How to repair inverters part 3

The image below was taken from a two weeks old china inverter which failed when 600W of load was plugged on it. The power rating of the inverter was 3000W with 6000W surge capacity.





Two stages switching inverter board

Facts about china switching inverters.

All these inverters are two stages high frequency-low frequency inverters.

All these inverters power their second stage using an auxiliary winding on one of the ferrite transformers in the first stage.

They all use mosfets connected in either H-Bridge or Half Bridge for their output.

Only few (about 10 percent) of these inverters uses micro controllers.

The most common PWM IC used is SG3525, SG3524 OR TL494.

Now that we know these about the inverters that we are going to repair, please follow the tutorial below carefully. Please read it until you get the whole concept since its the basis of all two stage inverters seen on our markets.

HOW TWO STAGES HIGH FREQUENCY INVERTER WORKS

Every high frequency inverter consists of:

DC to DC boost stage

low frequency AC stage.

Let's start by talking about the first stage of a high frequency inverter which is the DC to DC boost stage.

At this stage, the input DC battery is switched across a ferrite transformer at frequency between 25kHz. to 60khz. using a pulse width modulation driver IC such as 3524,3525, Tl491 etc. The switching topology used for this stage is mostly push pull using N channel mosfets.

The output high frequency square wave its rectified using fast switching diodes connected to form a bridge rectifier and a filter capacitor of 400V. The resulting output voltage ranges from 270V DC to 330V DC with best value of 310V which is 220V multiplied by the square root of 2 (RMS value).

Feedback is applied to the driver stage using either a separate winding from one of the switching ferrite transformers or using an optocoupler. The feedback ensures that the output voltage of this first stage is always stable.

low frequency AC stage.

This is the final stage of a high frequency inverter and this stage brings the difference between two inverters from different manufacturers. The kind of switching topology employed here will determine the quality, efficiency and durability of the inverter.

At this stage, the boost DC voltage obtained from the first stage is converted to 50Hz or 60Hz. AC which is either:

Pure sine wave
modified sine wave
square wave

using either H bridge topology or Half bridge topology.The switching mosfets used here are high voltage N mosfets or IGBT with minimum voltage of 400V.

The type of wave obtained at this stage which is the inverter output depends on the driving circuit and technique employed.

Note that at each section, there will be op amps or decision making ICs which monitors max current and high or low voltage.

I think i have given a fair idea about the subject. Your comments are welcome.

READ ABOUT MPPT

SOLAR BATTERY CHARGER CIRCUIT

Solar charge controller circuit that is used to charge  inverter batteries and car batteries using the solar energy.

12V, 24V, 36V, 48V, 72V SOLAR SOLAR BATTERY CHARGER CIRCUIT.

A solar battery charger is a charge controller device that controls the charging state of a battery when using solar panels. Solar chargers comes in different forms and types. But the most important aspect of it is to get your batteries charged without over charging them or under charging them since any of the two conditions can reduce your battery life. Also, the charger should not waste power and in that case must be highly efficient.

Today am going to show you how to make for your self a highly efficient two stages solar charger (solar charge controller).

HOW THE SOLAR BATTERY CHARGER CIRCUIT WORKS

The circuit is built around LM358, CD4013 and N-Mosfets. The charging current of this circuit is 60A but can be increased to handle 80A by using appropriate Mosfets or connecting more mosfets in parallel.

The op amp LM358 has two independent comparator within the 148 pins single package. Please download the data sheet from https://www.onsemi.com/pub/Collateral/LM358-D.PDF

The second  part of the op amp is configured to give a high output when the battery is low. The high output is sent to pin 5 and pin 9 of CD4013 digital IC which is called data pin. This causes pin one of the CD4013 to go high and turn optocoupler U10 on. When U10 turns on, Q13 also turns on and positive voltage is applied to the gates of the N-Mosfet to turn it on for charging to start.


