TVS Diodes for Relay Circuits
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    TVS Diodes for Relay Circuits

    If you are adding a relay to your tractor for any kind of electrical project, you will need a TVS diode for the circuit. This diode protects the contacts from arcing on the switch that controls the relay coil. Arcing will damage the contacts and reduce their useful life.

    In the GTT Technical Library, eepete provides a detailed technical explanation (in layman's terms) of how these diodes work and are used. LINK HERE.

    This is what the diode looks like:

    Click image for larger version. 

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    I recently installed a horn relay. The twin horns required 14 amps and I wasn't sure a push-button switch this large would fit on my dash, so I used a smaller, low-current switch to control a relay with 20 amp contacts.

    For my circuit, I selected an SA18CA TVS diode. This diode clamps any transients starting at 18 Volts. Normal voltages in a tractor don't exceed 15 volts, so the diode won't turn on for anything except for unwelcome transients. This diode is bi-directional, which means you can connect either end of it to either end of the relay coil. For a data sheet on the diode, simply type "SA18CA data sheet" into any internet search engine.


    You can see this diode in a real circuit diagram in this post. In the photo below, you can see one of these diodes installed on my horn relay.

    Click image for larger version. 

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    Click image for larger version. 

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    When I strip the wires that will be connected to the coil, I take the stripped insulation and put it on the diode leads, leaving enough exposed to crimp. Then I crimp the diode's leads into the same quick connects (along with the wire) that are used for the wires going to the coil terminal.

    This little component, in small quantities, can be purchased from someone like DigiKey for $0.54 each. But the shipping will dramatically increase the cost unless you need some other stuff that DigiKey offers. I found a deal on E-bay where I could buy 10 for $5.68, including shipping, from a surplus company. (I could buy 10 for less than I could buy 1.) I needed only 1 and I'm going to keep a couple for possible future projects, but the other 7 diodes need a good home. If you have a need for one, please PM me with your address. I'll put it in the mail for you.
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    Captain Hook Kennyd's Avatar
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    Good post Keane, thanks.

    Some relays have a diode built in I think, negating the need for an external one.
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    I followed the link back to eepete's outstanding explanation. It brought back a lot of memories. I can't add to the Tech Library post so I'll put it here.

    The energy left in the coil tries to go back on the circuit that energized it, either the contacts of another relay in a complicated control circuit (circa 40's - 60's) or the switch that energized it (more today). It usually caused arcing on those contacts. The simple diodes were called antichatter diodes because sometimes all the energy excitement caused the controlled relay to "bounce" making new problems somewhere else.

    We used to make shockers with 3 parts: a battery, a coil and a switch all in series. From each side of the switch there was a wire or lead we would hand to someone. Close the switch, charge the coil. Open the switch and all that coil energy became a much higher voltage and ZAP.

    I know, I'm showing my age here.
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    I don't believe a thyrector (that's what this device is, essentially) does much to protect a switch. Not. at least, a switch sized properly for the device being controlled.

    The voltage spikes from a reactive load are far more significant when the controller is a solid state device like a transistor. By significant, I mean they are more likely to do damage since the reverse voltage spike upon shutoff of the device is more likely to exceed the Vce breakdown voltage. Typically this is suppressed by placing a conventional rectifier diode across the inductive load opposite to the circuit polarity.

    On the other hand, if you are hell bent on using miniature switches with gold plated contacts meant for small signal resistive loads, then yeah, you might consider some protection.

    Al
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    Quote Originally Posted by Kennyd View Post
    Good post Keane, thanks.

    Some relays have a diode built in I think, negating the need for an external one.
    This is the easy way to do it. There should be evidence of a built-in diode either on the device or on its data sheet.
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    Quote Originally Posted by AlKozak View Post
    I don't believe a thyrector (that's what this device is, essentially) does much to protect a switch. Not. at least, a switch sized properly for the device being controlled.

    The voltage spikes from a reactive load are far more significant when the controller is a solid state device like a transistor. By significant, I mean they are more likely to do damage since the reverse voltage spike upon shutoff of the device is more likely to exceed the Vce breakdown voltage. Typically this is suppressed by placing a conventional rectifier diode across the inductive load opposite to the circuit polarity.

    On the other hand, if you are hell bent on using miniature switches with gold plated contacts meant for small signal resistive loads, then yeah, you might consider some protection.

