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Thread: High current digital pot

  1. #11
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    LOL ya true Texas ...I seen many electricians touch live 120v ac with bare hands and tell me yes this outlet is hot/live...
    but I like my fingers to stay pinkish white .

    okay here is my schematic of a test and it didn't perform with results I was hoping for
    of course what i gained from this test is to have mosfets moved from the drawer to the breadboard
    and maybe back to the drawer ...I think I need to use some special relay or something to make this
    hill billy digital pot work ...but its fun learning how not to make something
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  2. #12
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    Mark, for those mosfets to work, you have to be switching a resistor junction to a common line that is common to the mosfet SOURCE connection (ground!) So, your program would be to successively "short out" parts of the resistor string to adjust the total value. That may require you to change the value of the ladder of resistors.

    Your choice of mosfet requires about 10 volts gate to source to get it to fully conduct. It probably won't be very active at less than 5 volts in any configuration.
    - Tex
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    "Truth lies dormant in our future history." ― Tex Clodhopper LXV
    “Confidence is ignorance. If you're feeling cocky, it's because there's something you don't know.” ― Eoin Colfer, Artemis Fowl

  3. #13
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    thanks texas I knew something was wrong but couldn't name the "something" ... I am trying to find micro relays ...but all I get is solid state ones dc to dc...
    which i don't think it would work for this application either...once I get relays for a test then I should be able to control
    the digital pot with binary ...it will be interesting.

  4. #14
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    And still, I don't know if you are building a golden shovel to pick your nose.
    - Tex
    __________________________________________________ _______________________
    "Truth lies dormant in our future history." ― Tex Clodhopper LXV
    “Confidence is ignorance. If you're feeling cocky, it's because there's something you don't know.” ― Eoin Colfer, Artemis Fowl

  5. #15
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    Well texas its real secret ...because i don't even know what i am building
    my idea was to make a digital pot using 74ch595 with micro relays and resistors
    now since I found a small relay like this one..(or maybe something better)
    https://www.alliedelec.com/m/d/1e9bf...075b80bba3.pdf
    az850-5
    I am going to use two 74ch595 with resistors like 1,2,4,8,16,...
    the whole project is learning ...my result at the end of the year should have a
    digital pot from 0 ohms to 65,565 ohms .5W ..why? whats the point?
    because I think it would be cool to build one from scratch
    the resistors might be a pain to get but I will keep looking.
    maybe its a horrible idea but I want to see if I can pull it off

  6. #16
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    If we are moving away from the welder application and on to just a generic resistance box then I would say relays are probably a good option; just switch resistors in the chain in and out as required.

    I would check on what the current ratings are for a 74HC595 but it should be easy enough to add ULN2803A Darlington drivers if required.

    Getting resistors in non E-series values might prove difficult but you can probably daisy chain or parallel standard resistors to get what you need.

    It would also be an opportunity to go further, perhaps have a rotary encoder and an LCD/OLED display to show what resistance has been selected.

  7. #17
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    Using E12 series ...

    1 = 1R
    2 = 1R + 1R
    4 = 12R | 12R | 12R
    8 = 12R | ( 12R + 12R )
    16 = 10R + ( 12R | 12R )
    32 = 10R + 22R
    64 = 27R + 27R + 10R
    128 = 18R + ( 220R | 220R )
    256 = 100R + 100R + 56R
    512 = 12R + ( 1K | 1K )
    1024 = 12R + 12R + 1K
    2048 = 68R + 180R + 1K8
    4096 = 3K9 + ( 220R | 1K8 )
    8192 = 10R + ( 15K | 18K )
    16384 = 15K + ( 1K5 | 18K )
    32768 = 6K8 + ( 680K | 27K )
    65536 = 68K | ( 820K + 1M )

  8. #18
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    Hi,

    Note that that relay is rated to only 30 watts, 1 Amp switching and 50 milliohms "initial" contact resistance. So not too good for a Welder and 16 relays might give you almost one ohm before you start.

    Quote Originally Posted by newplumber View Post
    ...should have a digital pot from 0 ohms to 65,565 ohms .5W .
    Beware that a "variable resistor", a "potentiometer" and a "voltage divider", might do much the same thing but are fundamentally quite different. A potentiometer has three terminals and two resistive parts (from the wiper to each end), but of course you can use only one (part) if you wish. In some respects the variable resistor is actually the most difficult, for example, will you be able to find a set of resistors such that the 32.768k is one ohm more than the total of 1 + 2 + 4 + 8 + 16 + 32 + 64 + 128+ 256 + 512 + 1.024k + 2.048k + 4.096k + 8.192k + 16.384k ? (Hint: you won't).

    An advantage of a "Pot" (Potentiometer = Potential Divider) is that if the resistance material (e.g. carbon or metal film) changes with temperature, then both parts change proportionally and any "divided" voltage won't change (by as much). However there is still the problem that "32.768 kohms" probably won't be 1 ohm more than "32.767 kohms".

    But if you make a "Potential (i.e. voltage) Divider" using PWM with a low-pass filter, there is quite a good chance that "V * 32768 / 65536" will be V / 65536 larger than "V * 32767 / 65536".

    Cheers, Alan.

  9. #19
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    Hi newplumber,
    Just a thought.
    http://diy-welder.com/simple.shtml
    Good luck.
    Regards, bear...

  10. #20
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    Hi sorry i was weekend vacation

    @hippy thanks for the info and time saving table ...yes definitely on the LCD ..i bought a bunch of the cheap china ones...since the forums helped me get it to work.
    ("moving away from welder") yes I think maybe I will start a new thread for the welder if I still have a welder after i open the box and think i know what i am doing

    @allycat... thanks Alan your kinda making my light at the end of the tunnel with a sound of a loud train...but thanks for explaining... I (simple Simon)
    totally forgot about temperature change...but my heated garage stays somewhat stable in temp ...but again I in a little bit
    see problems you mentioned ...I guess i was just thinking how easy it would be to PE code it...maybe I will start with just one
    74ch595 and see if I can read ohms from 0-255 ...and see how close it can be in real world.
    when I get my relays I will let you know in my uneducated guesses

    @ the bear good thought and that is cool...i have to look into that thanks

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