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Thread: why not to use zener to make 12V DC??

  1. #1
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    Post why not to use zener to make 12V DC??

    Why I havn't seen any zener regulated ps's to make stable 12V DC ??
    I know that it needs some huge wattage for zener because there is ~3 A

    P = U*I
    min is 12 watts if using two or three zeners (one for mb & one for hd)
    But is there any schematics, because I can't remember those R values!?

    [again messy topic]
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  2. #2
    Retired Admin Aaron Cake's Avatar
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    Cool

    Setting up a zener diode and transistor as a regulator is more complicated and expensive then just slapping on a 78xx or larger regulator.
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  3. #3
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    Yea, but normally car voltage is 12-14,5 V
    12.4-12.0 when off
    ~11 when starting
    ~14 when running

    So 7812 would need Vin atleast 13 V so it won't work when car is parked and off!
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  4. #4
    Retired Admin Aaron Cake's Avatar
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    Cool

    The low dropout version of the regulator will work under these conditions.

    However, neither a zener nor a "real" regulator will be any good if the voltage falls below 12V, which is why a DC-DC is used.
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  5. #5
    FLAC
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    Post

    Originally posted by embedded:
    <STRONG>Yea, but normally car voltage is 12-14,5 V
    12.4-12.0 when off
    ~11 when starting
    ~14 when running

    So 7812 would need Vin atleast 13 V so it won't work when car is parked and off!</STRONG>
    If you use a Zener, you will also need a power transistor to amplify the current. The way a zener works is that when it has a reversed biased voltage put across it, it will have a specified voltage. Most zeners come in little packages that can't sink much heat. You don't want to put more than say 50 milliamps through a zener.

    What you do is hook the positive terminal of the 12 volt zener to ground. Hook the negitive terminal of zener to "point A" use a 1 k resistor and hook it between "point A" and 12 volts. Then hook "point A" to the base of an NPN power transistor. Hook the collector of the power transistor to 12 volts. Your emitter will now put out 12-0.7 volts or 11.3 volts. You will lose .7 volts by going through the power transistor. There is not much you can do about that. As your input goes from 8 to 14 volts, your output will go from 8 to 11.3 it will stay at 11.3 as long as the input is at or above 12 volts. So you can see this method only gives you overvoltage protection. If you used a 12.7 volt zener, you would get 8 to 12 volts as you sweeped the input, but again, your input would have to be above 12.7 volts for this to work. If you want to keep a constant voltage, you need some kind of stored charge based switched mode power supply. This is the same for the 7812. Think of the 7812 haveing a 1 volt drop instead of the typical diode drop of .7 for a power transisitor.
    This all depends on the type of components you choose to do this kind of design.

    Jeff_
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