Help with a car lighter inverter

[eccerr0r]

Re: Help with a car lighter inverter

UGH.. I think I found the problem in my inverter. I found a "103" (10KΩ?) 0603 resistor that measures infinite...

Now I need to find a replacement... I have a 1005 "1002" resistor in my spare parts dump which is a bit too big

Found one on a dead cdrom board... Soldered it in. After at least 5 years of this project backburnered, I see life in it. The LED turned green once more when it had never turned on ever since it failed. I need to resolder the transformer now and see if this thing lives once more.

What really pissed me off is that I bought this inverter to open and see what was inside. It worked at first. When I opened it to peek at its guts, it stopped working. I never got it working again. Being an honest tinkerer, I did not return the inverter despite it was under warranty because I could have damaged it... Now that I debugged it, I should have returned it, this was 100% manufacturing defect. Live and learn?

Allrighty my little 80W inverter is reassembled and working like new. That was one of the longer projects that took forever to rootcause *sigh* ... luckily I was able to find all the original screws and case parts despite it being disassembled for so many years. Tested it with my small bench PSU, quiescent current flow: ~250mA. With a 4W night light bulb, ~550mA. Regulation: horrible as expected. I could see the brightness of the night light bulb change as I adjusted the input voltage. The overvoltage and undervoltage inlet shutdown works, LED turns red and shuts down when I go over or under the limit...

[eccerr0r]

Re: Help with a car lighter inverter

For reference for my inverter, sorry for the threadjack:

Hazard Fraught Tools ... err Harbor Freight Tools / Chicago Electric Power Systems 80W direct plug in power inverter Item 66972

Design: modified square wave inverter using two KA7500 (TL494), two stage/H-bridge, isolated output. Open loop/No regulation.

Symptom: Inverter doesn't invert, fan turns on, LED remains red

Repair: Found R9 to be open, replacing with good 10KΩ resistor restored operation to normal. (The C1815 TO-92 transistor is there just for comparison.)

[eccerr0r]

Re: Help with a car lighter inverter

Apologies again for threadjack.

I hooked up this crappy inverter to my scope with a 10:1. Directly (hooray for isolation!) Yes, it's modified square wave, and I have to take back the observation it's not regulated - it does sort of regulate by changing duty cycle!

---

So, what's the current status of your inverter? Any luck?

[Mandraco]

Re: Help with a car lighter inverter

Sorry for taking so much time to respond. You're correct, there are instances when you get the car lighter plug shorted and there's no need to disassemble the device entirely, I've seen a few cases like that. That was the first thing I did, thanks for it's great advise. I will post the results of the tests as quick as I can today. I was a bit busy with other stuff, for example my lap top PC turns on but wont boot and I was fixing my clothes washer. As it was damaged during huricane Maria's stay over my country. I'm glad you got to fix your inverter :-)

[eccerr0r]

Re: Help with a car lighter inverter

Thanks. I just realized you're in PR and things like these inverters are probably lifesaving with the slow recovery... need to get these fixed, will do what we can to help!

I think I found your specific inverter on a website ... It appears to also have 5V USB like my little 80W - and likely also connected in parallel with 12V. Don't discount this segment causing the short, granted the max current that can pass through this is much lower than the main inverter.

[Agent24]

Re: Help with a car lighter inverter

And here is the picture of the plate I mentioned, Agent24. Notice the curved line on the right, it is like raised. If you pass the finger along the surface the line feels like a very fine bump. I said it seems like it made a cast but it could also be residues of the transistor's casing, I don't know. :-/

I would sandpaper\file it smooth, or flip it over before you use it again, just in case it caused the crack. With a crack that big in a transistor, usually it is because the part shorted out and exploded. I would expect a lot of soot residue from that, and since there isn't any, I think that plate/clamp may have been over tightened or was not smooth and it caused the crack.

If nothing else looks burnt, then my guess is that the crack has probably led to the transistor failing in some way which is causing the inverter to operate incorrectly and blow the fuse.

Have you tested the cracked transistor? Do you get the expected readings?

