Solenoids and clamping diodes

[FlipFlops]

I'm using a microcontroller to control the length of time a solenoid is kept energized, in combination with an n-channel mosfet. I understand the importance of a clamping diode to handle the inductive voltage created when the magnetic field collapses.

However, I've also noticed that the presence of the clamping diode reduces the speed at which the solenoid opens and closes, and this has an effect on my timing and cyclic rate. Is there a certain kind of diode that is better suited for this scenario? So far I've been using schottky fast recovery diodes. If there's some way I can protect the mosfet without slowing down the solenoid, that would be even better.

Thanks in advance.

 

[Ubergeek63]

The diode might delay the opening time but should not effect the closing time. You could try a zener in series with the diode.

That way the inductance can discharge into a higher voltage so the field can collapse quicker. For instance if you have a 12V rail and a 50V FET put in say a 25V zener.

 

[FlipFlops]

Uber,

Thanks for the info. Would a zener diode by itself suffice?

 

[dknguyen]

You can use a resistor and capacitor instead of a diode. You can then adjust the R and C to change the opening and closing time.

If you used a TVS diode of the right clamping voltage then it should improve things. It's like a zener diode, but much faster. A zener diode is designed to clamp steady voltage, and a TVS diode is designed to clamp fast transients.

You might also try using a bunch of your fast recovery diodes in series so the clamp voltage is higher but still within safe limits. That should also reduce the opening time.

 

[crutschow]

Thanks for the info. Would a zener diode by itself suffice?

No. The zener polarity has to be opposite polarity to the normal diode. Otherwise the zener would just act as a normal diode.