I've got an automotive relay that contains a resistor in parallel with the coil, and I measured what effect having a freewheel diode would have.
The relay is standard cube one. The coil is about 120 Ohms, and has a 680 Ohm resistor in parallel, so the total resistance is around 98 Ohms and it takes about 115 mA when supplied with 12 V. I supplied the relay through a pushbutton switch and I had a 1 kOhm pull-up on the contacts to monitor how long the contacts took to open.
Here is the waveform:-
The voltage peaked at around 65 V and the relay took about 2.6 ms to open. It's interesting to see that the current increased slightly as the armature moved.
With a diode in parallel with the coil, it's very different:-
The voltage stayed at 0.7 V and the current took 25 - 30 ms to die down. It took around 8 ms before the contacts started to open. I found it interesting that the contacts bounced for around 2 ms with at least two false openings before the finally opening. The bounce was quite consistent and happened much like that every time the relay released.
My take from this is not to use a diode to suppress the inductive spike on a relay if the contacts are taking anywhere near their maximum rating. Any sort of heavy load or inductive load on the relay is going to damage the contacts more than turning the relay off quickly. The coil suppression almost inevitably slows the relay release, so the amount of suppression is a compromise between protecting whatever is controlling the coil and protecting the contacts.
Of course, some suppression is vital in most applications. This is what it looked like when I disconnected the resistor:-
It's even faster than with the resistor but the inductive spikes are huge.
The relay is standard cube one. The coil is about 120 Ohms, and has a 680 Ohm resistor in parallel, so the total resistance is around 98 Ohms and it takes about 115 mA when supplied with 12 V. I supplied the relay through a pushbutton switch and I had a 1 kOhm pull-up on the contacts to monitor how long the contacts took to open.
Here is the waveform:-
The voltage peaked at around 65 V and the relay took about 2.6 ms to open. It's interesting to see that the current increased slightly as the armature moved.
With a diode in parallel with the coil, it's very different:-
The voltage stayed at 0.7 V and the current took 25 - 30 ms to die down. It took around 8 ms before the contacts started to open. I found it interesting that the contacts bounced for around 2 ms with at least two false openings before the finally opening. The bounce was quite consistent and happened much like that every time the relay released.
My take from this is not to use a diode to suppress the inductive spike on a relay if the contacts are taking anywhere near their maximum rating. Any sort of heavy load or inductive load on the relay is going to damage the contacts more than turning the relay off quickly. The coil suppression almost inevitably slows the relay release, so the amount of suppression is a compromise between protecting whatever is controlling the coil and protecting the contacts.
Of course, some suppression is vital in most applications. This is what it looked like when I disconnected the resistor:-
It's even faster than with the resistor but the inductive spikes are huge.