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Choosing the right zener diode? Answered

Hi Guys,

I want to make a power bank kind of thing that can charge my laptop. Its not at all efficient and a powerbank or rechargable batteries would be better but its a challenge kinda thing. Due to my research I found out that zener diodes can not only prevent backflow but also limit the current. So I was wondering if you guys could help me choose a zener diode. I have attached a picture of the schematics. Please don't comment saying that its not efficient and stuff, I know that. ( Extra info: laptop requires 20 volts 2 amps)

Thanks in advance :).

-Prickly Potato

5 Replies

Downunder35mBest Answer (author)2015-03-31

AA batteries won't give you 2A, or at least not for much longer than a few minutes.
A Zener for 20V would not help you either, as there is no use for a diode/resistor heater during use.

Get two proper 12V batteries and a 2A voltage regulator.
Better the regulator and a power transistor on the output as it might be hard to find an adjustable regulator for 2A+

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iceng (author)2015-03-31

You really need to understand a zener is like a diode in the direction of the arrow.

BUT in the reverse blocking direction it will hold a fixed voltage like 1.9Vz, 4.7Vz, 5Vz to 33Vz.

It does this by swallowing current (that is why a leading resistor is needed) otherwise it will burn up trying to suck down a power supply.

Yes there are different power ratings for zener diodes from 1/2 watt to a stud mounted 30 watt and each has a different 1Nxxxx.A B or C....

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The Prickly Potato (author)iceng2015-04-02

Thanks alot bro. I can't unmark best answer but if I could I would have chosen yours :)

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iceng (author)iceng2015-03-31

Try the water analogy.

The bigger the faucet the more the overflow the zener has to absorb.

See the zener voltage regulator circuit, unlike 3pin regulators, the zener can only regulate a load that varies less then the zener max current.

Say you buy a one watt 5.1 Volt zener the max zener current is 1/5.1 = 196 ma..

How about a two watt 24 Volt zener with a max current of 2/24 = 83 ma..

NOTE the higher the zener voltage the lower the regulation current ability.

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-max- (author)2015-03-31

Yeah, thats NOT going to work. I can garuntee you you will just try your zener diode, no matter what.

They are not designed to handle a lot of power, or current. Just a few mA's. On top of that, the input to the laptop should be regulated. That requires advanced regulating circuitry. Google how voltage regulators work, and the switch mode regulators if you are really into learning the slightly more advanced operation of them. (the main idea behind them all is to maintain a fixed voltage on the output regardless of the input voltage or the current draw. of the computer in this case. That requires some more advanced circuitry, a zener diode by itself will not do that, at least not the way you have it in the schematic.)

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