Need help in designing a power bank for mobile phone powered by Wind generator

[MEERA Meera]

Hello friends, for my engineering final year project i had to design a 5v mobile charger which is powered by a renewable source (say 12v DC Wind generator) with 6v battery for backup power.
im using 2 linear voltage regulators such as LM7806 for charging the 6v backp bt from the wind generator and LM 7805 for charging the mobile phone which requires 5v..

When the charger is connected im facing the heating issues with the mobile even it shows "charging"
Please help me to design a better circuit !!

 

[Tony Stewart]

LDO's are quite lossy due to the wide range of drop voltage at full load current and are not good choices.

There are two separate control systems.
1. Primary Battery charger that does not stall the turbine or over charge the battery.
2. Secondary Battery charger that does not drain the primary battery or exceed the V+ for the mobile,

Each requires high efficiency and protection for over/under voltage and over current.
Normally batteries prefer a 3 stage charge with CC, then CV then shutoff or float depending on type of chemistry.

Similar to PMMT regulators for PV panels, wind power turbines also have an optimum RPM for max power transfer which means these transfer functions need to be defined. A secondary sensor for wind speed is useful but not essential if the turbine has mechanical speed brakes for over speed and the batteries can handle the max charge current up to this speed.

It is always best to start with a design spec to define all the inputs, processes and outputs. The Mobile is assumed to regulate its own battery, but when many outputs are created from one source, it is necessary to include PTC current limiters rated for holding current in case of faults.
These include: a nominal and tolerance or min/max
V, I, P, OVP, OCP, UVP, Thermal protection
Generator
Primary battery
Secondary loads ( mobile loads) with possible different requirements.

It gets a bit overwhelming, but breaking each function down into smaller chunks makes design easier to do.

In the end, it may end up looking like a PV PMMT power controller, which uses Buck type regulators with extra features for MPT control.

Back in 1975, I started work as a designer with a small part of an automated weather station with a 750W Wind Turbine and battery packs in piles, floating in the Artic on an ice flow. The Generator designer went on to become quite prominant owner of RF electronic design & mfg. But at the time he was just green and scared because he kept burning up $200 power transistors due to transients exceeding the safe operating area. So this was how we learned by learning to detect the root cause then fix it.
Now you have the internet. But we always started with our own design spec.

 

[AnalogKid]

The circuit has problems. But first, since this is your assignment, please describe the way you think it should work, with voltages at each major circuit node, the output voltage and current, etc. Also, what you mean by "the heating issues"? Is the phone overheating, the charger, or both?

ak

 

[Tony Stewart]

Hello friends, for my engineering final year project i had to design a 5v mobile charger which is powered by a renewable source (say 12v DC Wind generator) with 6v battery for backup power.
im using 2 linear voltage regulators such as LM7806 for charging the 6v backp bt from the wind generator and LM 7805 for charging the mobile phone which requires 5v..

When the charger is connected im facing the heating issues with the mobile even it shows "charging"
Please help me to design a better circuit !!

Dont use these LDO's. THey have a minimum drop at 2V and at 12V in 8V out @1A they will dissipate 4Watts then the thermal resistance for Rja ( junction to ambient ) is high so it needs a large heatsink.

The problem is the 1st battery equivalent circuit is a huge capacitor ( xx Farads) with an Effective Series Resistance (ESR) in millohms so the regulator current limits and gets hot then shuts down and then cools and repeats until charging prevents OTP or over temp protection but still is too hot from lack of heatsink.

Consider a low milliohm MOSFET and high current coil choke with fast recovery diode clamps to regulate the current with pulses instead. ( i.e. a Buck regulator) after you define your specs.

 

[MEERA Meera]

The circuit has problems. But first, since this is your assignment, please describe the way you think it should work, with voltages at each major circuit node, the output voltage and current, etc. Also, what you mean by "the heating issues"? Is the phone overheating, the charger, or both?

ak

input is fed by a DC GENERATOR 8-12 v
i need to charge a battery for back up (not sure what to use..here i had used 6v 1300mah Mi-cd battery)
from the backup power im going to charge the moblile phone which requires 5v..with 500ma

please find me a better circuit which may be more helpful to me...

 

[Tony Stewart]

Those specs are incomplete. How do you determine practical power and storage capacity requirements? And what protection for aafety and damage? What efficiency is adequate? Make it like a commercial list of engineering specs.

 

[Les Jones]

Here are some comments about your schematic. I assume your 6 volts Ni-Mh battery is made up of 5 cells. To charge 5 cells you will need more than 6 volts as a cell on charge near its fully charged state will be about 1.4 volts. As you have 5 cells you will need 7 volts. As your 6 volt regulator is followed by a diode the voltage available after the diode will only be about 5.3 to 5.4 volts. Ni-Mh cells are normally charged at constant current and the end point detected by negative delta V or monitoring the cell temperature. Your circuit is a constant voltage charger with no end point detection. You say that the pone is getting hot when charging. Have you checked that the output voltage of your unit is correct ? I cannot understand how the LM7805 can work the way it is connected. Can you explain the theory behind connecting it in the way shown in your schematic.

Les.

 

[analog1]

First of all battery should not be charged with specific voltages. Batteries needs I current to charge and specifically at a certain rate. Car batteries for instance are never 12v change the temperature also changes the voltage across the battery. Fast charging and deep discharges effect the battery. If the battery are ion-lithium then more considerations are involved for a long use full life. Best you can do use simple lm7805 and use it as a current source for whatever current the battery needs for charging Do add a diode across the lm7805 for protection the extra volt to 6 volts or whatever is an offset to the adj pin. basically for a current source of 0.5 amps you need 5v/.5I=100 ohms the voltage offset it can be 0.001 ma or 1v/0.001= 1k to ground Now the adj pin becomes your connection to the battery. Simple 4 components. Of course it can be complicated but why? There are some things that you must learn more parts more trouble less reliable.

 

[tomizett]

Before we get too carried away here folks, can we clarify what level of project this needs to be?

Meera, you say in your original post

...my engineering final year project ...

. I think we're all assuming that this is a university degree final year project in electrical/electronic engineering (what in the UK we would call a BSc or BEng). If that's not right, could you give us a guide as to the kind of course you're on (perhaps electronics is only a minor part of it?) and the kind of circuits you've studied? Is the schematic you've posted typical of the kind of thing you've been looking at in class?

What you've suggested certainly isn't the approach a professional engineer would take to designing a commercial product, but perhaps that's not what your assessors are expecting?