Connect with us

2M+ ohm resistors not resisting

Discussion in 'General Electronics Discussion' started by schmidtbag, Dec 9, 2012.

Scroll to continue with content
  1. schmidtbag

    schmidtbag

    36
    0
    Nov 8, 2012
    2M+ ohm resistors not resisting *SOLVED*

    Just as a FYI, I'm not that great with electronics so if it seems like the answer is too obvious, its probably right.

    Anyways, I recently bought this DC voltage converter to convert 12V 20A to hopefully around 24V 4A. Unfortunately, I somehow missed the fact that it said stepped up to 35V-60V. The product works but the 2M ohm pot on it just isn't high enough. So, I removed it to place my own resistor in place (I don't need the pot, I just want a 24V output). I have some 4.7M ohm resistors (I believe they're 1/4W, although they could be 1/8W too) which I'm not sure would get me to 24V but would obviously get me closer.

    But, for some reason, the resistors don't seem to do anything at all. The power goes straight through them, and the power source puts on its overload protector. I tried 10M resistors too (which I think are also 1/4W) which also failed. I tried putting both kinds of resistors in parallel and serial, which didn't change anything.

    What I find interesting is if I try a 10 ohm, 1W resistor, everything works fine aside from the fact that I'm basically still well above 50V. I don't have any higher 1W resistors on me right now. So I'm just wondering, why don't the 4.7M and 10M resistors work, even in parallel? Is there a way I can use them? And if not, what would you recommend to get me to 24V?
     
    Last edited: Dec 9, 2012
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    The voltage adjustment trimpot in that module isn't 2M; it's 10k. This is indicated by the marking "103" on it, which means "1, 0, then three zeros", i.e. 10000 ohms or 10 kilohms.

    The module may not do what you want. Certain components, mainly the large inductor, are chosen to work over certain ranges of input and output voltage and current, and the board may not be able to convert 12V to 24V. Also, it may work but not be able to deliver the full output current. But you can try.

    If you want to change the output range, you'll probably need to change a resistor on the board. You should refit the original trimpot first, and adjust the output to minimum voltage.

    Now look on the underside of the board, at the thin track that comes from the single pin of the trimpot. You need to follow that signal. It is the "voltage feedback" signal that the control IC uses to regulate the output voltage. Probably it goes to one end of a resistor, and the other end of the resistor probably goes to another resistor that goes to ground, and to a pin on the IC.

    You need to decrease the value of the first resistor. That's the one between the trimpot and the IC pin. Figure out its value by reading the three-digit or four-digit markings, remembering that the last digit is the number of zeros to add after the first two or three digits. Then you need to replace that resistor with one with a lower resistance.

    Try a value of roughly half the present value. If that doesn't reduce the output far enough, use a lower value again.

    If you need more help, post a clear closeup photo of the area of the board that includes the trimpot's pins and the control IC.
     
  3. KJ6EAD

    KJ6EAD

    1,114
    159
    Aug 13, 2011
    The trimmer potentiometer in the product photo is 10k. Is yours really 2M? Is the part number on the SOIC-8 package on the bottom of the board MC34063A or something like that?

    Edit: Kris is way ahead of me on this. While I'm still typing, he posts a book, has it published and applies for a copyright. :rolleyes:
     
    Last edited: Dec 9, 2012
  4. schmidtbag

    schmidtbag

    36
    0
    Nov 8, 2012
    The trimmer is 3296, I checked the datasheet and it says it goes up to 2M. I too thought 2M was weird, however, the there is 60V running through the pins of the pot, so I have a feeling that a 10K resistor would not drop that down to 35V.

    Just in case I somehow misread the 3296 datasheet, I'll try something like a 20K resistor and see if that'll get me somewhere.
     
    Last edited: Dec 9, 2012
  5. KJ6EAD

    KJ6EAD

    1,114
    159
    Aug 13, 2011
    What are you talking about "3296"? The datasheet probably just indicates that the highest value in the series is a 2M pot. It has little to do with the one you have.
     
  6. KJ6EAD

    KJ6EAD

    1,114
    159
    Aug 13, 2011
    Bourns, that's what I thought. So which pot value do you actually have and what control chip?
     
  7. schmidtbag

    schmidtbag

    36
    0
    Nov 8, 2012
    If I turn the pot up all the way, I get a little more than 35V. If I turn it down all the way, it gets close to 60V, just as the product is advertised.

    There's 2 different ICs on the board and they're both really hard to read. As I said before, the voltage is already converted to 60V by the time it reaches the pot, so the ICs themselves aren't going to fail due to lack of power. I'm guessing that 1 IC increases the voltage while the other one is a voltage regulator.

    EDIT:
    I just tried a 22K resistor, and I'm still getting the same problem as the 4.7M. I should probably point out too that if I cut the power when the pot (or the 1W 10 ohm resistor) is in place, the LED on the converter slowly fades to off. But, if I use any of my skinnier resistors or leave nothing connected in that spot at all, the LED takes a couple minutes to fade but doesn't turn off completely until about 10 minutes later.
     
