Just thought I'd post this, from what I remember when I followed the Open College electronics course. This is almost like a FAQ page, explaining how exactly a PSU works - for those new to the subject, and those wanting to learn about electronics.
As you know, the electricity coming in from the mains is high-voltage AC (alternating current), moving first one way and then the other. Computer motherboards, CPUs and other peripherals only work with low-voltage DC (direct current), moving one way all the time. The role of the PSU is to convert the 240v 50hz (or 120v 60hz) AC mains into a stable DC supply.
The AC first goes into a transformer, which transforms the 240v/120v AC into a lower voltage AC supply, such as 12v. Transformers rely on moving magnetic fields to do their job, so they will ONLY work with an AC supply.
Now we have low-voltage AC, we need to make it into DC. The simplest way of converting AC to DC is to use a diode, effectively a one-way gate for electricity. The electricity can flow through the diode one way, but is blocked when it tries to move the other way. Using a single diode in this way, you get the electricity only going in one direction - but you are only using half of the AC waveform, so it's called half-wave rectification.
You need to add a second diode pointing the other way, but connected to the same DC line, to use the other half of the AC waveform. Doing this, you are using the full power provided by the AC waveform, so it's called full-wave rectification.
In practice, 4 diodes are usually used - two to allow the electricity in and out for the positive half of the AC cycle, and two for the negative half of the AC cycle.
We now have DC, but if you look at it on an oscilloscope you'll see that it's a series of positive voltage pulses. Computer components need a smooth DC supply (straight line on the scope), not a series of pulses.
This is where capacitors come in. A capacitor in a PSU will charge and discharge rapidly to smooth out the DC current. In effect, it's storing up charge during the voltage peaks, and putting it back into the circuit during the troughs. The result is a smooth, DC supply that can be used by most electrical appliances.
However, the output of the capacitor segment is still not smooth enough to be used by certain sensitive devices, such as computer CPUs. If you look at the trace on an oscilloscope, you will see that the line still has 'wrinkles' in it, since the capacitors can't fully smooth out the supply.
The next stage is to use Zener diodes. These are just the same as normal diodes in that they allow current to flow in one direction, but not another. However, that's only true up to a certain critical value. Once the voltage reaches that value, the Zener will start allowing a small current to flow in the 'wrong' direction. The bigger the reverse push, the more current the Zener passes through.
The practical upshot of this is that if you place a Zener across the positive and negative lines coming out of the capacitor segment, in reverse polarity, the voltage across the Zener will remain constant - no matter how the power supply output varies. Therefore, you take the power output not from the capacitor segment, but from across the Zener diode itself. This should, in theory, give you stabilized, constant, DC power for sensitive components.
Zener diodes are available in a wide range of voltage ratings, and are commonly found in PSUs in the form of a voltage regulator. This is a little black box containing a Zener diode, and also some internal circuitry that protects it against short circuits and overloads.
There is one disadvantage of this though - the output voltage is only stabilized as long as the voltage rating of the Zener is greater than the output of the power supply itself. If the load gets too great, and the power supply output dips BELOW the voltage rating of the Zener, then the Zener will go back to being a normal diode (stopping all current), and the output voltage is no longer stabilized.
Experience has shown that the PSU output voltage is rarely a clear, stable trace on the oscilloscope - even the best PSUs have what's called 'ripple current'. Also, if the PSU capacitors fail, the Zener will no longer be able to stabilize the fluctuating voltage, and you'll get 'bad cap' symptoms even if your board capacitors are fine.
That's in a nutshell how a normal PSU works. If you know where they are, you can measure the outputs of the various stages of the PSU on your own oscilloscope.
As you know, the electricity coming in from the mains is high-voltage AC (alternating current), moving first one way and then the other. Computer motherboards, CPUs and other peripherals only work with low-voltage DC (direct current), moving one way all the time. The role of the PSU is to convert the 240v 50hz (or 120v 60hz) AC mains into a stable DC supply.
The AC first goes into a transformer, which transforms the 240v/120v AC into a lower voltage AC supply, such as 12v. Transformers rely on moving magnetic fields to do their job, so they will ONLY work with an AC supply.
Now we have low-voltage AC, we need to make it into DC. The simplest way of converting AC to DC is to use a diode, effectively a one-way gate for electricity. The electricity can flow through the diode one way, but is blocked when it tries to move the other way. Using a single diode in this way, you get the electricity only going in one direction - but you are only using half of the AC waveform, so it's called half-wave rectification.
You need to add a second diode pointing the other way, but connected to the same DC line, to use the other half of the AC waveform. Doing this, you are using the full power provided by the AC waveform, so it's called full-wave rectification.
In practice, 4 diodes are usually used - two to allow the electricity in and out for the positive half of the AC cycle, and two for the negative half of the AC cycle.
We now have DC, but if you look at it on an oscilloscope you'll see that it's a series of positive voltage pulses. Computer components need a smooth DC supply (straight line on the scope), not a series of pulses.
This is where capacitors come in. A capacitor in a PSU will charge and discharge rapidly to smooth out the DC current. In effect, it's storing up charge during the voltage peaks, and putting it back into the circuit during the troughs. The result is a smooth, DC supply that can be used by most electrical appliances.
However, the output of the capacitor segment is still not smooth enough to be used by certain sensitive devices, such as computer CPUs. If you look at the trace on an oscilloscope, you will see that the line still has 'wrinkles' in it, since the capacitors can't fully smooth out the supply.
The next stage is to use Zener diodes. These are just the same as normal diodes in that they allow current to flow in one direction, but not another. However, that's only true up to a certain critical value. Once the voltage reaches that value, the Zener will start allowing a small current to flow in the 'wrong' direction. The bigger the reverse push, the more current the Zener passes through.
The practical upshot of this is that if you place a Zener across the positive and negative lines coming out of the capacitor segment, in reverse polarity, the voltage across the Zener will remain constant - no matter how the power supply output varies. Therefore, you take the power output not from the capacitor segment, but from across the Zener diode itself. This should, in theory, give you stabilized, constant, DC power for sensitive components.
Zener diodes are available in a wide range of voltage ratings, and are commonly found in PSUs in the form of a voltage regulator. This is a little black box containing a Zener diode, and also some internal circuitry that protects it against short circuits and overloads.
There is one disadvantage of this though - the output voltage is only stabilized as long as the voltage rating of the Zener is greater than the output of the power supply itself. If the load gets too great, and the power supply output dips BELOW the voltage rating of the Zener, then the Zener will go back to being a normal diode (stopping all current), and the output voltage is no longer stabilized.
Experience has shown that the PSU output voltage is rarely a clear, stable trace on the oscilloscope - even the best PSUs have what's called 'ripple current'. Also, if the PSU capacitors fail, the Zener will no longer be able to stabilize the fluctuating voltage, and you'll get 'bad cap' symptoms even if your board capacitors are fine.
That's in a nutshell how a normal PSU works. If you know where they are, you can measure the outputs of the various stages of the PSU on your own oscilloscope.
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