# Overvoltage protection circuit specification. Zener diodes... what wattage?

#### ianjuk

Joined Oct 10, 2020
14
The zener needs to be protected from the current. Hence, a resistor will be needed.

The logger may only use microamps, but the value of the resistor still needs to be calculated so as to protect the zener from excessive current.

A 30V zener will only conduct when voltages reach or exceed the 30V (± its tolerance). Therefore, if the generator is putting out 50V and the zener is rated for 1 watt then Ohms Law needs to be applied. I'm using 1 watt as a reference point. Since the zener conducts at and above 30V then 50V - 30V = 20V. So 20V at 1W equals 50mA (or 0.05A). 20V ÷ 0.05A = 400Ω. That will conduct 1 watt when the generator is putting out 50 volts. If the generator goes higher - say 75V then:
1W ÷ (75V - 30V) = 0.0222•••A or (22mA)
45V ÷ 0.022A = 2045Ω. 2KΩ would be required to limit the wattage to 1 watt. Putting a 400Ω resistor through that voltage will mean the zener has to dissipate [45V ÷ 400Ω = 0.1125A. 45V x 0.1125A =] 5.0625W. You'll burn up the zener if it's rated for 1 watt and you push 5 watts through it. In this second case you would need a 2KΩ resistor to limit the current to slightly over 1 watt at 75V output. Using a higher wattage rated zener would improve matters.

Since your data logger won't be seeing more than 30V (due to the presence of the zener) it will be safe. Unless the zener burns out and goes open. Then at 75V, 2KΩ the logger could see 38mA. If your logger can handle that then no problem. If not - you'll need to replace the logger.
Thankyou for this, it's added a lot of clarity.

So going with a 30v 1w zener and a 1KΩ resistor should be enough to limit excess voltage of between 30v and 50v, but if I suspected (not likely) that a peak of 75v was possible, i'd need to up the resistor to 2KΩ?

Have I got that right?

#### dl324

Joined Mar 30, 2015
16,988
So going with a 30v 1w zener and a 1KΩ resistor should be enough to limit excess voltage of between 30v and 50v, but if I suspected (not likely) that a peak of 75v was possible, i'd need to up the resistor to 2KΩ?
TLDR - didn't read all of the posts...

If you use a TVS diode, you don't need to worry as much about power dissipation. They're designed to withstand high surge currents. If you want to protect the TVS diode, put a fuse in series with it. If you can tolerate enough fuse resistance, you can use a PTC that will be self resetting.

#### ianjuk

Joined Oct 10, 2020
14
TLDR - didn't read all of the posts...

If you use a TVS diode, you don't need to worry as much about power dissipation. They're designed to withstand high surge currents. If you want to protect the TVS diode, put a fuse in series with it. If you can tolerate enough fuse resistance, you can use a PTC that will be self resetting.
You're confusing me with someone who knows what they're talking about

What's a TVS diode?

#### dl324

Joined Mar 30, 2015
16,988
What's a TVS diode?
It's a Transient Voltage Suppressor. Practically, it's a high power zener diode that will survive surge currents of 10-100+ amps for diodes rated under 100V.

#### Tonyr1084

Joined Sep 24, 2015
7,943
Have I got that right?
Essentially Yes. BUT!

I don't have experience with zener's and am not familiar with their wattage capabilities. IF I were to build such a circuit it would start with understanding the parts available and their associated data sheets. Would calculate the max current the device might see and then go with 1.5 to 2 times the wattage rating needed.

Some are mentioning TVS's. Essentially they're zener diodes with high current capabilities. They can handle more than 1 watt. Remember, I picked that number for the sake of making the point that a resistor is needed in such a circuit.

Think of a TVS or Zener as a dam. As long as the water level (voltage) doesn't rise above the top of the dam no water flows down stream. But if the water level (voltage) goes above the top of the dam then whatever TOPS the dam is spilled off. The water flow (current) runs down stream and returns to the sea (back to the source). If the dam is barely overtopped the water runoff will be low (current). If the overtopping is much greater then the runoff will be much higher (higher current). If the stream it flows into can not handle the flow (current) then the water will spill over the edge and destroy the neighboring homes (blow your circuit).

