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Old May 18, 2014, 07:17 AM
Dave the Rave
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Current limiting resistor for zener diode

I have a small circuit that uses a PIC12F675 that I normally plug directly into my receiver, and the chip runs fine off the receiver's battery. But I want to try it on a plane that uses a high voltage receiver, and the voltage coming from the receiver is going to be from a 2-cell Li-po, so it will be up around 8 volts when the battery is fully charged. I know the chip won't tolerate that voltage for long, so I have to knock it down a bit.

I could use a small voltage regulator, but the way this circuit is built, it would be easier to add a zener diode. But I think I also need a resistor to limit how much current the zener passes, and I don't know how to calculate that.

Can someone please explain to me how that is done? How do I know what size resistor (ohms and watts) to use with a zener diode to limit voltage to around 5 volts DC? Thanks in advance!
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Old May 18, 2014, 03:50 PM
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Here's link that should answer all your questions about the value and power rating of both the resistor and the zener diode.

http://www.kennethkuhn.com/students/...ors_zeners.pdf
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Old May 19, 2014, 07:33 AM
Dave the Rave
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Fantastic! Thanks!
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Old Jul 21, 2014, 05:31 PM
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Although using a zener and a resistor is a simpler and easier solution, I wouldn’t really recommend that because no matter how much you try to optimise the value of the resistor, you are going to lose power anyways. It is way better to use a small IC which might be a little bigger than a zener and a resistor but it’ll definitely protect your circuit and won’t let the power to be lost.

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Old Jul 21, 2014, 08:29 PM
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Quote:
Originally Posted by oliver89 View Post
Although using a zener and a resistor is a simpler and easier solution, I wouldn’t really recommend that because no matter how much you try to optimise the value of the resistor, you are going to lose power anyways. It is way better to use a small IC which might be a little bigger than a zener and a resistor but it’ll definitely protect your circuit and won’t let the power to be lost.
Oliver,

Please give us an example (simple schematic) using an IC based upon the criteria outlined in Post #1. Also identify the IC you would use to replace the zener an resistor.
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Old Jul 23, 2014, 10:54 AM
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I too would not use a zener - the voltage it clamps at actually depends on the current taken (look at the datasheet graphs) so as Oliver says, it is hard to get the resistor just right.
There are soooo many voltage regulator ICs available these days - it is a better way to do it, and not too much bigger. Ones which are straight-forward and easy-to-buy (assuming you will buy on the 'high street') would be a fixed voltage 7805 (5V), or LM317 if you want to tweak the voltage. And remember that lower voltage isn't necessarily better than higher - these are linear regulators so whatever voltage/power they need to knock off will simply be wasted in heat (though not as bad as a zener). A simple regulator will have minimum 'dropout' of a volt or so - so bear that in mind, it'll depend how low a voltage your circuit can run at (that PIC is >2V).
Also, don't ignore the recommended smoothing capacitors (one on input, one on output, is common) - they don't just smooth noise, they help keep the regulator stable.
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Old Jul 23, 2014, 06:04 PM
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if the mcu draws very little current then a zener does make sense here, specially if thats only what you have on hand. dropping 8v to 5v wastes very little energy too. cost and component count is considerably less than using a regulator. also note that in many cases if load is constant and due to non-linear current draw for devices like avr or pic its possible to use a resistor only w/o need for the zener. choosing the proper value becomes a little more critical though.
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