Assignment 1: trickle charger power supply

2009

Zahidur Rahman

Tshwane University of Technology: PJT101 T

2/27/2009

Assignment 1: Power Supply trickle charger


PJT101T: assignment 1

Zahidur Rahman

Table of contents

1) Introduction

2) Power supply

Transformers Step down transformer Step up transformer Center tapped transformer

Rectification Full wave rectifier; center tapped transformer Full wave rectifier; diode bridge

FilterRegulatorOverview

3) Trickle charger circuit

Circuit diagnosticsTrickle charger power supplyOverview

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1) Introduction

Power supplies are the root of all electronic devices, without an adequate flow of a desirable pulse, the circuit is fruitless. This assignment studies the details of a power supply that is enabled to trickle charge a battery such as a nickel-cadmium battery or a nickel metal hydride battery such that a constant capacity is maintained inside the battery even if it is under use.

Similar examples of such applications are such as an alternator inside a motor vehicle, as the vehicle uses the battery for applications such as sound devices or even the electric windows; the engine rotates the alternator armature to constantly recharge the battery which implies a virtual battery of infinity charge.

Concerning power supplies

The power supply can be built in different ways, but is always run under the same principals. There are 4 main stages in a power supply when converting an AC (alternating Current) signal to a DC (Direct Current) signal. The methods are named in respective to their levels: Transformer, Rectification, filter and Regulator.

Concerning chargers

There are two methods of charging a battery at a constant rate correspondent to its discharge rate. Float charging and trickle charging are very much similar, float charging is much safer because it does not overcharge the battery which causes a potential hazard to the battery. Trickle charging is more preferred if the load used on the battery is under constant use so that there is always a discharge to prevent an overcharge.


http://en.wikipedia.org/wiki/Nickel_metal_hydride_battery

http://en.wikipedia.org/wiki/Nickel_metal_hydride_battery



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2) The Power Supply

Transformers

Transformers are basically used in adjusting the intensity of AC, making it more controllable and regular.

Step up transformer: sometimes, in rear cases, an AC signal generated from the source is not sufficient for application, a step up transformer is used to increase the values, but it is rear to need them in usual electronics applications.

Step down transformer: in maximum cases of electrical applications, it would be ideal to have the source as to being more than the required amount. A step down transformer is used to decrease the input signal (for example, the wall socket of 230V, 50Hz) to a more acceptable value, preferably a little higher than the exact required value such that we can compensate for the voltage losses throughout the circuitry of the power supply.

This is what happens on a step down transformer

Note that the voltage has decreases but the frequency remains constant. Frquency does not really matter much except for when you reach the filter section in this document. At the end, the frequency will still seize to exist in a DC value.

Center tapped transformer: in some cases, rarely, a centre tapped transformer is used for doing two jobs at once, it is a transformer and also used in the rectification process rarely. The structure




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analysis of the device shows two windings in the transformer, though the second windings differ from the primary because there is a connection in the middle of the coil, thus we divide the output into two halves of the transformed signal in the secondary windings. The following sketch illustrates the signals generated:

This method can be used in a power supply but is not quite economic. This is a reliable component since the transformer is a solid component whilst a diode bridge (discussed later in this document) can sometimes fail due to over exposure to raw current that is maybe not “stepped down” enough, which may cause in heating the component and possible damage to the circuit.

Rectification

A rectifier is used to process the signal and channel it into an all positive output wave. The rectifier can either use one half of the input cycle (half wave rectifier) or it can use the full cycle and make it positive and pulsating at the rate of double the frequency.

We have multiple choices on making our rectifier circuit, it would be preferable to employ a full wave rectifier as it has a greater frequency that is better to work with when we reach the filter phase.

Full wave rectifier; center tapped transformer: when we use a center tapped transformer, as mentioned earlier, we use two halves of the secondary wave, if we rectify it, we use two diodes to make use of the




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positive half cycle of each half secondary wave. The following circuit diagram will allustrate what goes on.

Refer to next page for diagram

To truly understand what goes on, we must study the circuit throughly, if we take into account the first half of the wave along both terminals, we can see that the positive cycle from V1 is forward biased into D1 and then coming to the output, whilst V2’s positive cycle enters ground and does not influence the output voltage. Also, since the circuit that works on D2 is reverse biased, V2 is actually inactive and there is a Positive Inverse Voltage (PIV) that acts upon D2. Note the red line to show the active part of the cycle:




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In the second half of the cycle, the negative value of V2 is forward biased by D2 whilst –V1 reverse biases D1 and there is a (PIV) acting upon D2.