When solar battery Charging is complete, pin 7 of LM358 which is the op amp 2nd output pin goes low. As the op amp goes low, pin 2 of CD4013 goes high whiles its pin 1 goes Low. The high output from Pin 2 of CD4013 turns U14 on and this turns of the mosfet to stop the charging. I did not add hysteresis because i want the output to be always stable at my set battery full value.


whiles pins 1 and 2 of CD4013 takes care of switching the mosfets, pins 13 and 12 of CD4013 will turn on LED 2 and LED 1 to indicate charging and floating respectively.


The second op amp is configured to generate square wave clock signal and its output is fed to the clock pins of CD4013 flip-flop IC. Download CD4013 datasheet.When clocking is working, output pin 1 of the op amp will measure 2.5V or close.

12V 24V 48V 96V

SOLAR BATTERY CHARGER CIRCUIT

Updated Circuit

To set floating level ( battery full level), connect the battery to the circuit and connect your solar panels to the part labeled panel. Please ensure correct polarity though the circuit has reversed polarity protection.

monitor the charge level with a voltmeter and when battery reaches your desired battery full level, adjust U13 variable 10k resistor until float indicator turns on. In that case, the battery voltage will neither go up nor down but stable. That is called floating, this kind of charging ensures that your battery is always charged.


Other voltage designers should leave their designing voltages in the comment section below and i will be ready to support them. All other comments are also welcome.

Update: U5 should be replaced with LM7812 or any 12V voltage regulator.

OVERLOAD PROTECTION CIRCUIT AND LOW BATTERY ALARM

How to connect the low battery protection circuit and an overload protection circuit to any circuit that uses pulse width modulation IC such as SG3525, SG3524, UC3842, 43,44, TL494 etc.

Its Good to know and understand the functions of the pins of the PWM IC that you are going to add this function to. I therefore advice that you google the IC number and download its datasheet in PDF or any readable format.

Every pulse width modulation IC has either of these functional pins: Shutdown or current sense (Is) or even both in some ICs. 

 This post was requested by Mr. Amon from Asamankese (E/R). 

LOW BATTERY AND OVERLOAD PROTECTION FOR PWM IC

FUNCTIONS OF SHUTDOWN PIN

The function of the shutdown pin of any pulse width modulation (PWM) IC is to safely stop or turn off the PWM IC when the pin senses voltage higher than its operating point (threshold). After the IC has turned off, the system will turn on only when the cause for increased pin voltage is removed or reset.

FUNCTION OF CURRENT SENSE (Is) PIN

The current sense pin works the same way as the shutdown pin. When the pin dedicated for current sensing sees voltage higher or equal as the threshold voltage, the IC temporally, stops operating in other to save the system. The IC will begin to work normal again after the cause of overload is removed.


Current sense and shutdown pins responds to voltage within 0.3V to 5V with little current in milli ampere (mA). This means that if we are able to provide a circuit that will deliver 0.3V to 5V or above during low battery or overload, our aim of designing a low battery and overload protection circuit will be fulfilled.
 

HOW TO SENSE OVERLOAD CURRENT

Many people have the problem of sensing overload current in power system circuits. Their main problem has been either they are not getting a small resistor value which is usually called SHUNT or They are not getting a good length of AWG to provide a resistance value in Milli ohm. Personally, i prefer using AWG copper since its always common than real shunt resistors.


Overload current can be sensed either at the DC side or AC side. Its advisable to sense inverter overloads at the DC side. In that case, Mosfets can be used as Shunt resistors since they have some resistance in milli ohms even when they are fully turned on. Connecting the mosfets in parallel is same as connecting its internal resistance in parallel and hence forming low resistance shunt.


One thing to know is that before you can choose a good shunt value, you have to know the maximum current you want to switch and the voltage drop that must occur. This can be known using simple ohms law: V=IR where V is the maximum voltage drop that should occur at maximum current I. R is the shunt resistor value. Read the power drive stage of this post to learn how to calculate max current.


