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Why people not use zener instead of LM78xx linear regulator

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tellMeDaBasix

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lm78xx

I read in class that zener can be used to give fixed voltage.
But I see all over net people use LM78xx linear regulator always.

Why people not use zener instead of LM78xx linear regulator?

LM78xx linear regulator very waste power as heat but zener do not.

When to decide:

1. If to use zener or LM78xx linear regulator
2. How is regulation of LM78xx linear regulator versus zener
3. Why use not, for example, TL431 super zener instead of of LM78xx linear regulator

Important questions as I see people like to use ICs even maybe when not required!
 

chasxmd

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tl431 linear regulator

There are a lot of options for voltage regulation, but I think if you just need a generic bus voltage for a small project the 78XX series or 78LXX series is your best bet. They are simple to use and can handle a decent amount of current depending on your device.. generally you use them for less than 1 amp but if you need more you can use a current bridge with something like some 2N3055's..

The zener diode is okay for a fixed voltage reference on a device that doesn't require much current and it's current draw doesn't vary much. If you note they need a current limiting resistor so they don't self destruct.... which is were you loose your effiency.
 

doraemon

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zener regulator

Hello!

I think the main reason is because a regulator regulates, and a zener
does not. Zener can just give you a voltage reference if you feed it with
a constant current.
If you want to make a regulator with a zener, then you will also need a
ballast (transistor) and a few resistors. You will end up with a regulator
which takes too much space, and then you'll decide to use a 78xx which
is cheaper, smaller, etc...

I would be curious to see the topology of your zener regulator that does
not eat current. Basically a zener diode does only that: dissipating power.

Dora.

tellMeDaBasix said:
I read in class that zener can be used to give fixed voltage.
But I see all over net people use LM78xx linear regulator always.

Why people not use zener instead of LM78xx linear regulator?

LM78xx linear regulator very waste power as heat but zener do not.

When to decide:

1. If to use zener or LM78xx linear regulator
2. How is regulation of LM78xx linear regulator versus zener
3. Why use not, for example, TL431 super zener instead of of LM78xx linear regulator

Important questions as I see people like to use ICs even maybe when not required!
 

tellMeDaBasix

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how to use zener

I thought if place more than X volts across zener rated Y volts, then you get Y volts at output?

But now I understand - the input series resistance waste current :-(

Right?
 

betwixt

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lm78xx zener diode

The real reason is that 780x (and similar) regulators are series devices, they are in line with the load and control the amount of current they let through so the output is maintained under different load conditions. You can think of them as being 'self adjusting' resistors that drop just enough to make sure there is the desired voltage after them.

Zeners, are shunt devices, they are wired in parallel with the load so they 'absorb' the extra voltage by dumping current to ground.

From a practical point of view, Zeners are not perfect voltage stabilizers, their voltage increases slightly as the current through them is increased and they are not very temperature stable. That can be a major issue if you just use them in the 'X and Y' scenario you describe. If your load changed from say 0.5 to 1.0 amps, in order to maintain voltage the current would have to be kept at 1.0 amps by sinking the unused 0.5A through the Zener. They obey Ohms law like everything else so:

Vz * (max I - Min I) can equal a lot of Watts.

Their place in the world is as voltage reference providers where some other element actually does the hard work, this is what happens in normal regulators.

Brian.
 

dipnirvana

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linear regulator with 2n3055

The reason is load regulation. the more current u draw from zener or TL431 u go away with the volatge u promise.
But 74XX allow more current from them
 

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