# 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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This is a new photo :

• 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. – Shaydzmi Apr 18 '20 at 10:38
• Well, if there is something that needs addressing that hasn't been done so in the current answers, you should leave a comment asking for clarification. Asking for clarification on an already given answer surely must be the obvious route rather than waiting in vain for other answers to appear. – Andy aka Apr 18 '20 at 10:52
• After your edit we're still missing the part number and link to the datasheet for the switch and the value of the capacitor. – Transistor Apr 18 '20 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. – Shaydzmi Apr 18 '20 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. – Shaydzmi Apr 18 '20 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. – nanofarad Apr 18 '20 at 16:45
• You're welcome, Shaydzmi. And you've got your answer for the 34V! – vu2nan Apr 18 '20 at 16:54
• Thank you, Shaydzmi. – vu2nan Jun 22 '20 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. – Shaydzmi Apr 18 '20 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. – Transistor Apr 18 '20 at 10:47
• @Transistor, A freewheeling diode is a must to protect the magnetic switch! – vu2nan Apr 18 '20 at 10:47
• @vu2nan the bridge rectifier is a free-wheeling diode, if you will. – Marcus Müller Apr 18 '20 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. – Transistor Apr 18 '20 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. – Shaydzmi Apr 18 '20 at 10:42