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zener diode (limit 9v to 5v)? Answered

can i use zener diode  to limit voltage from 9v to 5v? ( i would use 5v zener diode )
can this work? what else do i need to use? and  is this way efficient?
i was thinking to power my phone with this or to charge my light

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Josehf MurchisonBest Answer (author)2013-08-25

With the help of a transistor yes.
Depending on the transistor and the voltage you want connect a 3.5 volt zener diode to the base of the transistor shunting the over voltage to ground.

Joe

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user

Nice trick! That won't cause any issues with the power source? Isn't that sorting the power source with the over voltage?

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user

By the way most of the time if the load current is under 100ma I use a LM78L05 voltage regulator it is the size of a 2N3904 transistor and it works from 7 to 20 volts.

A CJ78M05 for circuits 500 ma working voltage 7 to 25 volts.

An IP7805 for 1 amp circuits working voltage 7 to 35 voltage.

And a LM7805 for 1.5 amps working voltage 7 to 20 volts.

Adjustable voltage regulators I use LM317 up to 1.5 amp and LM138 for up to 5 amps working output voltage 1.2 volts up to 35 volts.

All the circuits are basically the same.

Joe

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tealk (author)Josehf Murchison2013-08-26

thanks for advice. i already used lm7805 and its great for my USB light. (it gives me 10 hours of light,which is great for the cost of 9v battery, 50 cents) and also it can charge my phone to 70% or even 100%.

do you have shematics for zener diode and transistor? ( i have one but i am not sure if its gonna work)

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Josehf Murchison (author)tealk2013-08-26

Since I don't know what your circuit is I'm not sure if it will run your circuit ether however you can make it work by upping the collector resistor.

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user

It doesn't cause issues as long as the impedance of the shunt circuit is equal to the load circuit. And yes it is shorting the overvoltage to ground, but that is what a zener diode does.

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iceng (author)2013-08-25

You can but you need a resistor for the design current to match the zener power and your load..

I suggest this simple 5V regulator.
That can work without the first three capacitors and does the work of a zener ++

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verence (author)iceng2013-08-26

Omitting the 100nF ceramic caps will work almost always. But every now and then, you will not get a voltage regulator but a MHz oscillator. These ceramics are cheap and small. Just put them very close to the regulator.

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iceng (author)verence2013-08-26

Excellent point !!
And that MHz oscillator,
when it happens it will mystify a beginner.

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steveastrouk (author)2013-08-26

Its horribly inefficient. There is a lot of of power wasted in the regulator - nearly half of what you have disppears as heat.

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mpilchfamily (author)2013-08-25

Your better off using a LM7805 voltage regulator. It your just trying to bleed off between .5V and 1.5V then diodes aren't a bad option. Anymore than that it's more efficient to use a voltage regulator.

BTW don't use a 9V battery to charge anything. They don't have a lot of capacity and will not do much to charge anything. A pair of AA batteries holds more power than a single 9V. Sure you only get 3V out of a pair of AA's but you put 6AAs together and you have nearly 6 times the power available for charging.

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tealk (author)mpilchfamily2013-08-26

thanks. could i charge my phone with it? And do i need to smooth voltage with capacitors?

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mpilchfamily (author)tealk2013-08-26

It's always a good idea to have some capacitors to help smooth out the voltage.

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