How to use a Zener Diode

[carbon.nanotube.capacitor]

So I read an article by Justin on ebike.ca that mentions using a zener diode to reduce the input voltage to a dc-dc converter. His example was to reduce 72 votls from the battery to 60 volts, which the dc-dc converters he sells can handle. The diode to use is stated as being a 12 volt zener diode. I read about zener diodes on Wikipedia but the information didn't really help me.
Do zener diodes reduce voltage? How would one connect one to a dc-dc converters input? Does the diode need to be of a certain power handling capacity? If I need to reduce the voltage by more than 12 volts can I use more than one diode?
There are a lot of half-watt zener diodes available on ebay.

Thanks!

 

[ZapPat]

So I read an article by Justin on ebike.ca that mentions using a zener diode to reduce the input voltage to a dc-dc converter. His example was to reduce 72 votls from the battery to 60 volts, which the dc-dc converters he sells can handle. The diode to use is stated as being a 12 volt zener diode. I read about zener diodes on Wikipedia but the information didn't really help me.
Do zener diodes reduce voltage? How would one connect one to a dc-dc converters input? Does the diode need to be of a certain power handling capacity? If I need to reduce the voltage by more than 12 volts can I use more than one diode?
There are a lot of half-watt zener diodes available on ebay.
Thanks!

Hi nanotube - The easy way of seing a zener is to know that it will drop it's zener rated voltage when it's cathode (the black line side) is connected to a positive voltage and it's anode to a negative voltage (it's biased reversed here). Remember that it still works as a regular diode if the voltage across it gets reversed (forward biased, so about 0.7V typical drop).

Since you want to connect it to get the zener voltage drop (Vz), you connect the cathode (black stripe) to the battery positve, and then you can assume that the anode (unmarked side) will be kept around the battery's voltage minus the zener's voltage.

As for the wattage required, you're going to know how much current your LED module requires (in Amps) and multiply this number by the zener's voltage drop (12V in your example). So a half watt zener could only take up to about 0.040 Amps before possible destruction (about 45 milli-amps). If this is not enough, you may take multiple lower voltage zeners and connect them in series as to distribute the heat over a number of parts. So two half watt 6V zeners in series would be able to go up to about 0.080A before death.

Hope this helps.

 

[carbon.nanotube.capacitor]

Thanks for your help ZapPat,

Unfortunately for me my dc-dc converter needs to output around 90 to 100 watts at one time, if all my lights are turned on at the same time, even though many of them are LED's. Does that mean I need zener diodes that can handle 90 to 100 watts? That could be expensive, as they only appear to be available up to 5 watts.

 

[gmouchawar]

The power rating should exceed the zenner voltage (in this case 12V) times the current consumed by the load. You don't state at what voltage is the 100 W consumed.
For example if 100W is consumed at 24V which will also be the output of the DC/DC then the current draw is approximately 4A. That means the Zenner would need to be rate at 12x4=48W.
You won't find that too easily. You would also need a large Al piece as a heat sink.
That solution is more suited for small current drains in the order of less than 0.5A. Keep looking for a DC/DC that can handle the full load.

 

[ZapPat]

Thanks for your help ZapPat,
Unfortunately for me my dc-dc converter needs to output around 90 to 100 watts at one time, if all my lights are turned on at the same time, even though many of them are LED's. Does that mean I need zener diodes that can handle 90 to 100 watts? That could be expensive, as they only appear to be available up to 5 watts.

By the sound of it, you'll need a higher Vin DC-DC converter for your application - 72V models are pretty easy to find on the net. I guess if you really wanted to use some kind of zener solution, you would need to use a transistor to go with it that can handle more current (and a small heatsink). Knuckles explains this simple circuit to use in the infinion controllers, so that the input voltage range is improved. All that is needed is one zener, one resistor and one transistor.

 

[rkosiorek]

The type of circuit is called an "Amplified Zener".



NOTES:
1 - the 1N4001 diodes are used to trim the voltage drop through the NPN transistor. they are not critical, almost any rectifier will do.
2 - the capacitor is used to filter any switching noise from the DC-DC converter.
3 - the NPN power transistor can be anything big enough to handle the power required. high gain types will work better.
4 - a heatsink for the transistor is required. it will get hot.

this circuit can handle up to about 4A with a proper heatsink. if you are trying to drop 84V down to 72V for the converter these values should work fine. most of these brick converters will use less than 2A on the high voltage input to deliver about 7A on the output.

to be safe and to allow for tolerances especially on the Zener i would probably use a higher voltage Zener like a 1N4744 (15V) or 1N4745 (16V) to make sure that the input to the converter is well below the maximum rating.

also remember that most batterypacks use a NOMINAL voltage rating. most 72V packs usualy read 84V no load. that means that part of the time your converter/zener combo will see that full no-load voltage and must be able to handle that. that means that if the pack actualy delivers 84V no load your zener would have to actualy be able to drop 24V from 84V down to 60V to be safe. these little fine points only become important when you are trying to run things right at the limits without any kind of safety margin.

rick

 

[ZapPat]

Ah, that's what nanocarbontube needs! I wrongly suggested a voltage regulating circuit (to get a fixed Vout), instead of Rick's circuit here that will give you fixed voltage drop, which is what you were looking for.

One little note though - You can eliminate the two regular diodes and use a higher voltage zener instead. Just add about 1.5V extra to your zener value to do this, and your circuit will be a bit simpler.

 

[rkosiorek]

but you could also use the diodes to "trim" the voltage in 0.6V steps. add a diode or two to raise the voltage up. or delete a diode or two to drop the voltage down.

sorry - i keep on forgeting that this is not an electronics experimenters corner and not everyone has spent years collecting drawers full of spare parts on hand.

rick

 

[fechter]

That will work, but it will dissipate a massive amount of heat.

It would be much better to use a dc-dc with a higher input rating.

 

[monster]

why dont you get higher voltage lights, if you want that much light?

or you could tap into your battery somewhere for lower voltage. or you could add a few lower AH cells in series with your main pack (for lights only) to boost the voltage up to meet the requirements for the dc-dc convertor. most have a minimum requirement or 90v.

 

[carbon.nanotube.capacitor]

Thank you everyone for your input. (I am Johnny 5). It would be fun to build the Amplified Zener circuit, however it would be best to use a DC-DC converter designed to accommodate higher input voltage. Especially since the Zener circuit will be wasting so much energy! Oh and ZapPat, you got my name wrong - it's CarboTubeNani or whatever, or something.