The circuit below shows how to sense overload DC current and connect it to the current sense pin of a PWM IC. 

ADDING LOW BATTERY PROTECTION TO PWM IC

I have already writing about low battery protection for battery powered devices and explained the circuit operation. I will therefore move on to show you how to connect this updated circuit to any pulse width modulation IC to shut your system off when battery runs low so that your battery doesn't over drain.
 

overload protection, low battery alarm and low battery shutdown circuit for inverters

A shunt resistor is a small value resistor or resistive device connected in a circuit mostly for the purpose of current sensing.

In the above circuit, the mosfet stage is connected to the shunt resistor which is connected in series with the negative supply of the battery. Please ensure that only the mosfets are connected to this part. all other sections of your inverter should be connected before your shunt. shunt size should be calculated by you based on your max current as discussed earlier.

Adjust U3 to set your low battery shutdown.

Adjust U2 to set your battery low alarm

POWER STABILIZER CIRCUIT

In this tutorial we are going to design an AC Voltage stabilizer using auto transformer. Auto transformer is a transformer with its primary and secondary connected together. This type of transformer is mostly used for AC voltage regulators (stabilizer).

100VA TO 10KVA POWER STABILIZER CIRCUIT

 

CLICK TO ENLARGE

Circuit Explanation

The circuit above is a Voltage stabilizer circuit designed and tested by me. The circuit uses an auto transformer for voltage stabilization. It has same useful functions as stabilizers currently on our markets. It has built in Delay, delay indicator, working indicator  and automatic switching.

The design accepts a wide range of input voltage ( 140V to 260V AC ). This does not mean that the circuit will fail when those voltage ranges are exceeded but its within these range that the AC voltage regulator will give constant 220V .

When power is applied to the unregulated input and S1 closed, Diodes D1 and D2 rectifies the 20V from the transformer and IC LM317 regulates its to 13V DC which is used to run the system. when the system starts working and the delay switch is closed, the output relay U2 will not switch until the delay time has passed. When delay time set by R1 and C5 is over, Q4 switches the output relay and the working indicator will now turn on to indicate working state.

To use the design for high power applications, use high current handling relays, else relay will fail.

HOW TO SET AUTOMATIC VOLTAGE REGULATION 

Automatic regulation is done using LM324 and LM358 configures as voltage comparators. To configure the system to work automatically, use a variable AC of 140V to 240V and adjust the variable resistors until their corresponding relays turn on at the voltages written on the variable.

Another way is to use a variable supply AC supply of 12V to 25V AC. when using this method, set your supply to 21.8V and adjust  the 240V variable until its relay turns on. Next, Adjust your variable supply to 20V and adjust 220V variable until its relay turns on. .....do the same for 200V at 18.1V, 180V at 16.4V, 160V at 14.5V and 140V at 12.7V AC. 

Please connect your variable supply at the 20V point.

Another way of setting up the system is to use variable DC supply of 33V-15V DC and adjusting the variable resistors to turn the relay on. At 31V adjust 240V variable until it turn on, at 28V adjust 220V, at 25.5V adjust 200V, at 23.2 adjust 180V variable, at 20.5DC adjust 160V variable and at 18V adjust 140V variable.

How to repair inverter part 1

Learn this simply way of repairing an inverter.

 

How to repair inverter

Inverter repairs  is not common and repairing a faulty inverter has not been an easy task for the local electronic repairers. This has made the few of us who does professional repairs to charge more especially when the inverter is a foreign made. I particularly do this to promote local inverter manufacturers as well as my products and discourage others from buying inferior foreign inverters which can even fail on same day. Lets start this first part of inverter repairs by knowing the types of inverters we currently have on market.

TYPES OF INVERTER

On the commercial market, the types of inverters seen are square wave two stages inverters, square wave single stage low frequency inverters, modified sine wave two stages inverters , modified sine wave single stage low frequency inverters, pure sine two stages inverters, pure sine single stage low frequency inverters.


