High Power 0-30v Voltage Regulator Circuit, Bench Power Supply
POWER SUPPLY CONTEST ENTRY
One of the most useful pieces of equipment for the electronic enthusiast is a good bench power supply. Buying one can be expensive, so most of us search the internet for circuits to copy, and just build our own.
But there is a disadvantage going this method. As the build will be just another "copy and paste" exercise, the builder does not understand how it works, and does not gain any knowledge.
With this Instructable, I hope I will be able to teach at least one person the basic operation of a variable power supply. I will break down the circuit, go through each step, and show how the components work, and how to calculate their values.
So what is a Bench Power Supply?
Well, it is simple, it is a power supply with a variable output voltage, and has adjustable current limit. No more sparks and burned out components when used correctly on your projects.
It consists of two parts:
1 - Unregulated part
This section converts the utility AC voltage to the required DC voltage for our power supply. The transformer performs two tasks:
It converts the utility voltage from a high voltage to a safe working voltage of your power supply
It gives electrical isolation between the utility network and your power supply output.
Rectifier DB1 converts the AC voltage to a DC voltage.
Lastly, capacitor C1 is used to filter out the 50/60Hz components present on the DC output.
See Figure 1.
2- Regulated part
Two things happen in this section. The ripple factor is reduced to as low as 1%, and the output voltage will be adjustable. Both facilities employ negative feedback. See Figure 2.
Care should be taken when designing a power supply. With the output set at its maximum output voltage, the output voltage must still be lower than the lowest voltage dips in the unregulated part of the supply. A good principal is to allow for at least 3 volt play. The idea is illustrated in Figure 3.
Lets design a 30V, 2A power supply.
In a moderate power supply like this, we can allow for a 10% peak-to-peak ripple voltage across the output. The peak voltage at the output will then be:
Vpeak = ( Vout + 3V margin + ( 0.1 x Vpeak ) ) x 1.2 safety margin
Vpeak = (30V + 3V + (0.1 x Vp)) x 1.2*
Vpeak = 45V
* 20% was added for transformer losses and diode forward voltage losses.
Earthing
When an electrical earthing point is available, it is always a good idea to connect the power supply case, as well as the core of the transformer to earth.
Depending on your requirements, you can leave the output of the power supply floating, or connect the 0V output to ground. I prefer to connect the 0V to earth via a 1Mohm resistor.
This section of the design is what controls the output of the power supply.
Looking at the transistors, we need to understand that the input voltage to the regulator will be fixed, and the output voltage can be varied by the user. The output voltage is determined by the bias current to the transistors. But there is one drawback. The input current will be the same as the output current, and the voltage across the transistors will be Vin - Vout. Thus, Ptransistors = (Vin - Vout) x I. Looking at Figure 3, this means that the area above the maximum output voltage, and below the unregulated voltage, is the energy that will have to be dissipated by the regulator, which is converted into heat.
Therefore, we need a decent power transistor, with a decent heat sink to dissipate this heat.
Transistor T1 will do the regulation, and must be able to handle the load current. Transistor T1 & T2 are connected as a darlington pair, and their combined gains will allow for a smaller biasing current. For this, lets use the old time favorite 2N3055 power transistor for T1. Transistor 2 can be a 2N3054.
The bias current for transistor T1 and T2 need to be stable, irrespective of the input voltage to the regulator. To do this, a constant current source can be used.
R4 and zener diode ZD1 form a constant current source. This current source must be high enough to cater for Ibase t2, as well as the voltage regulating circuit (not shown)..
Use BC179 transistors for T3.
The BC179 transistor has a typical gain of 100.
Our current source must deliver Ibase t2 as well as the regulation current. Lets limit the maximum regulation current to 1mA. The regulation current will be controlled by transistor T4, which we will discuss in the next step.
Take current through zener diode D1 as 2mA. This will ensure good regulation by the zener diode.
R5 = (average unregulated voltage - Vzener) / Izener
R5 = (45V - (4.5V/2) - 1.8V) / 2mA
R5 = 20.47K
R5 = 22K
With the constant current source complete, transistors T1 & T2 can now be switched to deliver the 2A of the power supply.
To be able to control the output voltage, we need a voltage feedback circuit. This is done via resistor R6, R7 and transistor T4. The circuit is set up as a negative feedback loop.
As the output voltage rises, transistor T4 is turned on harder, thus more current flows through T4. As the current source is constant, thus will result in less current to bias transistor T1 & T2. This results in a lower output voltage.
The next step is to calculate the voltage feedback components:
Use BC109 transistors for T3.
The BC109 transistor has a typical gain of 100.
Ice t4 max = Ice t3, or our maximum biasing current available from the constant current source.
Ice t4 = 1.6mA
Ibase t4 = Ice t4 / gain t4
Ibase t4 = 1.6mA / 100
Ibase t4 = 0.016mA.
This is the minimum current into the base of T4 that will allow the 1.6mA to flow.
This current needs to be supplied by the resistor network R6, R7 & R8. To be safe, current through these 3 resistors should be 10 times the base current needed on transistor T4.
As can be seen, there must always be an output voltage present to keep transistor T4 biased, Therefore, this type of power supply MUST have a minimum output voltage. Lets make the minimum output voltage 2V.
The minimum output voltage is 2V. Therfore, we can calculate the combined resistance value of R6, R7 and R8 at the minimum output voltage.
