Using a DB3 diode as a flyback for a 24vdc relay

Can I use the DB3 diode as a flyback for a relay that works on 24v dc?

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• Thank you Andy, all answers are helpful, I pushed the little triangle up, but I didn't mark any of them the right answer, because I didn't find the solution yet, I'm hoping for more answers. Thank you again, and I'm sorry for thi behavior. Apr 18, 2020 at 10:38
• After your edit we're still missing the part number and link to the datasheet for the switch and the value of the capacitor. Apr 18, 2020 at 16:48
• @Transistor, I don't have them, the only thing I can see 40v on the cap, because it's inside the glue. Apr 18, 2020 at 16:54

No, DB3 is not a diode. It is a diac, a non-directional semiconductor switch that can be turned on when its breakover voltage is exceeded.

Breakover voltage of DB3 is 28V. Your unregulated power supply voltage could touch 34V. The DB3 would get triggered and short circuit the power supply, thereby damaging itself and/or the power supply in the process.

A 1N4001 would suffice as a freewheeling diode for this application.

• Thank you for your answer, wow! How did you know that the voltage could touch 34v? Because when I measured the voltage before my question, I found 36v, and I thought that something was wrong with my multimeter. Apr 18, 2020 at 16:08
• @Shaydzmi Calculus will show that a 24 Vrms AC voltage must peak at approx. 34 V since $V_{peak} = V_{rms} * \sqrt{2}$ for a sine. Apr 18, 2020 at 16:45
• You're welcome, Shaydzmi. And you've got your answer for the 34V! Apr 18, 2020 at 16:54
• Thank you, Shaydzmi. Jun 22, 2020 at 10:42

You don't need a snubber diode.

simulate this circuit – Schematic created using CircuitLab

Figure 1. The bridge rectifier acts as a snubber for the relay.

When the AC switches off the relay current will continue to circulate through the diodes of the bridge rectifier.

Since you have clarified that there is a switch in the circuit but haven't given any details on it we can only suggest this circuit.

simulate this circuit

Figure 2. As @MarcusMüller has suggested, placing the switch in the AC side solves the problem.

• Thank you, no the snubber is needed to protect a proximity switch that drives the relay. Apr 18, 2020 at 10:44
• If that is the problem then please edit your question and add in the make, part number and link to datasheet for the proximity switch. I can see it now on your drawing but the image quality is poor and it wasn't obvious. Apr 18, 2020 at 10:47
• @Transistor, A freewheeling diode is a must to protect the magnetic switch! Apr 18, 2020 at 10:47
• @vu2nan the bridge rectifier is a free-wheeling diode, if you will. Apr 18, 2020 at 11:26
• @Marcus, if you look carefully at the circuit sketch you can just make out the "magnetic switch" on the positive rail (at the bottom) between the bridge and the relay. I'm still waiting for details on the switch. Apr 18, 2020 at 11:30

https://www.st.com/resource/en/datasheet/db3.pdf

Breakdown voltage 28V minimum Repetitive current limit: 2A,

Also its a back to back device, so there is no forward voltage, I would say this is a poor choice for a relay unless you needed the relay to close very quickly. if your supply voltage gets a little too high, or your circuit re-powers the relay before the breakdown has recovered, it will short out the supply to the relay may destroy itself from the excess amount of current flowing through it,

Avoid if at all possible for this application.

• Thank you for the answer, the relay is a part of an electromagnetic counter, so it closes for very short time, less than a half second. I didn't understand all of what you wrote unfortunately. Apr 18, 2020 at 10:42