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Using Zenner Diodes?

Discussion in 'General Electronics Chat' started by arrow, Aug 24, 2005.

  1. arrow

    arrow New Member

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    Hi All

    I am a newbie to electronics. I would like to setup a constant voltage without using a voltage regulator. My V supply is fluctuating.
    I thought of using a zenner diode by just connecting
    a zenner with a R=300kOhm across a fluctuating Voltage.

    My aim is to get a fixed voltage, say 1.5V where the R and zenner meet.
    Its not working :<

    Can someone please tell me what I am doing wrong?

    Thank you.
    arrow
  2. dipal_z

    dipal_z New Member

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    How much current you want to consume? Can you post the schematic?
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    The voltage across a zener is quite dependent on the current through it, you should aim to arrnage for the current through the zener to be constant (if possible), and for the current to be on the best part of the zeners curve.

    Also, 1.5V sounds rather low for a zener?.

    Another point is the resistor you mentioned?, 300KOhm, this sounds FAR too high a value, unless you are feeding it off hundreds of volts (or thousands?).

    You also don't mention what load you are using either?, really you don't give any of the required information?.

    For a general design, the zener would normally be designed to draw around five times the current the load takes!.

    So, the basic things you need to know:

    Supply voltage
    Zener voltage
    Load current
    Specification of zener diode

    From those you can work out the resistor and zener needed, and indeed if it's practical or not!.
  4. arrow

    arrow New Member

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    Hi Dipal_z

    I would like to draw as little current as possible since its a battery operated device.
    I also cant use a regulator.

    Thank you
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  5. arrow

    arrow New Member

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    Hi Nigel

    Thank you for your post.
    It now seems that a Zenner will not work for me.

    What I am trying to do is to set up a low battery detector similar to the MAX666. BUT without a delay.

    I have manged to construct the flip flop, but now I need to set up a constant voltage- independent of the supply voltage.
    The draw current must be as low as possible, especially since its just setting the V on a digital chip.

    I would appreciate any suggestions that you would make.
    All the best
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  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It depends a lot on the exact use, what voltage is the supply?, what sort of current does the device take?, for that matter what is the device?.

    What's the problem with a MAX666?.
  7. dipal_z

    dipal_z New Member

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    As suggested by Nigel you have to make sure that some current (depending on zener) must be flowing through the zener in order to make it work like regulator. If you want to minimize the current consumption from power supply (battery in your case), zener regulator is not a good choice because it might consume more current then your actual load just for regulation.
  8. arrow

    arrow New Member

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    Hi

    Thank you for your replies.
    The problem with the MAX666 is that there is a 140ms delay when the V high is reached. They say its for stabilization purposes.
    In my application that delay is completely undesireable.

    The current draw of the MAX666 is 1uA. I want to build an equivalent circuit to the 666, but with no delay.
    I need a predetermined voltage to be able to complete that circuit.

    Can anyone please help me to set up this constant V source- hopefully drawing as little current as possible?

    Thank you
    arrow
  9. ThermalRunaway

    ThermalRunaway New Member

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    You can buy reference voltage ICs specifically designed for the purpose you describe - providing a reference voltage. I'm not sure if you can get 1.5V fixed voltage reference ICs (I know that you can get 1.22V ones) but you can also get programmable reference voltage ICs which would possibly do the job you want. They cater for low current applications too, which would suit your battery operated project.

    Try a few good electronics suppliers and check out the voltage reference ICs they have on offer

    Brian

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