Quick and dirty 5v to 3.3v supply that's stable

Discussion in 'General Electronics Chat' started by programmer6502, Jan 9, 2016.

  1. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Hi,

    I'm working with a MCU and am interfacing with a chip that wants 3.3-4v VCC when my only power source is 5v. Initially I thought I could use a resistor ratio setup to drop 5v to 3.3v and it worked, but as soon as there's a load the voltage drops too low (which I guess should have been obvious). I also tried using a Mosfet setup, as well as PWM with the same results. I know that a regulator would do the trick, but I've already made two parts orders with this particular project and was wondering if there's a way to build a stable 3.3v "regulator" with common components like transistors, caps, resistors, etc. (This 3.3v source would only be required for one chip.)

    Yes? No?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Zener diode regulator with a series pass transistor. Google it.
     
  3. programmer6502

    Thread Starter Member

    Feb 1, 2014
    126
    6
    Where do you think I got the other three attempted methods from? ;)

    But thanks a bunch! I'll check that out!
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    How much current does your 3.3 volt circuit draw? What is the minimum and maximum current for it?
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    Two diodes will drop 5V to about 3.6V.
     
  6. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    I'll measure and find out. The chip was salvaged and I can't find any specific information for it. It's a decoder from an RC radio system that I reverse engineered.
     
  7. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    It appears to draw 20-30mA.
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    I don't know, but you didn't exactly give a definitive description of a zener regulator with a pass transistor. You did describe a couple of other hair-brained schemes, but what do I know? Where are the schematic drawings?
     
  9. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    True, but it was intended to be more of a general question and I just wanted to give enough details for the question to be answered, which thanks to you, was answered! That's what I was looking for, so I don't have or intend to provide schematics.

    I apologize if I was rude, I just research the best I can before coming here. Even if the answer was in plain sight lol.
     
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Here are 3 ideas:
    It is a simulation of 3 circuits. The outputs are shown at the top with a current step from .1 ma to 40 ma.
    1- is a transistor - similar to your resistor divider, but powered up by the transistor
    2- Some diodes in series with the 5 volts to drop it down.
    3- What is called a shunt regulator where the diodes clamps the 5 volts to a lower voltage.

    The diodes don't let you set the voltage of course.
    The transistor requires your 5 volt supply to be solid. If it varies some of that variation will be seen at the output.
     
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  11. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Circuit #1 did the trick with what I had on hand! Thanks a mill!

    I think it's about time I start using one of those circuit simulators. It would save a lot of time and I wouldn't risk destroying anything.
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    There is still a risk. I just smelled magic smoke this morning.
     
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  13. dl324

    Distinguished Member

    Mar 30, 2015
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    You still need to know what you're doing. I ran a simulation and LTspice happily operated a 2N3904 at 1A; 10X the rated current.

    I simulated a switching regulator using a linear regulator and the program was very picky about feedback ratios that would work.
     
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  14. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Few days ago I implemented a regulator with an LP2950, two caps plus a load resistor. Do not waste time in imprecise solutions and posting about them.

    EDIT /
    5 to 3V3
    /EDIT
     
  15. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    You have a point, it isn't practical but it was still a solution and it solved my problem:

    [​IMG]

    The point was to not make a 3rd parts order so I'm happy! Made from scratch using salvaged parts :D
     
  16. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Maybe I was too rough, and sorry for that. The reason is that in my case I was preparing to feed an RN42 which costs dearly here and the associated 8 level translators I need with mi 5V micro.

    Yesterday, the second took say, 3 minutes to implement and the output was 3.30 V. Well, my DMM says sow.
     
  17. programmer6502

    Thread Starter Member

    Feb 1, 2014
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    Yeah, I get what you're saying. My first choice would've been to pay 3x the normal cost to get a regulator at my local Radio Shack just to be "done", but I couldn't find anything standalone in the 3v range. They did however, have an adjustable regulator like you mentioned that I would have gotten if I needed provide power for more than one chip. But since I only needed to power one chip, I thought it would have been a little overkill. Not to mention that building that little transistor circuit took roughly the same time (5 minutes) it would've taken for me to implement the external circuitry for the adjustable regulator.

    So in my particular case I think it was a just decision, but in most circumstances I would do something like you did, especially if the chip in question was sensitive and of high value.

    If anything, I learned something new! :D
     
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  18. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I usually stick in a LM1117 and am done with it, no need to bodge your own.
     
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