I built a simple adj. current circuit, but don't understant it. Can we improve it?

Discussion in 'The Projects Forum' started by Hamlet, Aug 14, 2015.

  1. Hamlet

    Thread Starter Member

    Jun 10, 2015
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    Okay, this is my second thread, so I hope I can attach the schema, and everyone can read it.

    I recently got back into electronics as hobby, and have been working with linear power supply
    circuits, learning what I can and can't do with an LM317, by-pass transistors, and the challenge
    of controlling power > 1 amp, at voltages <30v.

    I'm attempting to make a rough and ready bench-top power supply. I'm trying to keep it simple, later I will get to LM723, L200, switchers, etc.

    I know how to make an adj. current regulator with LM317/LM338. For large currents, you need a large rheostat. So, I am trying something else.

    I found a power supply circuit, and the current adjusting part used a single high power, NPN, TIP142 Darlington. I don't have one of those, but
    I have several TIP102 Darlingtons in my junk box.

    Here is the original circuit:
    http://adjustablevoltagepower.blogspot.com/

    I breadboarded, then soldered up a modified version of just the current regulation portion. It seems to work with very fine adjustment control.

    Okay, that's all I've got. I can't figure out how to post my schematic jpg. If someone can help me get that posted, we can discuss it.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    No, you don't. You can use a normal low-power potentiometer/rheostat in conjunction with a power transistor.
    1) Reduce its size to a manageable <200k if possible
    2) Use the 'Upload a File' button at the foot of the text box
    3) Browse for the file.
     
  3. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Low-power pot. & power transistor? I tried this with a mosfet and a high value 1M pot, and it worked,
    sort-of, but the heating of the mosfet caused it's resistance to change and go wonky, and the current output was not stable.

    I tried the "Upload a File" button. I can see my folders, and text files, but I can't see any .png, .jpg, etc. to select.
    I'm using firefox.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You need feedback in a control loop if you want a stable output current. The MOSFET also needs a large heatsink if you are effectively using it as a replacement for a large rheostat.
    I believe there has been a site issue recently with uploading some file types.
     
  5. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Okay. I don't need to go the MOSFET route. Nice to know, thanks. Still can't upload
    a file, my schematic, so this thread is going nowhere. Maybe I'll try again sometime.
     
  6. tjohnson

    Active Member

    Dec 23, 2014
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    @Hamlet: If you're having problems uploading your attachment, I can PM you with my email address, and if you send it to me I'll upload it for you. Would you like to do that?
     
    Hamlet likes this.
  7. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Super! Hope I can see your pm...
     
  8. tjohnson

    Active Member

    Dec 23, 2014
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    Please check your Inbox (which can be accessed from the rightmost button that shows your avatar on the orange bar at the top of the page).
     
  9. tjohnson

    Active Member

    Dec 23, 2014
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    Here's the schematic that @Hamlet wanted to post:
    1. current-then-voltage regulator schema 1.jpg
    2. current-then-voltage regulator schema 2b.JPG
    3. current-then-voltage regulator schema 2c.jpg
     
    Last edited: Aug 15, 2015
  10. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Thanks TJ for getting those schematics posted for me!

    The top schematic is a power supply I lifted from the web. At the moment,
    we are concerned with the upper left portion, the current regulation circuitry,
    which I have enlarged in the second schematic. I removed the metering
    components, and added a few of my own. These changes are represented in the third
    schematic.

    I have this wired on a small chunk of perf, with power side consisting of a
    12v, 4A non-centertapped transformer, 8A diode bridge, & about 4400uF in caps.
    (Eventually, I might replace this with 10A transformer, and more caps.)

    I don't understand what the zener diode is doing. In or out, I couldn't note any difference,
    so I left it in.
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The zener sets the maximum that the current limit can be set to. With your modifications the pot R6 will get 8.2V max, so the max current will be (8.2- 1.2)/0.2 A through each Darlington (assuming the Vbe of the Darlington is 1.2V), = 35A per Darlington! Methinks your perf board is going to get a tad warm ;).
     
    Last edited: Aug 15, 2015
  12. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Ouch. That might not work if I try to test something big.

    8.2v - 1.2v/.2A =35A ?

    So, we subtract the voltage drop across the Darlingtons, which is two diode drops equivalent for each, or .6 + .6 =1.2

    Where do we get the .2A ?
     
  13. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    In full the equation should be (8.2V-1.2V)/0.2Ω = 35A.
    The 0.2Ω is the Darlington emitter resistor value.
    If your transformer is rated for 4A you ideally want a lower zener voltage and/or higher emitter resistor value so that the maximum adjusted current limit can't exceed 4A total (2A per Darlington).
     
    Last edited: Aug 15, 2015
  14. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Oh, I get that a little better now. Thank you. Current limiting is sort of my focus right now. I was just at my bench, it seems
    the zener is getting too hot, and the 0.2Ω resistors are toasty. I am going to see if I have larger ohm sand resistors. If I can't make this linear design work, I will look for another. Of course, I could purchase a switcher from ebay, but I'm not going to learn anything or have any satisfaction in that. I'm going to search online for a different linear solution. I don't want to give up and go back to using light bulbs for current limiting.
     
  15. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Alright, I added 1Ω each, for 1.2Ω. I need to dig around and find more sand resistors. The resistors are much cooler. However, at partial setting with modest load, the zener still gets hot.
     
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Try increasing R5 to, say, 47Ω. That should cool the zener, but the Darlingtons will run warmer.
     
  17. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Just tried it. Resistor smoked with a 1.5A resistive load on output. Also, need to check my zener, it might be fried. Response is hi/low, no longer linear. I'm need to buy a single power transistor, and simplify. I'm beginning to see where this is going...
    On another note, I found this schematic, posted by Wendy:[​IMG]
    http://forum.allaboutcircuits.com/t...t-between-op-amp-and-transistor-based.112277/

    It looks similar. I'll need some values to get me started. Then I'll order more parts (TIP147, power resistors, etc.)
    Uh, CR1 & CR2, are those zeners?
     
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