A power supply is a critical part of a particular measurement instrument. Almost all parts of a particular measurement instrument require electricity to run. The power supply in this case provides electricity to energize the system right from the transducer (sensor); we mean the passive one, goes to the microcontroller, and all the way down to the display (LCD) or another output for example an actuator. Without a power supply, the system will not work and stays off.
As a basic thing, a power supply is necessary to learn for beginners. In this post, we are going to explain what is power supply, its types, working principles, etc.
So, What is Power Supply?
A power supply is an electrical device that is used to energize (give the electricity) to electrical appliances. The energizing is done through several processes until the electricity is ready and suitable to use by the electrical appliances. The processes can be easily understood by the following block diagram.
Block diagram of a power supply. A typical power supply will have the following system blocks.
Types of Power Supply
There are three main types of power supply. The types are determined according to the blocks that build the overall system and the dc output signal that the power supply produces. Below are the types of power supply:
1. Unregulated Linear Power Supply
This power supply type consists of a step-down transformer, rectifier, filter capacitor for smoothing/filtering, and a drain (bleeder) resistor. It is low-cost and suitable for low-power devices. The only drawback is that its output is not constant. It means that there is a ripple that is not good and suitable for electronic devices. An inductor-capacitor (LC) filter can be used to reduce the ripple, but the cost will be higher.
2. Regulated Linear Power Supply
It is basically the unregulated linear power supply, but it is equipped with a regulator in the place of the drain resistor. The main purpose of this power supply type is to provide a constant dc output voltage over a wide range of different application loads.
The advantages of this type are simplicity, low-cost, reliability, and low noise level. On the other side, the disadvantages of it are the high heat loss, size, and low-efficiency level.
3. Switch Mode Power Supply (SMPS)
It is more complex and different than the previously explained types. The SMPS block diagram contains a rectifier, filter capacitor, series transistor, regulator, transformer, and drain resistor. The drawbacks of this type are that its complexity is not suitable for low-power applications and the transformers must be custom-made.
Working Principle of Power Supply
We have discussed that there are several types of power supplies. In this case, we are going to dig deeper into how it works specifically for the types of power supply we can easily make DIY for small projects.
1. Unregulated Linear Power Supply
The working mechanism can be easily and clearly explained per system block. The following is the explanation order:
- A step-down transformer. As its name implies, the transformer here has the function to step down the AC input voltage 220V into the required level of the rectifier.
- Rectifier. It is used to convert the AC to raw DC form. The circuit has two types that are half-wave rectifier and full-wave rectifier. It consists of the diodes arranged in a certain order.
- Filter capacitor. The produced DC from the rectifier is smoothened by the capacitor and reduces the unwanted ripples.
- A drain resistor. It is also known as a bleeder resistor and connected parallel to the filter capacitor to drain the stored charge so the system remains safe.
2. Regulated Linear Power Supply
It is already known that the regulated power supply is similar to the unregulated power supply except that the drain resistor is replaced by the regulator. The voltage regulator consists of BJT or MOSFET type transistors (active) and resistors (pass device either series or shunt) controlled by a high gain differential amplifier. Then, the voltage regulator here can be divided into two types.
- Series regulator. It is the most used regulator type for linear power supplies. Its place is series with the load to make sure that the output is linear to the current taken by the load.
- Shunt regulator. It is the least-used regulator type. Here, a source resistor is in series connection with the input, and a variable shunt resistor to ensure the constant voltage across the load.
3. Switch Mode Power Supply
The AC input voltage is rectified by the rectifier to an unregulated DC signal form with the series transistor and regulator. This DC signal is then chopped to be a constant high-frequency voltage. It means that the transformer size can be decreased and make a smaller size power supply. After being chopped and a constant high-frequency voltage, then the signal is smoothened by the filter capacitor and ready to use. A drain resistor is thereafter the filter resistor to keep the system safe.
Specifications of Power Supply
Prior to selecting a power supply, here are several things that are important to know.
1. Voltage and current specifications
These two parameters are the main and primary specifications you should consider. The power supply may have a fixed or variable output voltage. If it is a fixed output type, then it may need a certain adjustment to meet the required voltage. If it is a variable output type, make sure that the range covers the requirement you are about to fulfill. This rule also applies to the current specification.
2. Line regulation
It determines the regulation of the allowed input voltage changes. These changes then may be seen on the output. It is commonly written in millivolts of an input variable voltage.
3. Load regulation
It is similar to line regulation in terms of quotation in millivolts or a percentage of the maximum output voltage. It can be found that the output voltage drops slightly when a load is added or connected to the power supply output.
4. Ripple and noise
These two are considered as single specifications. The ripple frequency should be twice the line frequency for linear power supplies. As for the SMPS, the switching actions make the ripple and spike arise.
Noise and ripple should be more than 10mV for most good power supplies. As for the SMPS, 50mV or less should be available for many cases.
5. Temperature stability
The temperature can influence the output voltage changes. The information about the temperature can be obtained in the datasheet. It is measured as a percentage or absolute voltage change per degree C. For example, it may be in the region of 0.02%/oC or 2mV/oC.
6. Stability with time
The performance of an electronic device will change over time. It is often quoted in the overall power supply specifications. The stability with time is measured over a period of time under a constant load, input voltage, and the measured voltage drift. It is mostly in a few millivolts (5-10) over a ten-hour period.
7. Current limiting and over-voltage
To prevent damage in case of a failure, the protections are there to protect the power supplies. There are two types of it.
- Short circuit protection. It is needed when the short circuit happens or the taken current is beyond the initially designed current.
- Over-voltage protection. It is there to protect the device from the full pre-regulated output voltage. The device will be cut out by the over-voltage protection to prevent the full-over voltage condition from happening.