In this post we will try to learn how to diagnose and repair an inverter, by comprehensively learning the various stages of an inverter, and how a basic inverter functions.
Before we discuss how to repair an inverter it would be important for you to first get fully informed regarding the basic functioning of an inverter and its stages. The following content explains regarding the important aspects of an inverter.
Stages of an Inverter
As the name suggests DC to AC inverter is an electronic device which is able to "invert" a DC potential normally derived from a lead-acid battery into a stepped-up AC potential. The output from an inverter are normally quite comparable to the voltage that is found in our domestic AC Mains outlets.
Repairing sophisticated inverters are not easy due to their many involved complex stages and requires expertise in the field. Inverters which provide sine wave outputs or the ones which use PWM technology to generate modified sine wave can be difficult to diagnose and troubleshoot for the folks who are relatively new to electronics.
However, simpler inverter designs that involve basic operating principles can be repaired even by a person who is not specifically an expert with electronics.
Before we move into the fault finding details it would be important to discuss how does an inverter work and the different stages normally an inverter may comprise:
An inverter in its most basic form may be divided into three fundamental stages viz. oscillator, driver and the transformer output stage.
Oscillator:
This stage is basically responsible for the generation of oscillating pulses either through an IC circuit or a transistorized circuit.
These oscillations are basically the productions of alternate battery positive and negative (ground) voltage peaks with a particular specified frequency (number of positive peaks per second.) Such oscillations are generally in the form of square pillars and are termed as square waves, and the inverters operating with such oscillators are called square wave inverters.
The above generated square wave pulses though are too weak and can never be utilized to drive high current output transformers. Therefore these pulses are fed to the next amplifier stage for the required task.
For info on Inverter oscillators you can also refer to the complete tutorial which explains how to design an Inverter from the scratch
Booster or Amplifier (Driver):
Here the received oscillating frequency is suitably amplified to high current levels using either power transistors or Mosfets.
Though the boosted response is an AC, it is still at the battery supply voltage level and therefore cannot be used to operate electrical appliances which work at higher voltage AC potentials.
The amplified voltage is therefore finally applied to the output transformer secondary winding.
Output Power Transformer:
We all know how a transformer works; in AC/DC power supplies it is normally used to step-down the applied input mains AC to the lower specified AC levels through magnetic induction of its two windings.
In inverters a transformer is used for similar purpose but with just opposite orientation, i.e. here the low level AC from the above discussed electronic stages is applied to the secondary windings resulting in an induced stepped up voltage across the primary winding of the transformer.
This voltage is finally utilized for powering the various household electrical gadgets like lights, fans, mixers, soldering irons etc.
Basic Principle of Operation of an Inverter
The above diagram shows the most fundamental design of an inverter, the working principle becomes the back bone for all conventional inverter designs, from the simplest to the most sophisticated ones.
The functioning of the shown design may be understood from the following points:
1) The positive from the battery powers the oscillator IC (Vcc pin), and also the center tap of the transformer.
2) The oscillator IC when powered starts producing alternately switching Hi/lo pulses across its output pins PinA and PinB, at some given frequency rate, mostly at 50Hz, or 60Hz depending as per the country specs.
3) These pinouts can be seen connected with the relevant power devices #1, and #2, which could be mosfets or power BJTs.
3) At any instant when PinA is high, and PinB is low, the Power Device#1 is in the conducting mode, while Power Device#2 is held switched OFF.
4) This situation connects the upper tap of the transformer to ground via the power device#1, which in turn causes the battery positive to pass through upper half of the transformer, energizing this section of the transformer.
5) Identically, in the next instant when the pinB is high and PinA is low, the lower primary winding of the transformer becomes activated.
6) This cycle repeats continuously causing a push-pull high current conduction across the two halves of the transformer winding.
7) The above action within the transformer secondary causes an equivalent amount of voltage and current to switch across the secondary by means of magnetic induction, resulting in the production of the required 220V or the 120V AC across the secondary winding of the transformer, as indicated in the diagram.
DC to AC Inverter, Repairing Tips
In the above explanation a couple of things become very critical for obtaining correct results from an inverter.
1) First, the generation of the oscillations, due to which the power MOSFETs are switched ON/OFF, initiating the process of electromagnetic voltage induction across the primary/secondary winding of the transformer. Since the MOSFETs switch the primary of the transformer in a push-pull manner, this induces an alternating 220V or 120V AC across the secondary of the transformer.