This way, only the positive cycle of eachwave is active and extracted from the secondary coil. And thus we have arectified all positive signal that is pulsating.

Full wave rectifier; diode bridge: in electronics, making and buying a center tapped transformer is expencive, thus an alternative was inevitable, the diode bridge uses 4 diodes connected in parallel to eachother. The frequency in this circuit will double as a normal full wave rectifier does, but the amplitude of the wave will not be affected and will remain pure as the secondary coils produce them.

The opeartion proceeds as follows: when the positive cycle is active, D3 and D2 are forward biased and there is a PIV acting on D1 and D4. This way, the first half cycle is collected on the output terminal like shown in the next diagram:




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Note that the diodes tinted black are enduring a PIV, also that D2 seems to be reverse biased, but it is actually forward biased, the current flows from the top of the secondary coil through to D3, and then to ground, then it flows out of ground to the cathode of D2 and back to the coil.

The second half of the wave cycle does the same thing as the first half, the coils polarity is reversed and thus D4 is now forward biased and then D1 too. This causes a PIV to act upon D3 and D2. And the negative cycle is collected upon the output which adds up to the previous positive cycle as if it is pulsating. Refer to the following diagram for a graphical representation:

Filter

When we convert AC to DC, we try to controll a volatile signal into a smothe positive signal. After rectification, we have a signal that




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rapidly rises and falls, this fall can be controlled if we can add a capacitor that will discharge as the voltage drops and thus maintain a slower dropping signal that will be picked up by the following signal cycle. The process is caller filter and can be illustrated in the following diagram:

Refer to next page for diagram:

There are complex calculations involved when determining the voltage average of the ripple created in this circuit. depending on the capacitor and the resistor value, the discharge may be slow and thus the drop may be less steep.

Regulation

When we have a ripple voltage, we still don’t have a fully smothe flow of supply yet, using a voltage regulator in the last phase can help us get rid of all the unwanted variences, for this we can use a zener diode or just a normal voltage regulator for simplicity. We still endure a relatively large loss of voltage since for us to get rid of the ripple voltage, we must get a DC out voltage through the voltage regulator and the ripple additional voltage will be used by an extra load resistor.

Overview

The circuit power supply is as follows, we are using a normal step down transformer, diode bridge rectifier, capacitor filter and a voltage regulator.




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There has to be a load resistor. If this document is continued, you will notice that the load resistance is actually the trickle charging circuit for the battery.

3) Trickle charger circuit

Circuit diagnostics

The following circuit is a copy of a car battery charger that was extracted from http://www.aaroncake.net/Circuits/charger1.asp?showcomments=all. This circuit is said to charge most car lead acid batteries, to avoid over charging the battery, it is designed to switch off when the battery is fully charged, thus it is also a float charger that maintains the batteries health. Also, a heat sink is suggested to extract energy from the voltage regulator U2


http://www.aaroncake.net/Circuits/charger1.asp?showcomments=all




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Part Description

R1 500 Ohm 1/4 W Resistor

R2 3K 1/4 W Resistor





R7 0.2 Ohm 10 W Resistor

C1 0.1uF 25V Ceramic Capacitor

C2 1uF 25V Electrolytic Capacitor

C3 1000pF 25V Ceramic Capacitor

D1 1N457 Diode

Q1 2N2905 PNP Transistor

U1 LM350 Regulator

U2 LM301A Op Amp




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S1 Normally Open Push Button Switch

Trickle charger power supply

If the circuit shown is to be utilized, we must build a power supply that fruits 18V on the output before the regulator. The same website suggested a power supply that we can use.

Studiying the suggested power supply circuitry, a total list of real components are shown.

Part Description

C1 6800uF 25V Electrolytic Capacitor

T1 3A 15V Transformer

BR1 5A 50V Bridge Rectifier

S1 5A SPST Switch

F1 4A 250V Fuse




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Overview

Using the circuits given to us, we can build the complete circuit diagram of a car battery charger that can not only trickle charge, but also float charge at the same time. If we keep to the components list and the following circuit diagram:




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Bibliography

Books:

E.E Glasspoole Reg.Eng.Tech. “First Steps to Electronics”

book1/ book2/ book 3

© copyright 1990

Floyd, Thomas L. P. CM. “electronic devices/ Thomas L. Floyd.- 6th Edition”

© copyright 2002

World Wide Web addresses:


http://en.wikipedia.org/wiki/Trickle_charge


http://en.wikipedia.org/wiki/Trickle_charge


Documents

Assignment 1: trickle charger power supply