The most common method to speed up relay turn-off (AFAIK) is the zener diode.
Nope, a zener it is indeed. This is a pretty good description of the concept:http://relays.te.com/appnotes/app_pdfs/13c3264.pdf--The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected
The slowing of the release is small compared with the over all slowness of using a mechanical relay. It is not something that is significant in the context of what the OP is worrying about.
In the same article is written, one can speed up the release time by adding a resistor in serie to the diode. My problem is that I have no clue (and here the article didn't help me) how to calculate this resistor. And due to the fact that I cannot measure this time, I have no possibility to go via trial and error.
....So....if you have an IRF610 rated for 200V, that's not going to be a problem (MOSFET breakdown voltage). I'd worry more about the breakdown voltage of your relay coils. If you can't find that specification I'd say a safe number is the voltage rating of the coil itself under normal operation (18VDC). If you add a 12V-18V zener diode I think you will have a good result....
...A simple low value resistor, say 10 ohms or less will probably be sufficient to minimise hold-up time, but it will be a balance between decay time versus back emf....
...The slowing of the release is small compared with the over all slowness of using a mechanical relay. It is not something that is significant in the context of what the OP is worrying about....
I will try to get now a zener diode 18V to do some tests, and check whether I can see a change to the todays performance.
I will give this one also a trial. Advantage: Probably I have some resistors in the range of 10 Ohms available.
.... Max value of resistor = (((Vds voltage rating of mosfet)/(relay supply voltage)) - 1) * (resistance of relay coil)The higher the resistor, the faster the release time. Do the above calculation, for the highest relay supply voltage you are likely to get in your circuit, and using the resistance of the relay coil you measure when it is cold. Then use a value a bit lower than calculated to allow some margin.....
....A Zener diode is in theory faster, but if you have a 100v mosfet then you would need an 80v Zener diode, which will be hard to find.....
...It may even be safe to omit the kick back diode completely and allow the energy to be absorbed as avalanche energy in the mosfet (effectively, the mosfet itself behaves like a Zener diode), however to know whether this is safe you would need to know the amount of stored energy that is released when the relay opens. [You could use the Arduino to measure this.]....
...However, if you're trying to do water drop photographs, then mechanical relays may be too slow for what you want anyway.
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