2) The second important factor is the frequency of the oscillations, which is fixed as per the country’s specifications, for example countries that supply 230 V, generally have a working frequency of 50 Hz, in other countries where 120 V is specified mostly work at 60 Hz frequency.
3) Sophisticated electronic gadgets like TV sets, DVD players, computers etc. are never recommended to be operated with square wave inverters. The sharp rise and fall of the square waves are just not suitable for such applications.
4) However there are ways through more complex electronic circuits for modifying the square waves so that they become more favorable with the above discussed electronic equipment.
Inverters using further complex circuits are able to produce waveforms almost identical to the waveforms available at our domestic mains AC outlets.
How to Repair an Inverter
Once you get well versed with the different stages normally incorporated in an inverter unit as explained above, troubleshooting becomes relatively easy. The following tips will illustrate how to repair DC to AC inverter:
Inverter is “Dead”:
If your inverter is dead, do preliminary investigations such as checking battery voltage and connections, checking for a blown fuse, lose connections etc. If all these are OK, open the inverter outer cover and do the following steps:
1) Locate the oscillator section; disconnect its output from its MOSFET stage and using a frequency meter confirm whether or not it is generating the required frequency. Normally, for a 220V inverter this frequency will be 50 Hz, and for 120V inverter this will be 60 Hz. If your meter reads no frequency or a stable DC, it may indicate a possible fault with this oscillator stage. Check its IC and the associated components for the remedy.
2) In case you find the oscillator stage working fine, go for the next stage i.e. the current amplifier stage (power MOSFET). Isolate the MOSFETS from the transformer and check each device using a digital multimeter. Remember that you may have to completely remove the MOSFET or the BJT from the board while testing them with your DMM. If you find a particular device to be faulty, replace it with a new one, and check the response by switching ON the inverter. Preferably connect a high wattage DC bulb in series with the battery while testing the response, just to be on the safer side and prevent any undue damage to the battery
3) Occasionally, transformers can also become the major cause for a malfunction. You can check for an open winding or a loose internal connection in the associated transformer. If you find it to be suspicious, immediately change it with a new one.
Although it won't be that easy to learn everything about how to repair DC to AC inverter from this chapter itself, but definitely things will start "cooking" as you delve into the procedure through relentless practice, and some trial and error.
Still have doubts...feel free to post your specific questions here.
Hi, don’t exactly know how it happened, but there was some sort of a short working through the earth wire, which burned off the earth wire internally. This is on a 2kw digimark inverter. The unit is now faulty. What else could have blown in this case, any idea? In your opinion, is it worth sending this inverter in for repairs?
If the earth wire is burned it means the LIVE wire has somehow short circuited with the body of the inverter. Hopefully if the LIVE is directly shorted with the body then no parts could be damaged however if the short has happened through the circuit board then most probably a serious fault might have taken place on the circuit board.
I think what happened was that the borehole pump acted as a generator, and I plugged it out and put the plug on the ground, where one of the plug terminals then must have touched the ground strap. Because there is a second bore hole pump sharing a level switch, I am thinking that this is how the current then found a route through the circuit board. Do you reckon it may be repairable?
Yes, in that case it is repairable, according to me.
Thanks for the info
I accidentally shorted the battery cables that feed my psw inverter. It is dead. Any chance of bringing it back? The owners manual says it is protected from shorts.
If you shorted the battery cables then it is the battery which is supposed to get damaged, not the inverter. Please replace the cables and check the inverter again, it should be working fine.
If the inverter output is blinking when on load what is the solution
We bought an invetor which we just run the 2 tv on and wifi. Been working fine for months. My daughter plugged the wrong plugs in today s heater . We sat for an hour and the next minute a massive trip sound and all was dark. Is the invertor fixable. My husband is furious as it was so expensive.
If the trip sound was like an explosion then something might have burned inside. It will need to be checked practically to know the exact fault.
Pls I have a blue Gate inverter 2kva..it is not powering up.I have checked all the mosfets they seem to be ok from my DMM reading,what could be the likely issue here
Please check the fuses, if it is OK then check whether the battery supply is reaching the circuit board or not, then check whether oscillator section is working or not.
What happen when a transformer is giving heat
It means either there is an over voltage or an over load connected with the transformer.
My daughter turned on the utility supply on in the DB board before she disconnected the inverter. I’ve replaced the blown fuses and the inverter now turns on but does not supply any AC power. The fault light is not on
The utility supply could be completely destroyed the MOSFETs and other valuable devices of the inverter according to my knowledge, since the utility AC across the transformer primary might have caused an AC to be generated at the secondary of the transformer damaging the devices.
