Flyback circuit question

Hi all, newbie here so be gentle! I have some electronics knowledge but am very rusty.

My question is thus:

I intend on controlling a 24v solenoid valve via a photocoupled mosfet tlp241a The maximum power of the coil will be 20w (hopefully 10-15w when all is said and done)

Now because the solenoid is an inductive load i believe ill need a snubber.

I was origonally thinking a 1n5404 diode. Would this be sufficient? Or overkill?

Or am i better to look at a capacitor and resistor combo?

Or something else.

Im looking for a reliable, cheap and simple solution.

Thanks in advance to all who reply!

A single diode would be fine. Typically a 1N4001 or similar is enough in your situation.

Ok thanks. What about if i run 2x solenoids off the one fet? Should push the current up to 1.7amp

A 1N400x (1-4) would still be fine for simple on/off. Short duration peak current of that diode is about 25Amp from memory.

If you're going to PWM the solenoid, you might be better off with a schottky diode. Leo..

Thanks leo, no pwm on these. Just on/off, although the solenoids are potentially going to have a lot of on/off cycles over their lifetime. I wouldnt imagine that would bother the diodes tho...

Is a flyback diode really needed with this device? It has 2 mosfets and 2 diodes.

A flyback diode across a solenoid shorts the kickback when the transistor turns OFF. Without a diode, the collector drain would see many times the supply voltage of the solenoid. Leo..

Did you look at the device the OP is using?

And yes I know what a flyback diode does and that is why I raised the question.

I don't see what difference an opto fet makes. If you switch a solenoid with a common N-channel fet with integrated body diode, you still need a diode across the solenoid. Because that diode is in the wrong place. Leo..

The 2 Diodes in the optocoupler are for static protection, they won't work as kickback suppressors.

Thanks all for the discussion. Very informative. One more question, ive got a stack of 1n4007's lying around. Any drawbacks if i use them instead of a 1n400 1-4?

Ok to use the 1N4007.

The 4005-7 are AFAIK slightly slower to turn-on, but that does not matter here. Leo..

Here is an image of from the datasheet.tlp241a.JPG

tlp241a.JPG

Wawa: Ok to use the 1N4007.

The 4005-7 are AFAIK slightly slower to turn-on, but that does not matter here. Leo..

No diode fails to turn on fast enough for this use - though the forward voltage can be quite big (5, 10V even), for very short periods of time, this is well below the dangerous voltages the inductor would otherwise generate. The capacitance of the diode carries the initial current step until the pn-junction is full of carriers.

Turn-off time is a very big deal for rectifiers, but for inductive kick-back suppression it is immaterial.

Interesting to note, though it will not concern you, a flyback diode across a solenoid coil, or a relay for that matter, will keep the current circulating in the coil for a short spell before it finally decays causing it to stay energised for a slightly longer time that it normally would.

justone:
Here is an image of from the datasheet.tlp241a.JPG

I don't think you are seeing what function the flyback is doing. It's sole purpose is to mitigate the high transient spikes when the field in that solenoid collapses. Without it, the drain is going to see those spikes, and the body diodes are not meant to handle that. Sure , it might handle a few, maybe a hundred spikes before it gets damaged. You are simply asking it to do something it wasn't designed to do. That's where the big beefy flyback diode comes in

Say pin3 is solenoid supply negative, and the solenoid is connected between supply positive and pin4.

When the opto fet is "ON", current flows through the top fet and the bottom body diode. When the fet turns off, the solenoid produces a higher voltage on pin4. The top fet is exposed to a higher voltage. Both body diodes do nothing. Leo..

Isn't the device a common source which means it can do DC or AC which means voltage can flow in either direction?

justone: Isn't the device a common source which means it can do DC or AC which means voltage can flow in either direction?

When they are turned ON, sure. Take a look again and see what happens when you place a DC voltage on pin 3/4 with no channel formed from drain/source.

Ahhhh OK. So they do nothing to protect from ESD?