Power supply theory can be broken into two major parts, transformers and regulation. This first part will address transformers and then rectification and regulation. I should also mention I’m an Electrical Engineer and design power supply and transformer circuits on a regular basis, but I’m not a physicist.
Transformers are interesting devices as they operate in a weird area of electronics. They work by transferring energy from the primary side to the secondary side through magnetic flux or EMF. Flux is created when a current is passed through a conductor. Transformers only work with alternating current. The discussion of why is probably a separate posting by itself. As the alternating current passes through the primary windings, the magnetic flux increases and decreases in density too. This means the actual amount of magnetic field is always changing. The denser the flux, the more energy is available to be transferred. There is a limit to how much flux can be produced and stored in a transformer, and this limit is called saturation. These ideas get important later.
Transformers have two basic ratings. One we are all familiar with, and that is the input vs. output voltage. The ones we use are typically rated as 120v AC in (primary), and something like 6.3, 12.6, 18 or 24 volts out (secondary). These are called step down transformers. The ratio of input to output is the same as the turns ratio. Turns are the number of time the wire is wrapped around the core of the transformer. To keep the math easy, if you have 100 turns on the primary and 10 turns on the secondary, the output voltage will be 1/10 the input. If you put 120 volts into our example transformer, you would get 12 volts out. Many pinball transformers have multiple secondary windings sharing a single primary winding and metal core, saving space and weight. The turns math is the same for any given primary to secondary ratio being measured.
The second rating that is not commonly known is the power handling capability of the transformer. They are rated in Volt*Amps, or volts per amp. This is also one way Ohm’s Law calculates power. Simply put, this is how many amps a transformer can supply at a given output voltage. For example, a transformer rated at 24 volt secondary output and 48 VA can support a load of 2 amps at 24 volts AC. The manufacturer is guaranteeing the output voltage will not go below 24 volts AC at the 2 amp load. They are not guaranteeing how much higher than 24 volts it will be under any load conditions however. This also gets important later.
Now we can see why the input voltage to a power supply circuit is not always what the schematic says it should be. Let’s take a Bally solid state transformer board as an example, say the 12 volts that supply the input to the 5 volt regulator circuit on the solenoid board. Bally does not show the VA rating on the schematic, but we can guess at it based on the size of the fuse for that output. On my Lost World schematic, it is a 4 amp fuse. For our purposes, we’ll say the transformer is 12 v @ 4 amps for a rating of 48VA. You could also say it can supply a maximum of 48 Watts of power. This means the flux density of the transformer can transfer at least that much energy at a maximum. Flux density is created by the thickness of the core, the material it is made out of, the frequency of the AC current and other factors. Since the three most important; material, thickness and frequency cannot be changed once the game is built, the transformer must be able to create the maximum flux all the time.
The following example is a simplification of windings vs. inductance vs. load, but it’s close enough for this discussion. If the load on the secondary is only 2 amps, the unused energy (flux) reflects back into the primary windings. This makes the input voltage appear to go up. This means the secondary voltage will also go up. Most transformers we will see in this hobby have the same behavior. As the load from the game drops, the output voltage of the transformer will increase. If you unplug all outputs from the transformer board and measure the secondary connections at E11 and E12, you could see as much as 18 volts AC. And if you increase the load on the secondary, the output voltage will drop by some amount. If you apply a large enough load (like a short) beyond the rating of the transformer, the output voltage will drop to almost zero.
In summary, the schematics and voltages we work with should be considered guidelines more than absolutes when it comes to transformers. If the voltages are higher than you expect, maybe the circuit load is open. If it’s too low, maybe there is a high load or a short circuit.
Rectification and regulation of linear power supply circuits is next.