[eccerr0r]

Re: Help with a car lighter inverter

That holding plate is on the plastic side of the transistor so I would guess it's not critical to smooth it down. Perhaps take a soft plastic stick and rub over that line and see if it comes off, it may very well be made of condensed "magic smoke®". I would hope it's not necessary to tighten that retaining bar so tight to cause the plastic to crack, but it does need to provide some force to ensure good thermal contact is made on the other side of the transistors.

The high current path through inverters are through the primary of the transformer through the primary MOSFETs. Either MOSFET going through meltdown and shorting out will cause an effective short from power to GND. In fact I must blame MOSFET short out especially if you measure a short on the power input plug with less than 5V test voltage - since that would have a hard time turning the MOSFETs on and thus should not appear shorted.

[Agent24]

Re: Help with a car lighter inverter

I'm just surprised there isn't a ton of soot on the tab or legs of that cracked P55NF06.

[Mandraco]

Re: Help with a car lighter inverter

Hello again, I performed the test for 6 MOSFETs: The two P55NF06 and four IRF630B. I did the test using my multimeter's continuity/diode test mode. Results are bellow:

The cracked P55NF06: NEGATIVE lead to pin#3(SOURCE) and POSITIVE lead to pin#2(DRAIN)= shows a reading of 006.
With NEGATIVE lead to SOURCE and POSITIVE to GATE= 001 which is the same reading I get if I touch both leads together. I assume that is the "beep" in other DMMs with beep function.
For the MOSFET on the left of the first picture, NEGATIVE lead to SOURCE and POSITIVE to DRAIN does nothing. Then I touched POSITIVE lead to GATE while NEGATIVE is still on SOURCE, I get no reading but then I repeat NEGATIVE to source and POSITIVE to drain and get a reading slowly going up until about 2,000 and at that point the multimeter shows a "1" on the left side of screen as if there is open circuit/ no conduction.

For the others (IRF 630B I did the same. Neg to SOURCE and POSITIVE to DRAIN= nothing happens, then touch POSITIVE lead to GATE while the NEGATIVE is on the SOURCE, again nothing happens. I repeat NEGATIVE to source and POSITIVE to drain and this MOSFETs don't show a gradually increasing number, it just flashes a number around 1,500 and then goes to open circuit/ no continuity.

I also played with combinations like putting one lead to a pin and the other to the metal plate on the back of the transistor to see what happens and interchanging the leads. I think I read that the metal plate on the transistor is a ground, right?

I must say I also discovered that the fuse the inverter was using is 25 amps and the label states it should be a 20 amps fuse, the girl who gave me the inverter says it blew the fuses before she used a 25amps fuse. I'm currently waiting for the stores to open to get replacements to check if it blows with the MOSFETs off the boars, as you asked I do earlier. Around here during this week there are a few days off because it is a religious event.

Thanks for the patience, I'm sorry for taking so long to reply with the results :-)

[eccerr0r]

Re: Help with a car lighter inverter

It indeed looks like that the cracked P55NF06 has gone through meltdown and shorted, and this is why the inverter is shorted. Replace it and you may be up and running again, hopefully nothing else is wrong.

The other MOSFETs seem like they are okay but I get an uneasy feeling that they may or may not have been damaged by handling - they are VERY static sensitive and should not be handled out of circuit when not at an ESD safe work area... Hopefully they are okay since they seem to match or nearly match each other...

BTW the metal tab on the MOSFET with the hole on it is usually connected to the center pin - for MOSFETs this usually is the DRAIN and usually shouldn't be grounded. The rubbery thing is an insulator to prevent it being connected to the heatsink. This is the standard "TO-220" case.

[Mandraco]

Re: Help with a car lighter inverter

Hello, I was looking for the MOSFET, the P55NF06 and found a few sellers listing it "STP55NF06" while others list it without the "ST". Are they both the same? I thought ST could be a reference to a manufacturer (ST Microelectronics) but wanted to be sure.

