In a Previous Video: Converting an ATX Power Supply into a Lab Bench Power Supply

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The previous video showed you how to convert a common ATX power supply into a crude lab bench power supply, which features a 12-volt, 5-volt, and 3.3-volt output. Such a supply is definitely a decent start when it comes to testing your electronic circuits. However, it comes nowhere close to the convenience of a proper lab bench power supply with a variable voltage output.

Of course, you could add a buck-boost converter to the outputs of the ATX power supply and thus create a variable voltage output as well. However, this would require additional space inside the supply housing, which is not desirable. Therefore, there must be another solution to this problem.

That's why in this video, we will have a closer look at the circuit of a switched-mode power supply and find out whether it makes sense to modify it to achieve a variable output voltage as well.

Let's Get Started

First off, I removed all the case screws from my ATX power supply test subject and removed the top cover of the housing. At first glance, the circuit looks very big and confusing. However, when we consider the functional principle of a switch-mode power supply, it becomes much easier to understand.

On the left side, we have our 230-volt AC mains voltage, which firstly gets rectified by a full-bridge rectifier. And then smoothed out by capacitors in order to create a DC voltage of around 325 volts. This DC voltage is then switched on and off rapidly by some kind of transistor, thus creating a PWM signal with a high frequency of around 100 Kilo Hertz.

Next, this high-voltage high-frequency PWM voltage is connected to the primary side of a transformer, which therefore induces a low-voltage high-frequency PWM voltage into the secondary side. Last but not least, this voltage gets once again smoothed out by capacitors in order to create the initially targeted 12-volt DC.

Now we can check this output voltage by connecting the green wire of our supply with a black wire. Plugging in the power and measuring the voltage between a yellow and black wire. As you can see here, the voltage is around eleven point three five volts. And if we connect different loads to this 12-volt output, we can see that the voltage stays pretty much constant.

No matter how much current we draw from it. That means that if we go back to the switching transistor part of the circuit, there has to be some kind of feedback from the 12-volt output. Which tells the transistor to increase the duty cycle of the PWM signal if we draw more current. Because we need more energy on the output side to sustain the 12 volts.

So let's remove the circuit board from the housing and have a closer look at the ICs to find out which one was responsible for the PWM signal. The two ones in the top left corner were LM358 and thus operational amplifiers. While the third one in the same corner was a TPS3510, and thus a PC power supply protection IC.

The last IC was a UC3842, which according to its datasheet seems to be the IC we've been looking for. Sadly though, there was no direct feedback connection between 12-volt output and the pins of the IC. Instead, the 12-volt output was connected to an optocoupler which on the other side connected to pin 2 of the IC. This is a voltage feedback pin.

While measuring the voltage of this pin while the supply was powered on, I asserted a voltage of 2.5 volts. In conclusion that means that there exists a kind of voltage divider between the 12-volt output and the voltage feedback pin. Whose resistor values are chosen in a way that the 12 volts create a 2.5-voltage drop at the voltage feedback pin.

That means that by varying the resistor values, the feedback pin will regulate the duty cycles so that the output voltage will always create the 2.5 volts at the feedback pin and thus we can achieve a variable output voltage. So I had a closer look at the passive components around the IC and noticed a suspicious 10 kilo ohm SMD resistor.

After removing it, I soldered the two wires to a 50 kilo ohm potentiometer and soldered it to the pads of the SMD resistor. This would allow us to vary the voltage divider and thus achieve a variable output voltage.

Due to my sheer luck, this worked out quite well. So let's test the result by connecting different loads to the supply and measure the output voltage.

