Understanding rectifiers with actual signals from experiment

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An electronic training board

We know that a diode allows electric current in one direction and blocks electric current in another direction. We are using this principle to construct various types of rectifiers. A rectifier is nothing but a simple diode or group of diodes which converts the alternating current (AC) into direct current (DC). Rectifiers are classified into different types based on the number of diodes used in the circuit or arrangement of diodes in the circuit. The basic types of rectifiers are: half wave rectifier and full wave rectifier. In this topic, we will focus on the half-wave rectifier and full wave rectifier. Now we’ll take a look about its definition and how it works.

Half-wave Rectifier

Is the one that they called the “simplest form of a rectifier” where it only uses one diode for it to be constructed. It’s components are; an AC source where it supplies AC to a circuit, a transformer that reduces the AC voltage from higher to lower voltage , a resistor that serves as a load and a diode that serves as “switch” depending on the condition either forward bias or reverse bias. Half-wave rectifier is a type of a rectifier that only allows a half cycle either a positive half cycle or a negative cycle. If the positive half cycle is allowed to flow then the negative half cycle is blocked, it is also similar if the negative half cycle is allowed then the positive half cycle is blocked. One thing to remember in a half-wave rectifier is that it will not allow both half cycle at the same time thus it is either positive or negative half cycle of an AC input is taken in.

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Circuit used in the experiment.

In a transformer, it converts high AC voltage into a low AC voltage and when that low AC voltage is absorbed by the diode, and the condition of that diode is in forward bias which allows a current flow although, if during negative half cycle the condition of a certain diode is in reverse bias wherein there will be no presence of current flowing in a circuit. However, we cannot say that a positive half wave rectifier will entirely block all the negative half cycle, there will be always be a very small amount of negative current to flow wherein it come from a minority carriers inside the diode but it will not be showed or be displayed in a certain tester or oscilloscope. Likewise, it also the same process with the negative half cycle if it is allowed, the diode will be in forward bias and when in positive half cycle, it is in a reverse bias condition.

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Actual input signal registered in the oscilloscope

In a half-wave rectifier connected with a filter(either a capacitor or an inductor) for it produces pure and constant DC output because the voltage output is changing or varies from time to time, and as we all know that a capacitor is providing a higher resistive path to various DC components which have a low frequency signals than the AC components which have high frequency signals. A current will always choose to flow in a path where it has a lower resistance and once the current will reach to a certain filter, the DC component will be exposed to a higher resistance from the capacitor and the AC components will also be exposed to a lower resistance coming from the capacitor.

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Actual output signal registered in the oscilloscope

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Actual output signal when filtered as shown in the oscilloscope

Full-wave Rectifier

This is different from the half-wave rectifier mentioned earlier because this type of rectifier will convert both of the half cycles (positive half cycle and negative half cycle) of an AC signal into an output DC signal. The full wave rectifier is classified into two types; Center tapped full wave rectifier and a Full wave bridge rectifier.

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Circuit used in the full-wave experiment

So in center tapped full wave rectifier, it can be compared to a normal transformer that reduces or increases the voltage in an AC. By using this kind of full wave rectifier, we have an outcome of voltages that are in phase with each other, meaning to say is that by this type of full wave rectifier can also produce a current flow only in one direction. Whenever an input AC voltage is applied , the second winding of a center tapped full wave rectifier will divide that AC voltage into two which is the positive and the negative.


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Actual input signal registered in the oscilloscope for full-wave


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Actual output signal registered for full-wave rectifier in the oscilloscope

There are some similarity between a full wave rectifier with a filter and a half wave rectifier having also a filter. Unlike to a half wave rectifier, a full wave rectifier both the half cycles (positive or negative) will charge a capacitor wherein its duty is to shorten the ripples towards the ground and will blocks the pure DC in a DC components in order for it to flow across the alternate route will reach to the load output. Let’s say that there are two diodes connected in a full wave rectifier with a capacitor filter, whenever there is an applied AC voltage during the positive half cycle the diode D1 is in a forward bias state that will allow current to flow while the diode D2 is in a reverse bias state which blocks current flow. Similarly, during in negative half cycle the D2 will then be in forward bias state and will allow current to flow and the diode D1 which is in a reverse bias state will block the flow of current. Also, during that positive half cycle the current in a diode D1 will then reach to the filter and then charge the capacitor only if the AC voltage is greater than the voltage in a capacitor. If the capacitor is not charged then there is no voltage present in the plates of a capacitor and the charging of a capacitor will activate if the voltage is turned on. This is what they call the conduction period wherein the supplied voltage in a capacitor will be charged at a maximum value and the capacitor will also store it a maximum value thus making the voltage in a capacitor is equal to the supplied voltage. When a capacitor is discharging, the AC voltage is starting to decrease dramatically and this capacitor discharging has a slower process unlike when a capacitor is charging. But before that happens the charging process will take place and only half or more than half will be discharged.

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Full-wave rectifier circuit with filter


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Actual output signal when filtered as shown in the oscilloscope

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Oscilloscope registering the signal from the experiment

A capacitor only will allow AC components and block any DC components and whenever a direct current contains both of the components either a DC or AC components, as soon as it will reach the filter, a DC component will be experiencing a high resistance that comes from a capacitor and an AC component will experience a lower resistance. We all know that a current will flow to the path where there is low resistance this is also the same when an AC components will go to a capacitor wherein a DC components is blocked by the capacitor thus finding another path to an output load and that flow of AC component across a capacitor is likely the same as charging a capacitor.

In conclusion, a rectifier is made up of various diodes that is used to converts AC into a DC. One of the practical use of rectifiers is the adapter of a laptop. If it is connected to an AC source, the high AC voltage will be converted into a low DC voltage output and this will be supplied to the laptop that is why when we turn the laptop on manually, the laptop battery will start supplying DC current that will power up the laptop. There are also other electrical appliances that uses rectifiers it could be in our house, in school, in offices or anywhere for it have a great impact in terms of converting AC voltage into DC voltage and this rectifier is important because some of the appliances need DC current or DC voltage as an output for them to run or activate.


References

  1. Boylestad and Nachelsky, Electronic Devices and Circuit Theory
  2. Floyd, Electronics Fundamentals. Circuits, Devices, and Applications
  3. Schultz, Grob’s Basic Electronics

Note:

All images are from the author except with citation.



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3 comments
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Not a student of electronics or a fan of it. I think this is quite rich nevertheless, for those that care enough to read.

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