Before you undertake such a build you need to become familiar with the devices you're working with as well as the associated math in calculating the needed components. Failure to understand them may result in a failed circuit and damaged equipment. So far we know nothing about the generator other than sometimes it exceeds 30 volts DC. How much current is it capable of producing? If it can produce only one amp then there's not very much that can go wrong. But if it produces 100 amps - it can be much more dangerous if something goes wrong. So always plan for the worst case scenario then design for 1 1/2 to 2 times the greater strength.

How much current can your generator generate? Wattage is a function of voltage multiplied by amperage. If it can produce 100 amps at 75 volts then that's 7500 watts of power it can generate. That's a lot of heat energy that needs to be managed; and must be a crucial part of the design of any voltage regulation. And understanding of how components fail will also dictate design requirements. For instance, if your TVS or Zener shorts out - 7500 watts can easily start fires. So a part of the circuitry needed should be a fuse. Something that is sacrificial. If something goes wrong the fuse blows out and protects much of the circuit. Obviously for the fuse to blow will likely mean that some component has already failed. The fuse just stops the current from flowing so that excessive heat does not build up and start a fire.

Whatever circuitry you come up with to limit the input to 30VDC, consider all possibilities; failure, unexpected voltages, size of wires to conduct said energy and how you're going to shut things down in an emergency. Also, what happens if you're not present when the emergency arrives? How can the system protect itself?

So please answer the following question: What's the rated output of your generator? We're looking for voltage, amperage, wattage, whatever information you have on it. If it's home made - pictures can aim us in the right direction.

Finally, I'm in favor of @MrChips suggestion (post #10) of a voltage divider over regulation. Knowing your generator maxed out the reading capabilities of the logger only tells you that you had 30 volts OR MORE, but you don't know how much more. Using the voltage divider Chips recommended gives you twice the measuring range. Of course, you've said you don't want to have to double the reading. Convenient, but might not be the safest approach. ME? I'd consider a divide by 10 voltage divider network. Then whatever voltage I read, just multiply by 10. For instance, if the reading is 3.2V then it's 32 volts. That's above the 30 volt range without going over the range of the logger. And if you get 6.8V, then you know it saw 68 volts. 7.5 (as in my example) is equal to 75V. In theory you could read up to 300 volts, not just 30 volts. Then you'll have a good idea of what your generator is truly capable of. Sure, it requires a little math, but it's easier to fix the numbers than it is to fix the garage, or your neighbors roof or whatever may have gotten burned in the accident.

Last edited:

#### ianjuk

Joined Oct 10, 2020
14
Essentially Yes. BUT!

I don't have experience with zener's and am not familiar with their wattage capabilities. IF I were to build such a circuit it would start with understanding the parts available and their associated data sheets. Would calculate the max current the device might see and then go with 1.5 to 2 times the wattage rating needed.

Some are mentioning TVS's. Essentially they're zener diodes with high current capabilities. They can handle more than 1 watt. Remember, I picked that number for the sake of making the point that a resistor is needed in such a circuit.

Think of a TVS or Zener as a dam. As long as the water level (voltage) doesn't rise above the top of the dam no water flows down stream. But if the water level (voltage) goes above the top of the dam then whatever TOPS the dam is spilled off. The water flow (current) runs down stream and returns to the sea (back to the source). If the dam is barely overtopped the water runoff will be low (current). If the overtopping is much greater then the runoff will be much higher (higher current). If the stream it flows into can not handle the flow (current) then the water will spill over the edge and destroy the neighboring homes (blow your circuit).

Before you undertake such a build you need to become familiar with the devices you're working with as well as the associated math in calculating the needed components. Failure to understand them may result in a failed circuit and damaged equipment. So far we know nothing about the generator other than sometimes it exceeds 30 volts DC. How much current is it capable of producing? If it can produce only one amp then there's not very much that can go wrong. But if it produces 100 amps - it can be much more dangerous if something goes wrong. So always plan for the worst case scenario then design for 1 1/2 to 2 times the greater strength.