WEBVTTKind: captionsLanguage: enIn a previous video I showed you how to convert a common ATX power supply into a crude lab bench power supplyThat features a 12 volt 5 volt and 3.3 volt outputSuch a supply is definitely a decent start when it comes to testing your electronic circuitsBut it comes nowhere close to the convenience of a proper lab bench power supply with a variable voltage outputNow of course you could add a buck boost converter to the outputs of the atx power supply andand thus create a variable voltage output as wellOnly problem is that the converter takes up nonexistent additional space inside the supply housingSo there must be another solution to this problemThat is why in this videowe will have a closer look at the circuit of such a switched-mode power supply andfind out whether it makes sense to modify it in order to achieve a variable output voltage as wellLet's get startedFirst offI removed all the case screws from my atx power supply test subject and removed the top cover of the housingat first glance the circuit looks very big and confusingBut when we consider the functional principle of a switch mode power supply it becomes much easier to understandon the left side we got our230 volts Ac mains voltage which firstly gets rectified by full bridge rectifierAnd then smoothed out by capacitors in order to create a DC Voltage of around 325 voltsThis DC voltage is then switched on and off rapidly by some kind of transistor and Thus aPWM signal is created with a high frequency of around 100 Kilo HertzNext this high-voltage high-frequency PWM voltage is connected to the primary side of a transformerwhich therefore induces a low voltage high frequency PWM voltageinto the secondary sideLast but not least this voltage gets once again smoothed out by capacitors in order to create the initially targeted 12 volt DCNow we can check this output voltage by connecting the green wire of our supply with a black wirePlugging in the power and measuring the voltage between a yellow and black wireAs you can see here the voltage is around eleven point three five voltsand if we connect different loads to this 12 volt output we can see that the voltage stays pretty much constantNo matter how much current we draw from itThat means that if we go back to the switching transistor part of the circuitThere has to be some kind of feedback from the 12-volt outputWhich tells the transistor to increase the duty cycle of the PWM signal if we draw more currentBecause we need more energy on the output side to sustain the 12 voltsSo let's remove the circuit board from the housing and have a closer look at the ICs to find out which one was responsible forthe PWM signalThe two ones in the top left corner were LM358 and thus operational amplifiersWhile the third one in the same corner was a TPS3510and thus a PC power supply protection ICThe last IC was a UC3842Which according to its datasheet seems to be the IC we've been looking forSadly though there was no direct feedback connection between 12 volt output and the pins of the ICInstead the 12 volt output was connected to an optocoupler which on the other side connected to pin 2 of the ICAKA a the voltage feedback pin and while measuring the voltage of this pin while the supply was powered onI asserted a voltage of 2.5 voltsIn conclusion that means that there exists a kind of voltage divider between the 12 volt outputand the voltage feedback pinWhose resistor values are chosen in a way that the 12 volts create a 2.5 voltage drop at the voltage feedback pinThat means that by varying the resistor values the feedback pin will regulate the duty cycleso that the output voltage will always create the 2.5 volts at the feedback pinAnd thus we can achieve a variable output voltageSo I had a closer look at the passive components around the IC and noticed a suspicious 10 kilo ohm SMD resistorafter removing itI soldered the two wires to a 50 kilo ohm potentiometer and soldered it to the pads of the SMD resistorThen I set the potentiometer value to 10 kilo ohm turned on the supply and checked whether the 12 volt output still functioned correctlyWhich it did and by slowly varying the potentiometer value the output voltage did change accordinglyBut only upwards and only to a certain degreeSince the over voltage protection of the supply was still activeLuckily though another optocoupler close to the PWM IC was directly connected to pin 3 of the PCPower supply protection IC which was the full protection output pin whose job was to trigger the over voltage protectionTo get rid of this protection featureI simply shorted two pins of the optocoupler which resulted in a successful voltage adjustment up to 16 voltsWhich was the maximumSince the output capacitors were only rated for up to this voltageThe only remaining problem was that I was not able to lower the voltage beneath 12 voltsSo I unsoldered a second suspicious resistor and replaced it once again with a potentiometerThis time a 100 kilo ohm oneAnd due to my sheer "luck" the atx power supply was done for after this modificationWhich almost led me to give up on this modification attemptBut thankfully I had a backup supply laying around whose overall circuits looked a bit simplerand best of all its PWM IC the KA7500was directly connected to the 12 volt outputs through its pin 1That means all I had to do was to remove this one pin from the circuits prepare a new potentiometer with three wiresSolder its right wire to the 12 volt output the left wire to the ground outputand the middle wire to pin 1 of the ICNow by powering the supply I was capable of varying the output voltage easily between 3 volts and 12 voltsbut it seems like an over-voltage protection was still activeBut nevertheless after adding two more resistors to adjust the output voltage more fluently andAdding a proper load to the outputI put a stop to this modificationAs you can see it does workbut while tinkering on such a mains voltage circuit that has the potential to kill a personand constantly trying to disable protection features I realized that this is not the way to go if you want a variable power supplySo I do not recommend to replicate this modificationInstead buy yourself a beginner's lab bench power supplyThey are not that expensiveAnyway, I hope you enjoyed watching this modification attempt. Don't forget to like share and subscribeStay creative and I will see you next time