Transformers are interesting devices as they operate in a weird area of electronics. They work by transferring energy from the primary side to the secondary side through magnetic flux or EMF. Flux is created when a current is passed through a conductor. Transformers only work with alternating current. The discussion of why is probably a separate posting by itself. As the alternating current passes through the primary windings, the magnetic flux increases and decreases in density too. This means the actual amount of magnetic field is always changing. The denser the flux, the more energy is available to be transferred. There is a limit to how much flux can be produced and stored in a transformer, and this limit is called saturation. These ideas get important later.
Transformers have two basic ratings. One we are all familiar with, and that is the input vs. output voltage. The ones we use are typically rated as 120v AC in (primary), and something like 6.3, 12.6, 18 or 24 volts out (secondary). These are called step down transformers. The ratio of input to output is the same as the turns ratio. Turns are the number of time the wire is wrapped around the core of the transformer. To keep the math easy, if you have 100 turns on the primary and 10 turns on the secondary, the output voltage will be 1/10 the input. If you put 120 volts into our example transformer, you would get 12 volts out. Many pinball transformers have multiple secondary windings sharing a single primary winding and metal core, saving space and weight. The turns math is the same for any given primary to secondary ratio being measured.
The second rating that is not commonly known is the power handling capability of the transformer. They are rated in Volt*Amps, or volts per amp. This is also one way Ohm’s Law calculates power. Simply put, this is how many amps a transformer can supply at a given output voltage. For example, a transformer rated at 24 volt secondary output and 48 VA can support a load of 2 amps at 24 volts AC. The manufacturer is guaranteeing the output voltage will not go below 24 volts AC at the 2 amp load. They are not guaranteeing how much higher than 24 volts it will be under any load conditions however. This also gets important later.
Now we can see why the input voltage to a power supply circuit is not always what the schematic says it should be. Let’s take a Bally solid state transformer board as an example, say the 12 volts that supply the input to the 5 volt regulator circuit on the solenoid board. Bally does not show the VA rating on the schematic, but we can guess at it based on the size of the fuse for that output. On my Lost World schematic, it is a 4 amp fuse. For our purposes, we’ll say the transformer is 12 v @ 4 amps for a rating of 48VA. You could also say it can supply a maximum of 48 Watts of power. This means the flux density of the transformer can transfer at least that much energy at a maximum. Flux density is created by the thickness of the core, the material it is made out of, the frequency of the AC current and other factors. Since the three most important; material, thickness and frequency cannot be changed once the game is built, the transformer must be able to create the maximum flux all the time.
The following example is a simplification of windings vs. inductance vs. load, but it’s close enough for this discussion. If the load on the secondary is only 2 amps, the unused energy (flux) reflects back into the primary windings. This makes the input voltage appear to go up. This means the secondary voltage will also go up. Most transformers we will see in this hobby have the same behavior. As the load from the game drops, the output voltage of the transformer will increase. If you unplug all outputs from the transformer board and measure the secondary connections at E11 and E12, you could see as much as 18 volts AC. And if you increase the load on the secondary, the output voltage will drop by some amount. If you apply a large enough load (like a short) beyond the rating of the transformer, the output voltage will drop to almost zero.
In summary, the schematics and voltages we work with should be considered guidelines more than absolutes when it comes to transformers. If the voltages are higher than you expect, maybe the circuit load is open. If it’s too low, maybe there is a high load or a short circuit.
Rectification and regulation of linear power supply circuits is next.
Rectification
The next topic for power supplies is rectification. This is most commonly done using diodes. Most people think of a diode as a switch or one way valve. For the most part this is close enough. Diodes are made of different types of materials and have slightly different characteristics. Silicon and Germanium are the most common, but other types are also used. They are also considered non-linear devices, and we’ll cover that a bit later on.