High Power 0-30v Voltage Regulator Circuit, Bench Power Supply
POWER SUPPLY CONTEST ENTRY
One of the most useful pieces of equipment for the electronic enthusiast is a good bench power supply. Buying one can be expensive, so most of us search the internet for circuits to copy, and just build our own.
But there is a disadvantage going this method. As the build will be just another "copy and paste" exercise, the builder does not understand how it works, and does not gain any knowledge.
With this Instructable, I hope I will be able to teach at least one person the basic operation of a variable power supply. I will break down the circuit, go through each step, and show how the components work, and how to calculate their values.
So what is a Bench Power Supply?
Well, it is simple, it is a power supply with a variable output voltage, and has adjustable current limit. No more sparks and burned out components when used correctly on your projects.
It consists of two parts:
1 - Unregulated part
This section converts the utility AC voltage to the required DC voltage for our power supply. The transformer performs two tasks:
It converts the utility voltage from a high voltage to a safe working voltage of your power supply
It gives electrical isolation between the utility network and your power supply output.
Rectifier DB1 converts the AC voltage to a DC voltage.
Lastly, capacitor C1 is used to filter out the 50/60Hz components present on the DC output.
See Figure 1.
2- Regulated part
Two things happen in this section. The ripple factor is reduced to as low as 1%, and the output voltage will be adjustable. Both facilities employ negative feedback. See Figure 2.
Care should be taken when designing a power supply. With the output set at its maximum output voltage, the output voltage must still be lower than the lowest voltage dips in the unregulated part of the supply. A good principal is to allow for at least 3 volt play. The idea is illustrated in Figure 3.
Lets design a 30V, 2A power supply.
In a moderate power supply like this, we can allow for a 10% peak-to-peak ripple voltage across the output. The peak voltage at the output will then be:
Vpeak = ( Vout + 3V margin + ( 0.1 x Vpeak ) ) x 1.2 safety margin
Vpeak = (30V + 3V + (0.1 x Vp)) x 1.2*
Vpeak = 45V
* 20% was added for transformer losses and diode forward voltage losses.
Earthing
When an electrical earthing point is available, it is always a good idea to connect the power supply case, as well as the core of the transformer to earth.
Depending on your requirements, you can leave the output of the power supply floating, or connect the 0V output to ground. I prefer to connect the 0V to earth via a 1Mohm resistor.
This section of the design is what controls the output of the power supply.
Looking at the transistors, we need to understand that the input voltage to the regulator will be fixed, and the output voltage can be varied by the user. The output voltage is determined by the bias current to the transistors. But there is one drawback. The input current will be the same as the output current, and the voltage across the transistors will be Vin - Vout. Thus, Ptransistors = (Vin - Vout) x I. Looking at Figure 3, this means that the area above the maximum output voltage, and below the unregulated voltage, is the energy that will have to be dissipated by the regulator, which is converted into heat.
Therefore, we need a decent power transistor, with a decent heat sink to dissipate this heat.
Transistor T1 will do the regulation, and must be able to handle the load current. Transistor T1 & T2 are connected as a darlington pair, and their combined gains will allow for a smaller biasing current. For this, lets use the old time favorite 2N3055 power transistor for T1. Transistor 2 can be a 2N3054.
The bias current for transistor T1 and T2 need to be stable, irrespective of the input voltage to the regulator. To do this, a constant current source can be used.
R4 and zener diode ZD1 form a constant current source. This current source must be high enough to cater for Ibase t2, as well as the voltage regulating circuit (not shown)..
Use BC179 transistors for T3.
The BC179 transistor has a typical gain of 100.
Our current source must deliver Ibase t2 as well as the regulation current. Lets limit the maximum regulation current to 1mA. The regulation current will be controlled by transistor T4, which we will discuss in the next step.
Take current through zener diode D1 as 2mA. This will ensure good regulation by the zener diode.
R5 = (average unregulated voltage - Vzener) / Izener
R5 = (45V - (4.5V/2) - 1.8V) / 2mA
R5 = 20.47K
R5 = 22K
With the constant current source complete, transistors T1 & T2 can now be switched to deliver the 2A of the power supply.
To be able to control the output voltage, we need a voltage feedback circuit. This is done via resistor R6, R7 and transistor T4. The circuit is set up as a negative feedback loop.
As the output voltage rises, transistor T4 is turned on harder, thus more current flows through T4. As the current source is constant, thus will result in less current to bias transistor T1 & T2. This results in a lower output voltage.
The next step is to calculate the voltage feedback components:
Use BC109 transistors for T3.
The BC109 transistor has a typical gain of 100.
Ice t4 max = Ice t3, or our maximum biasing current available from the constant current source.
Ice t4 = 1.6mA
Ibase t4 = Ice t4 / gain t4
Ibase t4 = 1.6mA / 100
Ibase t4 = 0.016mA.
This is the minimum current into the base of T4 that will allow the 1.6mA to flow.
This current needs to be supplied by the resistor network R6, R7 & R8. To be safe, current through these 3 resistors should be 10 times the base current needed on transistor T4.
As can be seen, there must always be an output voltage present to keep transistor T4 biased, Therefore, this type of power supply MUST have a minimum output voltage. Lets make the minimum output voltage 2V.
The minimum output voltage is 2V. Therfore, we can calculate the combined resistance value of R6, R7 and R8 at the minimum output voltage.
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