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Could a 1n4007 diode be used as a protection diode for inductive loads like a relay?
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Yes.
But a 1N4148 would be better.
A schottky would be better still.
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The flyback diode must have a voltage rating no lower than the maximum supply voltage used to drive the relay, and a peak current rating no lower than the maximum relay coil current that is being switched. Any diode that meets those criteria will do. I would use a 1N4148 for a typical 5V or 12V relay.

The flyback diode is more critical when the inductive load is being PWM'd, especially when the PWM frequency is high. In that case, a Schottky diode or fast-recovery silicon diode is needed to help keep switching losses low.
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And of course the diode current rating must be sufficient for the relay current. So I would not use the 1N4148 or 1N914 small signal diodes as they are rated at only about 100 to 200mA.

Also, small signal diodes tend to fail open, whereas higher current diodes tend to fail short. In one case, it potentially destroys the driver transistor by presenting an overcurrent condition, in the other it may put a big voltage spike into the Vcc line and destroy other components. So I avoid using small signal diodes as flyback diodes.

The reason they fail differently, I think, is due to the bonding wires. I think in many cases the diode die fails shorted, but in the case of a small signal diode the bonding wires are very thin and so act like a little fuse. In addition, sudden heat may cause the glass to shatter, allowing atmospheric oxygen to burn the bonding wire. But with larger high current diodes, larger bonding wires are used or the leads are even bonded right to both sides of the diode and they typically use tougher plastic cases. And so the current limitation becomes the PCB traces, power source, or transistor driving the relay.

I am not saying that failure always happens this way, and there is no clear cut line between failure modes. And certainly lots of small signal diodes fail short circuit because they are typically used in low current parts of the circuit. But when higher currents are available, the above described tendency of small signal diodes to fail open circuit more often than higher current diodes has been my experience.

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ULN series of driver ICs and motor drivers like L293d have these diodes built into them....Good to one of the ULN seires if you are driving multiples relays
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And of course the diode current rating must be sufficient for the relay current. So I would not use the 1N4148 or 1N914 small signal diodes as they are rated at only about 100 to 200mA.

1N4148 is rated at 200mA continuous, 450mA peak repetitive @ 25C. So it's more than adequate for a typical relay that takes 100mA or less coil current. If I was using a big relay that took more than 150mA coil current, then of course I would use a bigger diode.

Your comments about the failure modes of diodes are interesting. I have not seen enough diode failures to draw any conclusions from experience.
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ULN line: Like.

BTW, relays take longer to release when using just a protection/flyback diode. This can sometimes cause problems with contacts burning. This is because the decay time constant for an RL circuit is t=L/R, so the lower R is, the longer Tau becomes. Tau is the time to decay to about 37% of the initial current.

As long as the driver transistor can withstand the extra voltage, you could put a resistor in series with the flyback diode. Using a resistor equal in value to the relay coil's resistance means it'll drop a voltage equal to Vcc, so the driver transistor must withstand about 2xVcc. A higher value resistor will cause a higher voltage, but faster decay.

There is also something called a "snubber circuit" that is designed to protect the relay contacts, consisting of a series capacitor and resistor. The two are connected across the contacts of the relay. This circuit is also used across Triacs and SSRs to protect them when driving inductive loads.
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I've helped a lot of friends with their circuits, repaired a wide variety of electronics, and made a lot of the same mistakes.

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Thanks but I wouldn't want to take any chances with that 1n4148 diode. I thought that was a signaling diode? But I use usually a 12 volt 600 ohm relay. That is about 20 milliamps? But thanks for your help. I try the 1n4148 diode too. But for speaker can I still use the 4007 diode?
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And of course the diode current rating must be sufficient for the relay current. So I would not use the 1N4148 or 1N914 small signal diodes as they are rated at only about 100 to 200mA.
200mA continuous, 450mA repetitive pulse rating, not too shabby really.
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Also, small signal diodes tend to fail open, whereas higher current diodes tend to fail short. In one case, it potentially destroys the driver transistor by presenting an overcurrent condition, in the other it may put a big voltage spike into the Vcc line and destroy other components. So I avoid using small signal diodes as flyback diodes.
Interesting datapoints there (surely the spike goes through transistor not Vcc though, not that it matters really)
Is there a good choice of schottky diode that fails closed then?
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If the diode fails open, the voltage spike on the coil appears across both the transistor and Vcc to ground.

The failure path seems to be first the diode die itself develops a low resistance, then something limits the current. In the case of a small glass signal diode, there is a very fine wire connecting one side of the die to the lead. It may act like a fuse and open. Or the die may heat and crack the glass.

With a higher current diode (schottky or not), since everything is built for a higher current, the hope is that the power supply will limit current in a non-catastrophic way.

In a circuit such as this, the chance of diode failure is low. I'm just saying I feel less than confident putting a relatively fragile (physically and electrically) small signal diode in there.
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