Most power supplies use silicon diodes. They have two leads, an anode and a cathode, with a depletion zone in between. The depletion zone has no electrons (-) or holes (+) and is electrically neutral, hence the name depletion zone. It is also the key to understanding how a diode works. The anode is made of mostly P type or positive material, and the cathode is mostly N type or negative material. In order for a current to flow, it must overcome the electrical resistance of the depletion zone. This is done by applying a voltage with the correct polarity to the diode. This is sometimes called the forward or biasing voltage. When you apply a voltage so the anode is more positive than the cathode, you add positive charge to the P type material and negative charge to the N type. Once the voltage gets large enough, it will overcome the resistance of the depletion zone and current will start to flow. They are considered non-linear because the amount of voltage it takes to ‘turn on’ a diode varies with the load current and temperature. For most silicon diodes, that voltage is around .7 volts, and we use that for doing circuit analysis in most cases.
If you reverse the voltage, the depletion zone actually gets larger. Applying a positive voltage to the cathode (and negative to the anode) draws the negative charged electrons away from the depletion zone, making it less able to conduct electricity. If you apply enough voltage though, you will eventually overcome this effect and the diode will conduct backwards. This is called the breakdown voltage. For example, the 400 volt rating of a 1N4004 diode means the manufacturer is guaranteeing the diode will withstand up to 400 volts of reverse voltage. Keep in mind this is a maximum voltage, and 400 volts AC has a peak value of 400*1.414 for a true voltage of 565 volts!
If a normal diode does go into reverse conduction, it is usually destroyed, since it not designed to work in that way. Specialty diodes called Zener diodes are intended to work backwards, but that is a topic for yet another tech blog.
The next topic for power supplies is rectification. This is most commonly done using diodes. Most people think of a diode as a switch or one way valve. For the most part this is close enough. Diodes are made of different types of materials and have slightly different characteristics. Silicon and Germanium are the most common, but other types are also used. They are also considered non-linear devices, and we’ll cover that a bit later on.
Most power supplies use silicon diodes. They have two leads, an anode and a cathode, with a depletion zone in between. The depletion zone has no electrons (-) or holes (+) and is electrically neutral, hence the name depletion zone. It is also the key to understanding how a diode works. The anode is made of mostly P type or positive material, and the cathode is mostly N type or negative material. In order for a current to flow, it must overcome the electrical resistance of the depletion zone. This is done by applying a voltage with the correct polarity to the diode. This is sometimes called the forward or biasing voltage. When you apply a voltage so the anode is more positive than the cathode, you add positive charge to the P type material and negative charge to the N type. Once the voltage gets large enough, it will overcome the resistance of the depletion zone and current will start to flow. They are considered non-linear because the amount of voltage it takes to ‘turn on’ a diode varies with the load current and temperature. For most silicon diodes, that voltage is around .7 volts, and we use that for doing circuit analysis in most cases.
If you reverse the voltage, the depletion zone actually gets larger. Applying a positive voltage to the cathode (and negative to the anode) draws the negative charged electrons away from the depletion zone, making it less able to conduct electricity. If you apply enough voltage though, you will eventually overcome this effect and the diode will conduct backwards. This is called the breakdown voltage. For example, the 400 volt rating of a 1N4004 diode means the manufacturer is guaranteeing the diode will withstand up to 400 volts of reverse voltage. Keep in mind this is a maximum voltage, and 400 volts AC has a peak value of 400*1.414 for a true voltage of 565 volts!
If a normal diode does go into reverse conduction, it is usually destroyed, since it not designed to work in that way. Specialty diodes called Zener diodes are intended to work backwards, but that is a topic for yet another tech blog.
There are two common methods of using diodes to rectify voltages, half wave and full wave. An AC signal does not have a constant value, it varies with time. Most folks have seen a graph of an AC waveform, but we’ll include one here as a starting point.
The voltage is relative to a reference point called neutral, sometimes incorrectly called ground as well. It is positive half the time and negative half the time. A half wave rectifier does just like it sounds, it rectifies half the AC wave. A picture of what the wave looks like is below.
A full wave rectifier will make both halves of the AC wave positive. It looks like the picture below.
The quick learner will notice in all three pictures the voltage is still changing with time, it is not a constant value (DC). That is where filtering and regulation come in, and we’ll cover that next time.