU is quite compact and very little space left for additional components. 

2. Banana plugs and binding post for the terminal connection. Banana plug is much easier and convenient to use especially for DC devices for testing purpose. Just plug and use.

3. Rotary switch - to toggle voltage measure between the 3 different voltages 3.3v, 5v, and 12v

4. Volt / Amp meter - to measure voltage and current. This is optional component and not necessary if you just need the power supply itself. However having the amp meter can be valuable as you could measure the amperage of power being consumed by different electrical devices especially in testing.

5. Cigarette lighter 12v female socket - Most people will use USB female port but I think 12v cigarette is more versatile and flexible. Beside the USB port is very messy and shaky to install on the casing later when  applying glue stick on it. Using the cigarette lighter female socket you could power 12v and use the cigarette lighter step down USB plug.

Tools to use in this project

1. Drills to mill holes for the binding posts, rotary switch and the 12v cigarette light female socket. 

2. Knife to mill hole for the volt / am meter. 

3. Wire stripper for fast and precise wire stripping

4. Solder iron / solder coil

5. Heat shrink tube to insulate exposed solder joints - prevent from accidental short circuit. And of course the lighter to shrink the tube.

6. Wire cutter

7. Rotary tool (optional) to cut metal for the ground rail. I prefer this way to make it easier for the ground connection because it is soldered 1 by 1 kind of tedious job therefore by using a rail all you need is solder all ground wire to the rail and you could easily install it using the binding post screw.

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