I notice the inverters are grounded in the refrigerators and the refrigerators have a ground. I’m getting 43 volt intermittent reading from the refrigerator ground to earth ground. Do these inverters use the unit ground as a French ground while the unit runs and creating this reading? I have checked the house wiring, it’s OK and this fridge is on it’s own circuit. Thanks for any in-put you may have.
Sorry, I have no idea about this situation, hope somebody else in this forum will be able to throw some light on this.
Good day,got a 300w Goldstone invertor/charger.fuse was blown.somebody tampered with switch wires inorder to fix it…how should the Invertor or Charge wires be connected?
Sorry, I have never opened a Goldstone inverter so solving your query can be difficult.
Power drive 1500. Normal voltage at rest. Loaded with water pump (5.5 amps) output voltage goes to 135.
Dear Swagatam,
My inverter has a continuous beep with code 7 indicating that it is an overload fault. But nothing is connected to it. Could there be an internal overload? The fan is working well.
Any suggestions to what I should look for?
Hi Claudius,
Error 7 might indicate an internal temperature sensor failure. But you are saying that the fan is working alright. So it may be difficult to judge the fault without a practical check.
If you think it is due to overload, you can refer to this article:
https://sakopower.com/wp-content/uploads/2022/03/How-to-check-error-code-07.pdf
Hello , I have a doxin 3000w which is coming on but the fault light comes up then goes out but no HV. MOSFETs and all of the usual suspects are ok. Any advice? Thanks for your time.
Hello, further practical tests will be needed to identify the fault, it cannot be judged without testing.
Can one convert an inverter to a normal ac?
I have a Chicago elec 2000/4000 inverter. The problem is:. Turns on and I push trigger on jigsaw. It runs for as long as I hold trigger. Once let go of trigger alarm sounds and voltmeter on front completely lights up passed 15v dc. Turn off and then back on can use again untill I let go of trigger
Sorry, no ideas! I do not have sufficient information regarding Chicago elec 2000/4000 inverter specifications and working!
Thanks for your time. Great site.
My pleasure!
Thanks for posting such an excellent article. It helped me a lot and I love the subject matter.
Very nice explanation..Thanks
Hello Swagatam , I have a 4000w inverter that I tapped into the 120v on my sailboat. It worked just fine. The line that I tapped into also runs to the shore power, so I disconnected the shore power from the boat. So when I wanted to run the inverter I would make sure that the shore power was not connected. I didn’t have the inverter turned on one day and went to plug the shore power in. I didn’t get any power, so I unplugged the shore power and went to turn the inverter on. The display came up but it didn’t show the AC voltage rising up to 120v. It just shows oo volts. Then an alarm sounds and I get a VDC overvoltage on the display. I removed the casing and found a bad cap(3300uf 35v), I replaced the cap, hoping that may have been it, it wasn’t. The overvoltage still comes on,with no AC voltage. Can you to the best of your ability let me know what you believe may be at fault.i looked the two boards over pretty close and could not fine anything burnt or singed or discolored. Thank you so very much, I’m very happy to have found your site and have bookmarked it. Thank you again, Richard.
Thank you Richard,
From your explanations what I understand is, the shore power and the inverter both are used to supply 120 V to the sailboat. So I am wondering how the problem occurred? Was it due to shore power entering the inverter 120 V socket while both were connected to the same line? I am not able to figure out this aspect.
You also said the display is showing over voltage. From where is this over voltage occurring, is it from the battery DC side or the 120 V AC side of the inverter transformer?
So the above doubts will need to be first clarified before we understand the actual issue with the inverter. I am glad you found this site useful. I hope your problem will be solved soon.
Hello Swagatam, yes, the inverter was connected at the sc terminal block to to house wiring, which continues to the shore power socket(both are 120v). The inverter was turned off,so when I plugged the shore power in, it had to have 120v at the terminal blocks on the front of the inverter. As for the display, the VDC overvoltage must be coming from the batteries12v.thats the only thing connected. There is nothing connected to the ac side, no wires or plugs.
Thank you Richard, could there have been a possibility of the 120 V from the shore power entering the inverter 120 V side? In that case the power devices of the inverter could have easily blown off.
The over voltage of the batteries is not supposed to affect the inverter working, it should be more related to an automatic battery charging cut off system.
With the battery fully charged the inverter should have no problems operating normally, since the battery is fully charged and ready to use.
It might be actually difficult to assess the exact fault without actually checking the device practically.