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Input on the proper use of a flyback diode, and determining its specs.

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redindia

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I've included a diagram to help convey what I have going on here. I'm mostly self taught so I can definitely some use feedback if anyone has any suggestions or if you see something I am doing wrong (ie: Am I using the right resistor for the base of the transistor? Is the transistor going to reach saturation?)

I have 2 channels of 12v solenoid valves which are being controlled via a micro-controller. I am using a TIP120 transistor for each channel. I am turning the valves on and off very quickly. Valve is open for ~50ms, then closed for 150ms, for a total of 5 cycles per second.

I need some flyback protection but I honestly have no idea what the best approach is and how much protection I need. I know a flyback diode will slow the rate at which I am currently controlling the valves and that may be a compromise I have to make.

Should I use a single diode for each channel, which is placed right after the transistor? This would make for a total of 2 diodes.

Or do I need a diode for each and every inductive load, making for a total of 19 diodes as they are attached directly at the valves?

How do I then go about calculating which diode will meet my needs without severely compromising the speed of which I can control the valves?
 

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1 for each set will be fine. I would use a 3 amp schottky diode just to be safe safe.
You probably won't notice the delay in turn off time.
The diodes should be rated over 12 volts.... maybe 25 volts would be common.
 
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The peak current in the diode is what was flowing in the inductor just prior to the switch opening (250mA) . The peak inverse voltage applied to the diode is the power supply voltage. To be most effective in preventing EMI to other circuits, each solenoid should have its own diode, and the leads between the coil and it should be as short as possible.

With a snubber diode across it, I worry a bit about how long the solenoid will take to release. Try it and see if the diode causes the solenoid to hang in too long. If so, there is another way of suppressing it which will release faster.
 
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