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I am part of a physics project group, and we are trying to design a relay driver.

Our Relay is rated at 24VDC (it is a Tyco Electronics 1432826-1). What zener diode (or regular diode if that would work better) would work best as a flywheel diode and why? Our Vin is also variable.

We are basing the setup off of this pictureenter image description here

Although, of course the zener diode and 9V are incorrect.

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    \$\begingroup\$ Use a zener AND diode as shown in part of the schematic at this answer. \$\endgroup\$ – jonk Apr 8 at 19:46
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You don't need to use a zener to obtain the flywheel functionality (assuming you only drive coil current in one direction). Any PN junction diode capable of handling your coil voltage as a reverse voltage, and the coil current (plus some margin) as its forward current, will do.

You could use a zener to simultaneously protect the drive circuit from flywheel currents, while protecting the relay from over-voltage. But you'd need to do something to limit current into the zener in overvoltage conditions and this would also affect the coil current during normal operation.

However, the coil is probably more robust to the kind of things zeners could protect it from than whatever is driving the coil, so I'd stick with just using a silicon PN diode for flywheel protection and not worry about a zener.

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    \$\begingroup\$ A zener diode would allow the magnetic field in the relay to collapse faster, affecting the relay performance a bit \$\endgroup\$ – uglyoldbob Apr 8 at 19:29
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    \$\begingroup\$ @uglyoldbob I was about to write just that. It's important to keep this in mind and should be mentioned every single time this subject is discussed in order to keep it in plain view on top of the table and not hidden away and forgotten. \$\endgroup\$ – jonk Apr 8 at 19:43
  • \$\begingroup\$ If power consumption is not a concern, putting a resistor across the coil can achieve the same effect. \$\endgroup\$ – EinarA Apr 9 at 0:52
  • \$\begingroup\$ This seems to have done the trick, as we ended up trying to use a 1N4148 and it worked perfectly. Thank you! \$\endgroup\$ – Crush Apr 12 at 16:34
  • \$\begingroup\$ @Crush, if you have decided which answer did most to solve your problem, you should accept that answer. (Also, the comments are correct that using a snubber able to support a larger voltage will quench the inductor current faster) \$\endgroup\$ – The Photon Apr 12 at 16:36
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You need to add a switch (at the battery) to your diagram to see the issue. When the switch opens, the inductance of the relay coil will cause the current to want to keep flowing. With nowhere to go, a huge negative voltage will form at the positive side of the relay. This voltage will stress whatever switch (mechanical or electrical) that you use. The diode provides a safe path.

Now that you understand the issue, you should be able to determine what type of diode will help.

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