1000W 12V/220V Power Inverter Repair with 5A Power Supply (10 seconds of short) - Page 1
Forum section: electronics / repair

author: donkey
author_rank: Regular Contributor
timestamp: December 05, 2018, 09:16:00 am
content: Hi there everyone!I have a 1000W 12V to 220V Power Inverter. I have found 2 bad transistors and replaced them with the new ones. The short signal on the transistors have gone. When i test the continuity between the input terminals i get a short signal reading between the input terminals only for about 10 seconds and after 10 seconds, the short switches to the other polarity. The both Power and the Fu led lights are not lighthing up. When i test the device with a 30V/5A Bench Power Supply with a fixed 12V regulation and maximum current output regulation the power supply displays 4.8 Ampere and 4.3 Volts.When there is no load is connected and there is no shorts at the output: however i thought that 4.8 Ampere has to be far more sufficient to turn on the inverter; i tried to connect the inverter to a 12V car battery with a 10 Ampere fuse to rule out if my power supply can not supply enough amperage to turn on the unit but the 10A fuse is blown immediately as i connected the therminals to the car battery.Input resistance is building up from 0 to 27 Mohms (in 10 seconds)Output resistance is 218 kohmsI have never used an inverter before and i do not know how much ampere it needs to start up.I have 2 questions:1) Is this still can be an amperage issue ? ( Is (10A) not enough to start up the inverter) Which fuse(A) i have to use to test the inverter?2) What may cause the short in the input terminals for 10 seconds and switches it to the other polarity (In a continues sequence). Is this means that there is still a short in somewhere else or is this a false short reading because of a capacitor or some other components?Thank you!

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 05, 2018, 09:55:00 am
content: short for some seconds is just a biiig capacitor chargingput your supply to 12V/200mA for example and attempt to start the inverter like thisin fact you have some short or incorrect commutation, when repairing stuff don't set max current on power supply limit never ever.you risk the DUT not to power up, but avoid burning again power transistors and adjacent stuff..OR you series something everyone has like a car bulb (phere in french, don't know in english the word)you missed something in control logic ,I guess,if short dissapears after 10 secs it'n not really 'shorted' power transistors but maybe IC powers up tre transistor(s) when powering the dut.you need to test if the ic really commutes the transistor(s)regards,pierre

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 05, 2018, 10:09:52 am
content: Quote from: donkey on December 05, 2018, 09:16:00 am1>no.start the inverter without load on his output.and he don't gonna need 10 amps for starting.is he has slow start, maybe 1 amp is ok.test gradually increasing you power supply current limit from 0.2A up2>test using diode tester, not resistance.and tell us results after waiting some seconds to stabilising the reading on multimeter.in one direction maybe you find a diode for protection (A on Gnd, Kathode on +12V, reding may be as low as 0.2V for shottky protection diode) and in opposite direction you'll 'see' the power stage (much bigger value, min 1.xxx V on diode tester)Enough with learning, let's hear the practicepierre

author: donkey
author_rank: Regular Contributor
timestamp: December 05, 2018, 10:30:17 am
content: Thank you for your answer Perie,I regulated the output of my power supply to maximum is because of the Current Control (C.C) light of my power supply unit is lighted up from 100 mA until the 5A.I have once tested a D-Link Network Switch with a wierd startup response which looks like this one. I set the voltage of my power supply to 12 Volts and started to regulate the Amperage from 100 mA to over 2 Amperes. Until i regulated the power supply until over 2 Amps, power supply acts like the switch has a short in it (The Current Control (C.C) light is lighting up and the current regulation is climbing as i regulate the amperage) but when it exceeded the 2 ampere readings the Current Control (C.C) light goes off and the unit starts to draw only 200mA from the power supply. I really do not know what is behind the scenes but for some reason however the unit draws only 200 mA after it fully boots up i had to supply the unit more then 2 Amps of current to make the device worked properly.I thought that may be there is a same boot up scenerio in this device.Thank you!

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 05, 2018, 10:37:40 am
content: Quote from: donkey on December 05, 2018, 10:30:17 amlet the LED light away.when the capacitors on inverter input charges they will trigger LED for crt limit until charging is done.normal.then the inverter starts doing his job.never test with full amperage, just put enough current limiting that the device can start.no need for amps.forget the LED....sometimes the device is not starting, you can increase the current checking who's getting hot on DUT, that's the one and only practice, no negociation!

author: donkey
author_rank: Regular Contributor
timestamp: December 05, 2018, 10:39:43 am
content: The input terminal reverse bias reading (A on Gnd, Kathode on +12V) is 0.47 VoltsForward bias reading is over 1 VoltsI regulated the Voltage to 13.3 Volts and regulated the Ampere to 0.3 Amps. As soon as i switch the DUT the Voltage readings of the power supply drops to 3.4 Volts.It has been 3 minutes the device is under test and nothing works and nothing smokesThank you!

author: donkey
author_rank: Regular Contributor
timestamp: December 05, 2018, 12:28:38 pm
content: Ok, there are two TL-494 Pulse Width Modulation IC's in the board. When i connected both IC's pin 7(Gnd) and pin 12 (Vcc) separately and directly to the power supply the fan starts working. Power led didn't lit up but what I assume was; if there was a short around, the fan did not work and it won't draw only 180mA.Am I correct?What can be the problem?Thank you!

author: donkey
author_rank: Regular Contributor
timestamp: December 05, 2018, 12:48:37 pm
content: Ok! I tried to connect both TL494 Pulse Width Modulation Control IC directly to the power supply at the same time. The Power led did not lit up (I know that the Power led is good because i checked it out). I read 4.9V AC readings at the output.What may be the problem with the Inverter ?Thank you!

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 05, 2018, 01:44:18 pm
content: Hi!How long is a piece of string?!Seriously tho' we need some good clear pictures of your unit:–1) Outside views showing make and model:–2) Internal views of pcb(s), heatsinked power transistors, control board if separate, main inverter transformer,3) If anything appears burnt or damaged, a clear close–up picture of the damage, we may be able to identify the failed parts;Directly applying 12V TL494 PWM controller rarely works because internal protective circuitry is in operation that may shut the output stage drive down, either directly at it's output pins or sometimes via the error amplifier pins, or disconnect the supply from driver/output stages via protection relays!You need to remove and test all the power semiconductor diodes and transistors for faults, and pay particular attention to the small electrolytics associated with the TL494s themselves!The theoretical input current at 12V would typically be 80 A but in practice, this sort of device would only provude about half that, 400–500W continuously, and don't forget the normal size of car–battery would only run these inverters up to about 300W maximum!(The current figures above are "theoretically" what the unit would need at 12V input to provide the "claimed!" power output, but in practice, you only need a 40/60W household filament lamp to provide a safe working load, and at this load the inverter would require 4–5A at 12V.Off–load with nothing connected it should take less than 1A at 12V)We can advise better when we've seen pictures!Have you looked for example 1kW inverter circuit designs on Google? – you might find your unit is based on a published example you can work from!Chris Williams

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 05, 2018, 06:40:05 pm
content: Quote from: donkey on December 05, 2018, 10:39:43 amSo 1'st step is ok (readings from multimeter says it looks ok).NextIncrease current and look if you have some output voltage with a load not too big, like a signaling bulb,5w.or a little bigger than minumum inverter load, if specified.Tell us the result.dont go more than 1 amp AND see if something worms.Applying more risk you burn power stage.If you see some voltage rising on output means 494 does his pwm job and maybe after you can fully start with a battery.we aren't yet there for apply 80 amps on repaired stage.Regards,pierreEnvoyé de mon iPad en utilisant Tapatalk