E-cig repair, need help identifying a voltage regulator

[raider988]

Hello everyone,

I'm trying to repair an Evic Primo Mini powered by an 18650 Li-Ion battery. I managed to destroy it myself while being careless in repairing a connector... Long story short, I created a short and burned two components. One was a simple coil that I was able to replace, the other is small SOT-23-5 IC that appears to be a voltage regulator.

I checked all google results for the number on the IC (SG4HK) and couldn't find anything.

Looking at the suggested circuits in the datasheets of different regulators, I came to the conclusion that it must be either a boost converter, a buck-boost or a SEPIC, but I can't be sure, since it's impossible to find a schematic, even though the device is already a couple of years old.

I did some measurements around and created a small schematic with the components in that area of the board, hoping that some of you might recognize the configuration. The only thing I know for sure is the function of the IC: it provides a seemingly small load (under 100 mA I believe) to power up the part of circuitry that reads the atomizer resistence.

I need to find a way to test the circuitry, in order to find out how much voltage is actually needed, but at the moment I don't even have a good idea of where the output is located.

I attached an image of the board and the small schematic I made by testing the components. Any help is really appreciated!

[stj]

Re: E-cig repair, need help identifying a voltage regulator

related to this maybe
http://www.utc-ic.com/2011/0916/LR1101.html

[raider988]

Re: E-cig repair, need help identifying a voltage regulator

I don't think so, the chip numbering looked promising, but the LR1101 only uses 4 pins and has no feedback, while the IC I need seems to have it. The application diagram is also missing the 10uH coil.

[R_J]

Re: E-cig repair, need help identifying a voltage regulator

I found SGM6601 that uses the marking SG4xx H= year (2018) K=Month (November) Low-Power dc-dc step-up converter

[raider988]

Re: E-cig repair, need help identifying a voltage regulator

Sounds promising, but I have a question. When calculating the output voltage with the formula 1.227*(1+R1/R2), I get an output of 2.7v. Considering I measured 4.2v as input, is it possible that when I solder the component, the input voltage goes down to less than 2.7?

There is also a similar circuit on the other side of the board that uses the same IC (it's necessary for the oled screen to lit up) but different resistors, I tried removing the coil and the oled stops working. Can I use that as definitive proof that the IC is a boost converter?

[R_J]

Re: E-cig repair, need help identifying a voltage regulator

On the other forum site you posted, they seem to think it is a 3.3 volt regulator, So I really can't be sure, If you have another one on the board, check its input and output voltages, maybe that will give a better idea.
The SGM6601 is a step-up so the voltage should be more than the input. so I guess its not the correct ic.

[raider988]

Re: E-cig repair, need help identifying a voltage regulator

Ok, I think most of the problem had to do with me having no idea how a boost regulator works and being unable to desolder small 0402 components.

First of all, I desoldered the coil on the other circuit (let's call it oled circuit) and measured the input of the IC, it's 4.2v. I checked continuity between the inputs of both ICs and the result was positive, so they have the same input.

Next I checked again if the oled was working without the coil desoldered and it didn't work. Then I put the work into understanding how a boost converter works and realized that SW (on the IC) is NOT the output, the other side of the diode connected to cap next to the SW is the output. I reconnected the coil on the oled circuit and measure the output, it was variable depending on what was on the oled screen, but generally between 6.5v and 12v.

I think all the evidence is pointing out to a boost converter. My only doubt is still about the values of the feedback resistors. Could it be that by measuring without delsoldering, I incorrectly measured a lower value on that 180k resistor due to other circuits on the board lowering the overall resistance? Like... 5 to 6 times lower?

[R_J]

Re: E-cig repair, need help identifying a voltage regulator

The resistors may show the wrong reading if left on the board, If you measure the resistors on the working boost converter, how do they compare?

[raider988]

Re: E-cig repair, need help identifying a voltage regulator

On the oled feedback circuit I get a stable 56K for the resistor connected between FB and GND and 150K (rising up to 450k) for the other one. Also, I checked my schematic again and found an error, I attach the correct one.

[R_J]

Re: E-cig repair, need help identifying a voltage regulator

I do not know what the diode is, are there any markings? you may need to clean the diode with alcohol to read any markings

[raider988]

Re: E-cig repair, need help identifying a voltage regulator

Cleaned it with IP, this is the best I could come up with the phone. The first seems to be T4 or J4. The second is T4.

[raider988]

Re: E-cig repair, need help identifying a voltage regulator

I found the solution to the puzzle...

Since most of the evidence was pointing to a boost converter (and now I knew where the output was located), I thought I'd venture into different parts of the board, hoping to identify what that supposed higher voltage was going to power. I found two 8-pin ICs and I though "Bingo! I'll just check if they need a voltage higher than 4.2".

Spent a good couple of hours searching the net with no luck, but at least I learned that there are databases out there that index IC's markings; good to know for the future.

Anyway I went back to the board to see if I missed any other ICs and maybe get better luck.
AND THERE IT WAS, on the other side of the board, a probe point for my IC's output. Next to it, a strand of hot glue and some markings on the silk layer.



So, there it is, I am an idiot. A lazy, inconsitent, idiot.
I measured at the proble point, 0 volts. I thought about injecting 8 volts there and see what happens. Since I didn't have boost converter at hand to slowly raise the voltage from 1 to 8 and keep it safe, I just connected two 18650 in series, tapped in the middle for powering on the cigarette. And it worked, the circuit came back to life giving me a reading of the resistance, the cigarette was firing again.

What I learned is to always look at the fucking evidence with much more care, probe points are there for a reason and any clue is a good clue, especially when you're desperate. Well I also learned how boost converters work, at least the basics. Still worth it.

Talking about definitive repair, the fact that I can't desolder the feedback resistors to measure their value doesn't make me confortable at all. I'm sure I have no chance in hell to desolder such small resistors with a hand-held iron, no matter how thin I can make the tip. I have no idea how a boost converter would behave once I solder it: the 8v line seems to power a lot of stuff on the board, I could risk getting to the point of no return.
Since I have a little bit of real estate available, I think I'll just get a small boost converter board and connect that to the IC's V+, the OUTPUT and GND.

I'll check on Aliexpress something that resembles the performace of the SGM6601. Thank you R_J for the valuable suggestions, if you think I'm missing something, feel free to chime in.

I'll post pictures of the finished repair once I'm done.

[R_J]

Re: E-cig repair, need help identifying a voltage regulator

Cleaned it with IP, this is the best I could come up with the phone. The first seems to be T4 or J4. The second is T4.

Both diodes look like they are T4

If the diode is J4 it could be a MMSZ5249B-F 18 volt zener
If it is T4 it could be 1N4148WT fast switching diode