I want to use an arduino power driver shield which contains the RFP30N06LE MOSFET to drive the 12V version of a solenoid valve which has a power consumption of 2.0 Watts (so 167 mA current draw I think?).

The power driver circuit does not appear to have any components to eliminate back EMF which I didn't even realize was an issue until I read some other attempts at driving solenoids with similar circuits (see: How to control a 12V solenoid valve with a mosfet?).

I'm planning to wire a flyback diode and zener diode as shown in the third circuit from the left in the first opening figure of: Can a Zener diode that protects a switch against inductance when the switch opens, affect turn on speed of the valve when you close it again?

So, my question is: Will the following flyback diode and zener diode that I've chosen work? Are there better options?

Flyback Diode (?): 1N4007 (probably overkill? But they're cheap)

Zener Diode (?): BZX85C36 (I'm a lot less sure about this one, but I think I need to choose a Zener Voltage that is high so that the solenoid discharges quickly but not over the MAX that the RFP30N06LE can handle?)

I'm quite inexperienced with electronics so any other tips would be greatly appreciated!

  • \$\begingroup\$ Thanks for all the links in the question, it shows respect for any answerer's time, and makes it clear exactly what components are being considered \$\endgroup\$
    – Neil_UK
    Oct 6, 2015 at 8:19
  • \$\begingroup\$ If I might ask, why is fast turn-off important to you? Do you have numbers to back up your requirements? How fast (at what frequency or what transition time) do you propose to cycle your valve? \$\endgroup\$ Oct 6, 2015 at 17:54
  • \$\begingroup\$ The solenoid data sheet says it has a response time of ~10 ms I'm assuming that it will probably take a bit longer. So I'm perfectly happy if I can get a response time of ~20-40 ms. \$\endgroup\$
    – NJM
    Oct 6, 2015 at 18:19

1 Answer 1


Can the zener affect the turn on time?

No. If it's not conducting, it's doing nothing. If it is conducting when you come to turn on, that means the relay has still some decaying current flowing, so it will turn on faster.

'The power driver circuit does not appear to have any components to eliminate back EMF'?

I agree, not on the face of it. However, follow your own RFP30... link to the data sheet. Look at figures 14 and 6. You will see that the FET itself is hardened against precisely the back EMF that an inductive load causes. Figure 6 shows that even starting at a die temperature of 150C, it will handle 3A for 1mS, 8A if it starts cool. During this time, the avalanche voltage across the device will be at least 60v. This is the voltage that's available for ramping down the solenoid current.

In order to use this information, you will need to know the inductance of your solenoid load, and the DC current it takes in operation. How long will it take to ramp down from its operating current at 60v? Will that fit inside the figure 6 lines? If it does, then there's no need for the protection components. To me, the RFP30... looks pretty tough.

But, you can use a diode and zener anyway, they won't do any harm. If you do, they must clamp before the 60v is reached, otherwise the FET will be protecting them, rather than the other way round! With a diode and zener, you'll have a lower turnoff voltage than using this FET alone. If you think you'd prefer a zener+diode because you don't want to do the sums on the FET, then don't forget, you should do the pulse-handling sums on the zener as well, how long can it conduct x current at 36v before overheating? You'll probably find the TO-220 FET is waaay harder than the zener!

The reason I went digging in the data sheet is the question 'how could SparkFun have a successful business selling switches that were going to break?'. Warning, not all FETs are this tough, and few BJTs are. Always assume you'll need back EMF protection until you've done the sums on the specific driver.

  • \$\begingroup\$ Hi user44635, many thanks for the detailed answer and explanation. It took me a bit more research and reading up on the terms you were talking about but it sounds like: 1) The Zener won't affect the turn on time (but it will affect the time it takes to turn off the solenoid correct?) 2) The RFP30 FET that I'm using has 'better' specs compared to the flyback+zener combo that I was suggesting, but that I should check the inductance of the solenoid-valve to ensure that the FET can handle the solenoid. \$\endgroup\$
    – NJM
    Oct 6, 2015 at 16:08
  • \$\begingroup\$ 3) If I do choose to use a zener+flyback combo I need to make sure the zener voltage is lower than 60v, otherwise the FET will be protecting the zener+flyback instead. Is that basically correct? \$\endgroup\$
    – NJM
    Oct 6, 2015 at 16:09
  • 2
    \$\begingroup\$ If you do choose to use a zener+diode combo, they won't be doing anything unless the clamp voltage is <60v, so you needn't have bothered, and they will slow down your turn-off. What I'm really saying is don't use zener+diode. \$\endgroup\$
    – Neil_UK
    Oct 6, 2015 at 16:35

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