[eccerr0r]

Re: Help with a car lighter inverter

Yes, ST usually denotes STMicro made it, but you can double check datasheets to make sure they're electrically equivalent. I'm pretty sure they are - this should be a semi JEDEC standardized part number.

(and yeah, there's a reason for a 20 amp fuse and not a 25 amp fuse...eek. That 20A fuse already seems a bit overrated (37%) as I have a 300W inverter that uses a 30A fuse (20%) and that 325W inverter that uses a 35A fuse (29% overrate).)

[Mandraco]

Re: Help with a car lighter inverter

Hello eccerr0r :-) when you say

That 20A fuse already seems a bit overrated (37%)

What do you mean?

[eccerr0r]

Re: Help with a car lighter inverter

That's because rated power output = 175W
Power input = 12V * 20A = 240W

(240-175)/175 = 37% -- part of this is inefficiency...heck all of this is either inefficiency or excess "overload" capacity. Bumping this to 25A gives it another 60W, not all of it can be converted, some went to... melting down the transistor.

I have a 400W pure sine inverter that has an even greater mismatch - it's got three 30A fuses in parallel on the 12V line for a total of over 1KW. Granted fuses in parallel is NOT a good solution hence it will not really go to 90A, but either way, that's well over 400W. I suppose they did it this way because of the parallel fuses not being ideal...

[Mandraco]

Re: Help with a car lighter inverter

eccerr0r, thanks for the response. It's been a while since last I wrote here, I was about to order the MOSFET for the inverter but found out a couple of cables that were soldered to the back of the board before are no longer in place and I don't know where is the place for all of them, I took a picture previous to manipulating the board but unfortunately it doesn't got all those cables so I was searching for a picture of that board online or a schematic. I also thought of ordering other parts I need for other devices, specifically some caps but want to ask if there is other specifications I need for the replacement capacitors other then voltage rating and µ (is it capacitance?). Can I ask it here or should I start another thread? Thanks in advance

[eccerr0r]

Re: Help with a car lighter inverter

Well if it's related to the car lighter inverter, why not? Perhaps posting pictures of the entire board - both sides, the wires to the devices they go, etc., maybe someone can figure out where they're supposed to go.

But if it's for some other device, it might be best to put it in its own thread so that people who know about the equipment can also chime in, you'll likely get more responses that way.

[Mandraco]

Re: Help with a car lighter inverter

O.K. I'll post a couple of pictures and the parts where those loose cable go to. About the caps my question is more in general, like asking what should I keep on mind when buying a replacement capacitor for any device, I already know the replacements should be the same voltage rating and that pico farads(I think that's what means the symbol "µ") but wonder if I would have to measure the capacitor's diameter and height. Thanks again!

[eccerr0r]

Re: Help with a car lighter inverter

Well there's a lot of information on this site, it's for capacitors after all!

There are a lot of characteristics for capacitors, you got some of them
Voltage, in volts
Capacitance, in farads. uF means "microfarads" or 1 millionth of a farad.
height
diameter/circumference/width/length
pin spacing/pin thickness
tolerance
temperature rating

There are more that aren't obvious by looking at them...
ESR/loss factor
liquid/solid electrolyte or separator, whichever it may be
leakage current (for long term storage, integrators, etc.)
polarization (is polarized OK or nonpolar is needed?)
shock sensitivity (ceramics are microphonic)
expected failure mode (X, Y-capacitors)
expected lifetime
cost...
more...

There's so many different parameters out there...

[Mandraco]

Re: Help with a car lighter inverter

Thanks for the lesson, eccerr0r. Of all those parameters I was only aware of capacitance, voltage and temp rating I think I need to check the capacitor's data sheet to get some of those. Thanks.

[Mandraco]

Re: Help with a car lighter inverter

Here is the first picture of the back of main board. The cables that go soldered somewhere on those solder points are the red in the left (marked with masking tape), the red and black on top right side and the blue one on the right( also marked with tape). I soldered the black and red ones on the right just to have them fixed to some place but I'm not sure if that's their place, I need to check before turning this thing on back again. The black and red on top left go to a board containing two USB ports.