    Last edited: Dec 9, 2012
  8. KJ6EAD

    KJ6EAD

    1,114
    159
    Aug 13, 2011
    That's why, when we ask for specific information to help you, it's in your interest to provide direct answers to the best of your ability. :confused:
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    The "3296" is the name of a RANGE of trimpots. Bourns manufactures trimpots in that range with values up to 2M, but that doesn't mean that the trimpot on YOUR board goes up to 2M. Most likely it's a lot lower than that. Look for a three-digit number like 103 or 203 that will tell you the resistance of THAT trimpot.

    The way these regulators work is through a "voltage feedback loop". The output voltage goes through a number of resistors and the adjustment trimpot, which act as a voltage divider, and the output of this voltage divider feeds the feedback input of the control IC.

    The best way to solve this quickly is to take a clear close-up photo of the board that includes the trimpot pins and the control IC (or both ICs if you're not sure which one is which). KJ6EAD or I will be able to suggest a specific change that may allow you to reduce the output voltage to 24V.
     
  10. davenn

    davenn Moderator

    13,837
    1,952
    Sep 5, 2009
    yup probably true cuz its still 12k higher than the 10k pot thats in the circuit !! :)

    As Kris has explained earlier, its more than just that trimpot that controls the voltage setting ... its its interaction with the switching chip etc

    Dave
     
  11. schmidtbag

    schmidtbag

    36
    0
    Nov 8, 2012
    [​IMG]

    [​IMG]

    LED is circled:
    [​IMG]

    pot solder joints in green square:
    [​IMG]

    In the first 2 pictures, you can see I have 2 black wires in place of the pot, where I've been attempting to place resistors. Just curious but I thought that if the resistance was too high, electricity simply doesn't flow through. But in my case, its the exact opposite where everything flows through and disregards the resistors entirely, unless its that fat 10 ohm resistor.
     

    Attached Files:

    Last edited: Dec 9, 2012
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    That looks exactly like the ebay picture.

    I need to see a picture that shows the trackwork clearly. It needs to be zoomed more closely around the area that includes the trimpot pins and the controller chip (the 8-pin one).
     
  13. schmidtbag

    schmidtbag

    36
    0
    Nov 8, 2012
    So, I tried a 2.2K resistor and now the power source won't fail. But, that still means that if I go much beyond 10K, which doesn't really make sense to me. So, would it work if I just put a 10K in place of the pot and then maybe another 10K at the output? Or is it not that simple?

    EDIT:
    Kris, the pictures are shruken in the forums. Considering it's night time where I am and I have poor lighting, I can't get much closer without the camera blurring until its morning.
     
    Last edited: Dec 9, 2012
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    OK LISTEN.

    Changing the resistance of the trimpot WON'T WORK.

    You are going to have to change one of the resistors on the PCB.

    I can tell you WHICH resistor you need to change, and what value to change it to.

    BUT I need to see a close-up of the tracks in the area that includes the trimpot pins and the 8-pin controller IC.

    There is no guarantee that the board will work properly at 24V output, because other components on the board may have been chosen to work over a higher voltage range.

    Also please answer this question: What is the three-digit resistance value marked on the trimpot?
     
    Last edited: Dec 9, 2012
  15. schmidtbag

    schmidtbag

    36
    0
    Nov 8, 2012
    I submitted 4 pictures, the first 2 show what models the ICs are if you look at them in full view. The reason I've been focusing on replacing the trimpot with larger resistors is because the trimpot is how the designers of this PCB intended to alter the voltage, so to me it made sense that I could just keep taking on more ohms until it would be 24v.
     
    Last edited: Dec 9, 2012
  16. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    I can't read any numbers on the controller IC.

    I cannot tell where the tracks go in the area between the trimpot and the controller IC.

    You may be able to measure those tracks with a continuity tester but this will take a lot of time and so far you haven't shown much inclination to read my posts carefully and follow my instructions.

    I really need to be able to see where all the tracks go in the area that includes the trimpot pins and the 8-pin controller IC.

    Put a bright light on it, and perhaps try using a magnifying glass with your camera. Put the light and camera at a slight angle to avoid reflections.
     
  17. KJ6EAD

    KJ6EAD

    1,114
    159
    Aug 13, 2011
    Most cameras, even cell phones, have a macro setting available. It often has a menu icon that looks like a tulip. If you have one, use it.
     
  18. schmidtbag

    schmidtbag

    36
    0
    Nov 8, 2012
    I'm not sure how you can't see the pictures, their resolutions are in the mid to upper thousands and in the first 2 images the text on the ICs is perfectly legible. To double check that it wasn't just me, I viewed this topic in a proxy. Anyways to save time, I'll just send you the IC datasheets:

    http://www.alldatasheet.com/datasheet-pdf/pdf/24606/STMICROELECTRONICS/STPS20H100CT.html

    and

    http://www.alldatasheet.com/datasheet-pdf/pdf/23978/STMICROELECTRONICS/STP75NF75FP.html


    If for some reason you still can't view the closer image of the back, here's a direct link of it:
    https://www.electronicspoint.com/at...4-2m-ohm-resistors-not-resisting-100_2350.jpg
    If that STILL isn't close enough, I'll see what I can do.
     
    Last edited: Dec 9, 2012
  19. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    I said "8-pin controller IC". Eight pins.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-