    Al
    Your point about solid state electronics is a good one. My 1025R has a lot of that in the dash console control module. My old tractor had nothing of the kind. It could be good practice to stifle any transients just so you don't have to wonder if they can get into that module, directly or inductively. For example, when I tap into the reverse switch to add backup lights, I think that switch goes straight back to the control module. I don't know what kind of protection they have built into that module. At the cost to replace it, I don't want to acquire the knowledge the hard way.
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    Quote Originally Posted by RadarDon View Post
    I followed the link back to eepete's outstanding explanation. It brought back a lot of memories. I can't add to the Tech Library post so I'll put it here.

    The energy left in the coil tries to go back on the circuit that energized it, either the contacts of another relay in a complicated control circuit (circa 40's - 60's) or the switch that energized it (more today). It usually caused arcing on those contacts. The simple diodes were called antichatter diodes because sometimes all the energy excitement caused the controlled relay to "bounce" making new problems somewhere else.

    We used to make shockers with 3 parts: a battery, a coil and a switch all in series. From each side of the switch there was a wire or lead we would hand to someone. Close the switch, charge the coil. Open the switch and all that coil energy became a much higher voltage and ZAP.

    I know, I'm showing my age here.
    The kid next door to me made one of these "shockers" where I grew up. I was set to make one, too, but mother was not encouraging. He had a lot of fun with his. He wired his into the seat of a chair and then invited people to have a seat.
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    In the 60's I had a book with a foil covering that had a very suggestive title that was wired with a coil and a spring loaded magnetic switch that would jump up and down when the book was opened. You didn't get just one shock you got several in a row. It was very enlightening. I had a German Instructor in electronics trade school that spotted the book in my briefcase when walking the class during a test. He opened it up and the look on his face as the book popped him several times was priceless! He had to borrow the book for the day. I expect the other instructors got quite a charge out of reading it.
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    It seems as if I have tickled some other's memories. I'm delighted.

    One thought to keep in mind. The relay coil and contacts are isolated from each other by design. Any coil discharge can't get to the relay contacts so it goes back on the circuit it is connected to. But then there were the self latching relay connections....

    I remember a new AFC circuit modification that replaced a couple vacuum tubes with peanut tubes. They were smaller so you could put a more complicated circuit in the same space. Transistors were still kind of a new thing. The simple relay anti-chatter diode was appropriate for the old relay control circuits. Heck, I still have my contact burnishing tool. Never let a good tool go.

    Today, if you want to get fancy you can use semiconductors instead of relays. If you don't mind paying for it that is. Just think solid state relay. No coil spike at least.

    Our needs are simpler so a cheap (but quality) relay is really all we need. I should hope the tractor electronics are input protected as many other things can cause a spike on the line and potentially kill them. Still, not to tempt the demon Murphy, we should do our part to eliminate any spikes we might cause. Since I'm not current I won't try to recommend specific devices. I was mostly remembering the old days.

    I taught radar systems around the world for about 45 years. One pleasant surprise I had was that almost every electronic device ever invented was used in a radar somewhere. Some were used only in radars. Just like space programs, there were spinoffs. Think Radar Range or micowave oven. I am happy I was able to watch electronics grow up to young adulthood. I wonder what is next? Ah, sorry, the teacher in me goes on and on.
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    Interesting topic. One near and dear to my heart. That's probably for the many times I slogged a half mile or so through the mud to swap out a Microswitch with welded contacts on a center pivot tower. All because a 50˘ snubber circuit had a broken wire where it was attached to the contactor coil. These were AC circuits so used RC snubber circuits.

    Now for a real life DC example. Here are a couple of captures of the voltage across switch contacts that are driving a 24 VDC relay coil. This is when the switch opens. On the left is a relay coil with a reverse biased diode across the coil to provide surge suppression. Nothing special, just a 1N4004 or 1N4006 or similar from my "stack of stuff." There is no spike on the switch when the contacts open as the inductive energy dissipates through the diode.

    On the right is the same relay coil with no suppression The voltage spikes to about 228 VDC. That inductive energy has to go somewhere so ends up as an arc across the switch contacts. Same principles apply to 12 VDC.

    It's hard to see the decimal points on the screen captures. The left one is scaled automagically by the scope software from -18.8 to 31.2 volts. The right one is 10 times that or -188 to 312 volts.

    Click image for larger version. 

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