How much current can your generator generate? Wattage is a function of voltage multiplied by amperage. If it can produce 100 amps at 75 volts then that's 7500 watts of power it can generate. That's a lot of heat energy that needs to be managed; and must be a crucial part of the design of any voltage regulation. And understanding of how components fail will also dictate design requirements. For instance, if your TVS or Zener shorts out - 7500 watts can easily start fires. So a part of the circuitry needed should be a fuse. Something that is sacrificial. If something goes wrong the fuse blows out and protects much of the circuit. Obviously for the fuse to blow will likely mean that some component has already failed. The fuse just stops the current from flowing so that excessive heat does not build up and start a fire.

Whatever circuitry you come up with to limit the input to 30VDC, consider all possibilities; failure, unexpected voltages, size of wires to conduct said energy and how you're going to shut things down in an emergency. Also, what happens if you're not present when the emergency arrives? How can the system protect itself?

So please answer the following question: What's the rated output of your generator? We're looking for voltage, amperage, wattage, whatever information you have on it. If it's home made - pictures can aim us in the right direction.

Finally, I'm in favor of @MrChips suggestion (post #10) of a voltage divider over regulation. Knowing your generator maxed out the reading capabilities of the logger only tells you that you had 30 volts OR MORE, but you don't know how much more. Using the voltage divider Chips recommended gives you twice the measuring range. Of course, you've said you don't want to have to double the reading. Convenient, but might not be the safest approach. ME? I'd consider a divide by 10 voltage divider network. Then whatever voltage I read, just add a zero to it. For instance, if the reading is 3.2V then it's 32 volts. That's above the 30 volt range without going over the range of the logger. And if you get 6.8V, then you know it saw 68 volts. 7.5 (as in my example) is equal to 75V. In theory you could read up to 300 volts, not just 30 volts. Then you'll have a good idea of what your generator is truly capable of. Sure, it requires a little math, but it's easier to fix the numbers than it is to fix the garage, or your neighbors roof or whatever may have gotten burned in the accident.
Very concise and detailed, thankyou, esp the dam analogy

It/they are theoretically capable of generating 12v/500w (10a?) but unlikely to do so in anything other than pretty extreme conditions. They are basic Chinese made turbines, £100 a go. I'm not interested in their power generating capability, they're not charging anything. The research is a comparative study of blade designs, having moved from the lab to the field for an initial trial. They are literally on scaffold in said field so nothing's going to burn down, but I get your point. The reason for this is to protect the data loggers, or rather the data on them, as that's what's valuable. The loggers themselves are £60 so essentially disposable however we don't want to be replacing them every time the wind gets above eg: 40mph.

So, I need to make a "device" that will limit a data logger to exposure of voltages in excess of 30v. We're unlikely to see more than 40, but there's no meaningful load and therefore no brake so i'd rather be careful.

I started out, after researching, with the assumption a Zener diode solution was the way to go and was looking for help/clarification of the type I would need. This doesn't need to be particularly elegant, but it does need to be effective and consist of as few components as possible. We're not looking at high voltage or amperage, thankfully.

Hopefully that gives more context and information.

#### dl324

Joined Mar 30, 2015
16,988
the dam analogy
The water analogy causes more harm than good because it's not a good analog. Just learn how think about how things work with current.

Last edited:

#### Tonyr1084

Joined Sep 24, 2015
7,943
Blade efficiency may not always equate with RPM's. One blade may be capable of spinning faster, but not produce as much usable energy as another, slower speed blade. Remember, you're converting wind energy into electrical energy. Just because the blade spins faster doesn't mean it will capture as much energy as a slower blade. For meaningful comparisons, the generator should be loaded. Instead of measuring the voltage you should measure wattage, which is a truer measure of the efficiency of a blade design.

And the water analogy - - - sometimes it clarifies things for beginners.