The PSU and I

[Reloadron]

Based on several recent posts concerning the PSU and more so opening and modifying the PSU, I thought maybe a brief explanation of what is going on in there might be in order. Many computer enthusiasts know very little about the actual inner workings of the PSU and yet the PSU is really the heart of a system.

Manufacturers of Power Supply Units go through great lengths to make sure a PSU is pretty much enclosed. This is done for very good reasons. They also place little warnings on the PSU advising that it is unwise to open and tinker in a PSU and again for good reason. Just about all of the wires that exit a PSU into your computer are relatively harmless. They deliver low DC (Direct Current) voltages to the system. These voltages to name a few are 12, 5, and 3.3 Volts DC. There is nothing there that is going to hurt you. I am sure many of you have “tested” a simple 9 Volt battery by placing it on your tongue (something I don’t recommend) and felt the sting or tingle of a small electrical shock. However, placing your fingers across the same battery does nothing. This is just low voltage DC and certainly not enough to hurt you or cause any physical distress. So what’s in the box? What makes a PSU dangerous?

The PSU used in computers these days is what is known in Power Supply circles as a “Switching” power supply. The name comes from how it works and does its PSU thing. Power supplies are not unique to home computers; they have been around long before the computer was a gleam in anyone’s eye. They come in a variety of flavors designed for a variety of task. The switching type PSU is just one of many designs.

A switching type PSU as used in a home computer takes the AC line voltage from a source, like a wall outlet and converts it to the useable DC voltages used in your computer. Now how it does this depends to a point where you live. Those living in the US and countries like the US have a typical wall outlet delivering 120 Volts AC (Alternating Current) at a frequency of 60 Hz (Hertz). This just means an RMS (Root Mean Square) value of a sine wave voltage where the current flow is changing direction 60 times in a second. Everyday household current, the same voltage and current that light the lamps in your home and make other nice things work. Many of our friends across the pond have a household voltage of 240 VAC at 50 Hz. Either of these voltages very exposed inside the PSU are dangerous and in many cases can prove lethal. Read into that they will make you dead and ruin your day for good! Not to say getting bit by 120 or 240 VAC will kill you but in the case of a weak heart it has a good chance of doing just that with long enough contact as in not letting go real quick!

So what’s the big deal? Nobody is going to open a PSU when it is plugged in, just unplug the thing. Kill the power! Now it is 100% safe to open and work on. Right, we can assume this right? Nobody would open a PSU while it was plugged in! Nope, sometimes people do stupid things. However, even unplugged a PSU can present a clear and present danger. We will now look a little at the inner workings of a switching PSU.

The PSU takes the line voltage of either 120 or 240 VAC and immediately converts it to DC. That is the very first step in the process. There is a minor step here to look at because it is important. If the line voltage is 120 VAC the PSU will use a voltage doubling concept to create 240 VDC and if the incoming line voltage is 240 VAC the PSU will simply rectify it to 240 VDC. The bottom line being 240 VDC is derived from the incoming line voltage. That is the very first step. Some PSU units sense the line voltage and adjust accordingly and some PSU units have a rear panel switch where the user selects the line voltage. The bottom line here being we now have 240 VDC.

We are going to need to look at a little math here. I want to keep this short and sweet as I like simple and what is going on is not simple. I mentioned “RMS Value” earlier. The RMS value of a sine wave is .707 of the voltage peak of the waveform. This means 70.7% of the total voltage peak. Just hold that thought as we will see it real soon.

The 240 VDC is now fed to a few capacitors. Capacitors in their simplest terms oppose a voltage change. They take the bumps out and make a DC voltage “smooth” eliminating any AC ripple or garbage following rectification. Capacitors charge to a voltage level, that level being “E Peak” or the voltage peak not the RMS value. E peak or Voltage Peak is defined as the RMS value times the square root of 2 or simply stated E RMS X 1.414. Therefore we now have a DC potential of 240 X 1.414 = 340 Volts across the input capacitors. This is the ugly part. This section along with the input sections of the PSU is why the PSU is so enclosed. This section is the part that will most definitely place a hurt on you and your body! A typical PSU has a dozen or more capacitors. Probably 90% of these capacitors pose absolutely no threat to human life as we know it. However the two I mentioned pose a hell of a threat.

As we muddle into what happens next, we need to dispel a myth or two. Unplugging or simply turning the PSU off does not discharge these two capacitors. These capacitors are discharged by what I learned as a “Bleeder” resistor placed across them. I learned this as a bleeder resistor and at 57 years old am not about to quibble about terminology. It is a resistor placed across the capacitors affording a discharge path when power is removed. This is done by design. Bleeder resistors serve no other purpose other than to discharge the system capacitors. That is their job and purpose in life. The theory or belief that when the PSU fan stops turning the PSU is safe to open is pure nonsense. These capacitors have nothing to do with the output of the PSU. They are on the primary side of the main PSU transformer(s), and they have nothing to do with the output side of the PSU. Do not for a single moment believe that when the fans stop the PSU is dead. The PSU is only dead when unplugged and the voltage across those primary side capacitors is bled to a level not harmful to life is reached.

So what about these bleeder resistors and how long before I can get my hot little fingers in there? Hard to say how long before things are safe. Rumor has it the Romans charged capacitors that to this day hold a charge capable of toasting a person, so think long and hard.

OK, just kidding but it sounded good. The time it takes is determined by what is known as a RC Time Constant where Time is equal to R X C. We multiply the Resistance times the Capacitance to derive a number representing time. Without getting into charts and graphs we can assume a RC time constant times 5 will render a safe system to open and work on. Typically these capacitors have a value of 1,000 to 2,000 uF (Micro Farads) and the bleeder is about 220,000 Ohms. Without beating the math to death and trying to teach EL-101 we can assume after about 5 to 10 seconds things should be dead and the PSU is safe to open, assuming it is unplugged from the wall. This at this point also assumes the presence of a bleeder resistor in the PSU, also a working bleeder resistor. Remember ASSUME = Ass U Me as assumption can be a very bad thing. I assumed the power was off, I assumed things were safe, and the list goes on. Get the idea?

So what to do at this point? The point is that unless you are familiar with the inner workings of a PSU and how it does what it does it is best not to overly screw with it. The point is if you have this deep burning desire to modify a PSU in any way it is best to wait quite some time before opening it once power has been removed. Quite some time may be defined as overnight or a few minuets depending on how comfortable one is inside a PSU. There is no simple answer.

Should anyone have questions concerning the PSU and how it does what it does beyond this simple safety posting please do ask. I will be more than happy to answer what I can. I did not want to get into the theory aspect as much as the safety aspect.

[Beetle Bailey]

Very nice write up. Hopefully some people will read this and not show up to work the next morning with charred fingers. Thanks again, Bailey.