TWO STAGES INVERTERS



Two stages inverters are inverters that first converts the input battery voltage to 300V - 310V DC using high frequency. This is achieved by switching ferrite transformers at high frequency. The square wave output of the ferrite transformer is then rectified using diode bridge rectifier and filtered using 400V electrolytic capacitors. This ends the first stage of the two stages inverter.
Before we go to the second stage, please know that the components in the second stage is powered by a separate 15- 25V winding on the ferrite transformer used for the 310V switching.
The second stage of two stages inverters works by converting the 310V DC to 220V AC pure sine or PWM or Square Wave using mosfet bridge or mosfet half bridge with an appropriate control drive circuit.


SINGLE STAGE (LOW FREQUENCY CONVERTERS)


This type of inverters consists of a driver stage (oscillator stage) and mosfet switches arranged in either a push pull ( for center tap transformer inverters ) or H-bridge ( for inverters without center tap transformers. This type has mosfets fixed on three different heat sinks.)

OSCILLATOR STAGES OF ALL CONVERTERS

Every inverter is made up of a control signal generation stage called the oscillator. The oscillator in lay mans term can be explained as an electronic signal that goes on and off at a set frequency. For inverters, the set frequency may be either 50 times in one second (50Hz) or 60 times in a second (60Hz)

Currently, inverters on market uses either a pulse width modulation (PWM) IC for their oscillation stage or a microcontroller with built in PWM.

COMMON INVERTER OSCILLIATOR ICS


commercial inverters and UPS are built around the following ICs.


SG3525

SG3524

TL494

CD40..

MICRO CONTROLLERS

Common Decision making ICs (OP AMPS) used in commercial inverters



LM324

LM358

LM339

LM471

LM258

ETC.
FUNCTIONS OF THE OP AMPS IN INVERTERS

Every good inverter has a number of operational amplifiers (OP AMPS) configured or designed to take simple decisions for the inverter. These decisions may include:


Battery Low

Battery Full

overload in some cases

high and low temperature

Fault.

In How to repair inverter part 2, I will tell you how these op amps can cause fault and how to remove or solve the faults in few minutes.
Now that you have idea of the compositions of an inverter, lets start our troubleshooting and repairs by reading INVERTER REPAIRS 2.

LOW BATTERY ALARM CIRCUIT

The circuit below will provide you a beeping alarm when your battery is low (below your low battery set point) and also turn off any circuit or device connected to it when battery goes below your second set point.

The beeping circuit consists of two transistor astable multi-vibrator which turns the buzzer on in short pulses avoiding continues high pitch noise.

Low Battery protection and alarm for Inverters and battery powered applications

LOW BATTERY PROTECTION AND ALARM CIRCUIT USING P-MOSFET

LOW BATTERY PROTECTION AND ALARM CIRCUIT USING RELAY

The circuit is designed around LM358 operational amplifier configured as comparator (click here to download data sheet). The IC has two different op amps (A and B) in a single package. Both op amps are given a common reference voltage of 4.7V or 5.1V using zener diode D2. Variable resistors U3 and U4 are used to set the operating points (thresholds) of their corresponding op amps.

low battery alarm Setting

When setting low battery alarm, monitor the battery voltage using voltmeter. Now let the battery run down to the point where you always want to be notified and adjust U4 until the buzzer start beeping and LED2 starts blinking.
Setting low battery protection or shutdown

After the alarm, let the battery drain some little more and adjust U3 until the output light and the alarm goes off. These settings are done once and it will always work for u as set.


Please note that, when the system turns off, it may not be able to turn on instantly unless the battery level has increased beyond a certain level which is set by R7. (Hysteresis). This functions sees to it that the circuit does not turn on and off unnecessarily.

To increase the turn off / turn on time, please decrease or increase R7.