TVS diodes

  • Thread starter thomsonm
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  • #1
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Main Question or Discussion Point

Hey, I'm trying to find a suitable diode to protect against some back EMF in my circuit, but I don't know much about them and so i'm having a really hard time finding one. If someone could look at what i need and then possibly send me a link i'd be so grateful.

Requirements:
back EMF = 137mV
must be TVS diode with component leads
must be unidirectional

Thank you so much if you can help me with this, i'm at my wits end with this project!
 

Answers and Replies

  • #2
berkeman
Mentor
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Hey, I'm trying to find a suitable diode to protect against some back EMF in my circuit, but I don't know much about them and so i'm having a really hard time finding one. If someone could look at what i need and then possibly send me a link i'd be so grateful.

Requirements:
back EMF = 137mV
must be TVS diode with component leads
must be unidirectional

Thank you so much if you can help me with this, i'm at my wits end with this project!
137mV is a pretty small voltage. What is the application?
 
  • #3
13
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I'm trying to make a circuit for an electromagnet so that it pulses when a switch is triggered. Obviously once it's pulsed the magnetic field will collapse and create a back EMF.

The component i'm trying to save is a microchip that i'm using as a switch.
 
  • #4
vk6kro
Science Advisor
4,081
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If you use a transistor to do the switching, you can drive it from your micro. This takes the expensive micro away from any danger and it also allows you to switch more current if you choose a suitable transistor.

Back EMFs are still a problem, and these can be hundreds of volts. This is enough to destroy many transistors.
You can put a diode across the inductor so that it will conduct when there is an EMF and this may be enough to save the transistor.

In the following diagram, resistor R must be chosen to suit your transistor.

[PLAIN]http://dl.dropbox.com/u/4222062/electromagnet%20driver.JPG [Broken]
 
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  • #5
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Yeah that's fantastic, but I'm really just looking for a link to a website where I can buy the diode in question, I'm not looking for alternatives.

Thanks for the response, but it's more than I needed
 
  • #6
vk6kro
Science Advisor
4,081
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137 mV is only 1 / 7 th of a volt so it is no threat to anything.

So you won't find anyone making suppression devices for this voltage.

Could you post your circuit and give your calculations for how you arrived at this figure?


That voltage happens to be about the voltage you would get across a Schottky diode conducting in the forward direction. Maybe that is what you mean?
 
  • #7
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Yeah that's fantastic, but I'm really just looking for a link to a website where I can buy the diode in question, I'm not looking for alternatives.

Thanks for the response, but it's more than I needed
If you need to limit back emf to 137mV, it is going to be hard to find a TVS that accurate. Say if your magnet is hooking to 10V, you cannot find a TVS that turn on at 10.137V. I use a lot of TVS, you cannot control that. You'll be lucky to find one even to within 1V.

You better off looking at your design and relax the requirement. I design a few magnet controllers including a 40A high speed high precision controller, never even run into anything close to this requirement. What VK6kro has is the most common way. You can substitude with a schottky diode to limit the overshoot ( back emf) to about 0.4V. This is it, any lower than that, you're going to be beating up your head.

A TVS is just like a high speed zener diode that turn on very quick when you pass certain voltage. They are designed for arc, lighting protection. I don't think you should use it for this purpose. They are not accurate at all. Good luck on finding one that is within 1V accuracy.

Like VK6kro said, 137mV is not going hurt anything. If a design cannot take this back emf, it is the design you need to look at. Post the schematic, I am sure a lot of people here can come up with a solution in no time. Trust me, full filling your requirement is going to be 10 times harder than to redesign a new controller that don't require this.
 

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