TV wont work from Inverter - Page 1
Forum section: electronics / repair

author: hazmoment
author_rank: Contributor
timestamp: November 22, 2018, 06:57:30 pm
content: Hello, Ive been asked to check out why a friends tv has stopped working from an inverter. It used to go, and have proved the inverter is working by plugging in a resistive load (lamp) seperately and at the same time to ensure enough loading on the inverter to output.Tv works sweet on the mains. I have even tried a replacement Power supply board in the tv.Is this something to do with modified sine waves and the TV power supply not liking it? If so I cant understand how this has ever worked.. Its not overloading the inverter, and will power my lamp ok. Friend has tried another inverter to find the same problem.Im hoping to try to get my hands on a better quality inverter to see if that makes any difference. like a pure instead of modified.Any smart ideas out there, things to check??Cheers!Harry

author: wraper
author_rank: Unknown
timestamp: November 22, 2018, 07:03:14 pm
content: Have you thought that inverter is faulty? Testing inverter with resistive load is not better than testing TV from mains voltage.

author: hazmoment
author_rank: Contributor
timestamp: November 22, 2018, 07:05:29 pm
content: Hi, Yeah i have considered the inverter being faulty, but it does power up other devices, like satellite decoder, incandescent lamps etc.The tv has also been proven to work fine when plugged directly into 240V mains..

author: wraper
author_rank: Unknown
timestamp: November 22, 2018, 07:08:24 pm
content: Modern TV likely has PFC, sattelite decoder doesn't.

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 22, 2018, 08:03:37 pm
content: wraper has a likely explanation.Has your friend recently changed anything in his setup that may have reduced the line inductance between inverter and TV?What model is the TV? Did it not run with the replacement PS board?If the PSU schematic is out there it would be interesting to figure how to repair or modify it to run on modsin inverter.A first step could be to check the inverter's output amplitude in case it is causing an UVLO or OVLO in the TV PSU, then you would be looking for capacitors that could have degraded faster due to the "square" nature of the supply, or problems caused by ringing.Have you considered trying to feed the TV through a large series inductor just as a test?

author: coromonadalix
author_rank: Super Contributor
timestamp: November 22, 2018, 09:23:06 pm
content: If your inverter has a pseudo square - sinusoidal sine wave some electronic devices simply wont work .....I have a Lenovo adapter who refuses this kind of sine wave, if i use a pure sinewave it work flawlessly ..... tried with microwave, tv's portable adapters, vacuum, even some power tools reject this, trashed the pseudo sinewave inverter and bought a 3kw Xantrex pure sinewave, never had any problems since.Lots of people dont know that, and most sellers dont ...........

author: langwadt
author_rank: Super Contributor
timestamp: November 22, 2018, 10:16:18 pm
content: AvE showed his fancy coffee machine would only work on an inverter when a blender was running, the motor helpingto "sinusify" the squarewaves

author: wraper
author_rank: Unknown
timestamp: November 22, 2018, 10:35:13 pm
content: Forgot to mention one thing about PFC. While high voltage capacitor is charging, it can take high current even when input voltage is low. Thus many UPS and inverters don't like them and shut down. Also generally PFC don't go well with "modified sine".

author: hazmoment
author_rank: Contributor
timestamp: November 22, 2018, 10:51:49 pm
content: Sent from my iPhone using Tapatalk

author: hazmoment
author_rank: Contributor
timestamp: November 22, 2018, 10:59:11 pm
content: Pics above: First one is the power supply board in question in the tv.The second pic is probing on the input of the diode bridge on mains connection, then on inverter. Then probing after diode bridge on main filter cap mains, then probed again on inverter.As can be seen there is around 44v drop between mains and inverter.What would be your suggestion to try an inductor in circuit? Just any coil in series with the main voltage in? It looks like there are a pair of inductors one for phase and one for neutral coming in on the pcb?Really keen to understand what the problem is here as I have tried a different tv with the inverter and powers up no worries.Sent from my iPhone using Tapatalk

author: chris_leyson
author_rank: Super Contributor
timestamp: November 23, 2018, 01:20:03 am
content: What does your TV instruction manual say about mains operating voltage range ? If you've only got 290VDC across your bulk mains smoothing cap and then that is eqivalent to 200VAC from a sinewave source. If your TV is specified to work from lets say 210VAC to 250VAC for example the the under volts lock out on the primary side switcher will say no go at 200VAC. For 230VAC mains you should expect something like 340VDC across your primary side bulk smoothing cap.From the scope measurements it looks like your quasi sine inverter isn't really up to the job, the peak voltage isn't high enough it should be 340VDC not 290VDC. If there is a volts adjust pot in the inverter try tweaking it up a little bit but do it on an incandescent bulb load first.As regards the TV psu, the primary circuit is very simple to trace out, it's marked out on to the top side silk screen, fuse, X cap, 2 Y caps, common mode choke, 2 Y caps, X cap, common mode choke, bridge rectifier and bulk smoothing cap with a few series ferrite beads added in for good measure.

