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zener voltage regulation

Discussion in 'Power Electronics' started by hobby, Aug 15, 2019 at 1:45 AM.

  1. hobby

    hobby

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    Thursday
    I thought zener diodes make fairly good voltage regulators but my simple project failed. I tried to get 3v from 5v source. I connected 100 ohm 1/2W resistor and 3.3v 5W zener diode to ground. I expected 3v at the resistor diode junction but I get only 2.2v with no load. Why?
     
  2. Audioguru

    Audioguru

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    Sep 24, 2016
    5V through 100 ohms to a 3.3V Zener diode is a current of 95V - 3.3V)/100 ohms= 17mA. A load on the Zener diode will reduce its current.
    A 5W zener diode might have its voltage rated at 300mA or more. Post its part number and datasheet and look at them.

    I looked up a 5W 3.3V Zener diode and found the datasheet for a 1N5333B. It is 3.3V when its current is 380mA. Then your resistor should be (5V - 3.3V)/380mA= 4.5 ohms. What a huge waste of current!

    How much current does your load need?
     
  3. hobby

    hobby

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    Thursday
    That's the diode I got at Digikey. The "B" should be 5% but that's all the specs I have. So this is no-go. I don't need high current. I was looking to run LCD clock from supercapacitor instead of 3v coin battery and charge the cap from USB charger. Normally I would prefer voltage regulator but the 1.25v drop won't get me 3v from capacitor. At least not long enough for the clock to run a week or two. Besides, I could not find in datasheet what's the minimum voltage input for LM317. Some voltage regulators set the minimum as 4.5v.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
  5. hobby

    hobby

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    Thursday
    I have to look up the LDO voltage regulators or something that will work. Then I'll try tiny solar cell in addition to usb charging. Small solar cell rated 5v 30mA should put out 3v and few mA average to charge two 4F 5.5v capacitors. I make some dummy batteries coin and AAA from piece of plastic and 2 wires to see what works and what doesn't work.
     
  6. Audioguru

    Audioguru

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    Sep 24, 2016
    A capacitor is a poor choice to power anything because its voltage drops very quickly at the beginning of its discharge.
    A modern AAA alkaline or Lithium battery voltage drops very slowly for most of its discharge.
    Most clocks are powered for a long time from a replaceable battery. I have never seen a clock use a rechargeable battery unless it has a current-hungry motor driving its heavy hands.
     

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  7. hobby

    hobby

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    Thursday
    You are correct. For that reason I would make dummy battery slug and not even opening the powered gizmo and not installing "or" tied diodes. If the capacitor is charged by USB or solar cell it may work when the battery is dead. Pull battery out, put in the dummy and look for replacement battery. Eventually things will run that way. There is energy harvesting research for IoT, so someone will get it done. I see some new car dashcam video recorders use capacitors to save file and shut down when the ignition is turned off.
    Maxim makes some LDO voltage regulators with P-channel pass through for minimal quiescent current. It is just hobby right now but helps to learn new things thanks to support like you provide. Thanks.
     
  8. Audioguru

    Audioguru

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    Sep 24, 2016
    The datasheet of an LM317 shows that its idle current is 5mA to 10mA all the time which is at least 10 times the current of an LCD clock. Its datasheet also shows that its input must be at least 1.5V higher than its output voltage when its load current is 20mA or less.

    All low-dropout voltage regulators made in the last 20 years or more use a PNP pass transistor that does not have the voltage drop of an emitter-follower NPN.
     
  9. hobby

    hobby

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    Thursday
    Maxim claims that MAX8882/8883 are improvement over PNP. The P-channel doesn't use bias current. They come in several voltage outputs that may be used to charge single cell battery, 2.7v supercap or 3v coin battery and accept 6.5v input which is just right for USB power source. This may be the part for me to play with. It doesn't require special PCB with copper for exposed pad which is hard to hand solder.
     
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