author: hazmoment
author_rank: Contributor
timestamp: November 23, 2018, 08:28:04 am
content: Hey thanks for your suggestions!I decided to go back in the workshop and run the tv up on the variac, it will boot up and operate on anything above 160vac RMS. So not satisfied that the low voltage reading I got from the Invertor is my issue. Checked the output of the Invertor with my DMM and getting 236volts with no load.Definitely something still to do with the tv's circuitry and the modified sine wave the invertor is providing. Weird. Looked for a volts adjustment pot within the Sp1000 invertor and there are 3 trim pots but nothing is marked up.at a bit of a loose end, My next task will be to track down a loan invertor to try out.

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 23, 2018, 08:28:43 am
content: Maybe something like the primary of a large laminated transformer for series inductance, the point being to smooth the squareness a little.Your friend's TV must be a smallish model there is no active PFC.The UVLO could be something to explore rather than readjusting the inverter, you could find the lowest start voltage with a lab supply or variac if you have one.The downside to upping the inverter's output amplitude is that you may end up with an RMS output well above 240V, that could be a problem for other loads.

author: hazmoment
author_rank: Contributor
timestamp: November 23, 2018, 08:37:09 am
content: Thanks. Cool, I will be able to try this series inductor suggestion with an old power transformer for a valve radio i have. Would it be sufficient to just wire the phase conductor through the inductor, or would the neutral require the same treatment?I tried your suggestion with the variac and was suprised at how low in voltage the tv will operate at.It is a small 32" Led tv. Its probably not worth all of the hassle I am going to but I cant give up without finding out why!

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 23, 2018, 08:50:00 am
content: Can we see the copper side of the PSU? (PWM IC used and how the start supply is made)With the variac check the voltage you have at the PWM's Vcc when you reach starting voltage, check same voltage when using inverter paying extra attention to ripple or noise (AC coupling on the scope.)The X capacitors in the line filter may have aged prematurely due to the faster rise time of the inverter.Another thought (again as a consequence of the faster rise time of the inverter) is higher leakage current transformer and Y caps maybe bogging the feedback loop.

author: hazmoment
author_rank: Contributor
timestamp: November 25, 2018, 06:56:49 am
content: So I managed to get the tv to start up from the inverter by using the series inductor suggested!I wired a choke up in series with the main from the inverter into the tv. Boots up no worries.As the power pcb in the tv has been replaced this takes away any possibility of degraded components in the tv being the cause of the issue and more towards the inverter being the issue. What could have degraded in the inverter leading to this fault? As this inverter has aways done the job until recently.Would caps be the immediate suspect in this case? see attached pic of ugly wave form after choke filteringSent from my iPhone using Tapatalk

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: November 25, 2018, 10:20:41 am
content: Quote from: hazmoment on November 25, 2018, 06:56:49 amCoil oscillator I would say, lol.

author: hazmoment
author_rank: Contributor
timestamp: November 25, 2018, 05:23:09 pm
content: Would it look this nasty because I probed the output without any load connected?Sent from my iPhone using Tapatalk

author: wraper
author_rank: Unknown
timestamp: November 25, 2018, 07:46:12 pm
content: Quote from: hazmoment on November 25, 2018, 05:23:09 pmOf curse, probing without a load is completely useless.

author: hazmoment
author_rank: Contributor
timestamp: November 26, 2018, 09:13:54 am
content: Thanks for the help. I will probe it again with the load connected and put up another pic.Sent from my iPhone using Tapatalk

author: hazmoment
author_rank: Contributor
timestamp: November 29, 2018, 05:28:03 pm
content: So I tried the tv with another inverter (actually a UPS) and it didnt like this either.This is the waveform from the ups, under load from the tv.Will I notice a difference with a “pure” sinewave inverter over these modified/simulated inverters? Whats the difference in the two types?Sent from my iPhone using Tapatalk

author: langwadt
author_rank: Super Contributor
timestamp: November 29, 2018, 06:19:01 pm
content: Quote from: hazmoment on November 29, 2018, 05:28:03 pmas the name implies pure sinewave inverters make sine waves, modified sine wave is what your waveforms show; equal time positive, zero, negative, zero. It is not a sine wave but it has less harmonics than a pure square wave