Transistor isn switching enough current to drive solenoid!

Re: transistor problems

So, I was reading in some paintball website that a MOSFET would be ideal for switching the solenoid on. So I got a few-- the model for it is rfp12n10l. I have a voltage divider coming off the output of the 555 timer going to the gate. I have the source hooked up to ground. And I have the drain hooked to one side of the solenoid. Everything looks to work perfectly when I am just testing the solenoid but when I hook the solenoid back up to the sear and the grip frame I have problems because the bolt puts a lot of force on the sear which is directly controlled by the solenoid. So it looks like I need even more current yet going to the solenoid. The MOSFET I have now is rated for 10 amps and the MOSFET gets pretty hot after using it a while so I'm assuming I just need a MOSFET that can handle more current. I looked at a couple and the one's that were rated for 30 amps cost like 5 bucks a piece. I was wondering if you guys might know which one I should get because I don't want to waste my money on something that doesn't work. Thank you much.

Re: transistor problems

Hi Rambo,

Does the gun work now, even if the MOSFET gets hot?

The RFP12N10L has an rDS(ON) of 0,2Ω so if 10A flows through the transistor
it will dissipate P=RI²=0,2*100=20W - quite a lot...

Obviously you cannot decrease the current so your only option is to find a
MOSFET with lower rDS(ON), e.g IRF540N with an rDS(ON)=0,044Ω => 4,4W of
dissipation.
Another common trick to decrease the resistance is to connect several MOSFETs
in parallel. If space permits you could try that too.

/Rambo

Re: transistor problems

Actually I tried using two of them in parallel and that solved the heat issue- they didn't seem to get much hotter after prolonged use. But the problem is I still don't have enough current going through the solenoid to fire the gun. So I am thinking that I need a mosfet rated for an even higher current. That is why I am wondering which one I should get.

Re: transistor problems

With two in parallel the resistance is 0,1Ω and if 10A is flowing through them
the voltage drop over the transistors will be U=RI=0,1*10=1V
As your battery is 9V you are loosing about 10% of the voltage in the MOSFETS.
Doesn't sound that bad, but the solenoid might be designed so that it needs every
last Volt.

What is the resistance of the solenoid? If you knew the inductance too, it would
be helpful.

The IRF540N will handle a continuous current of 33A and a pulsed one of 110A -
it should do the trick... :wink:

/Rambo

PS By the way - you say you have a 555 driving the transistor(s). How long is the
pulse that it generates? If it it too short the current through the solenoid will be
cut off prematurely.

Re: transistor problems

I actually was looking at the stock board and I found on it that the solenoid is being driven by a hexfet mosfet just like the IRF540 you just reccomended. I also looked at the current rating for it and it was like 35A whereas the one I was using now is only rated for 10A. So, it looks like that mosfet you just reccomended should work good. Plus, I found a spot that sells them for .99. And yes, I already checked to make sure that the issue wasn't that of the pulse being too small. I just rigged the 555 up so that it was always "on" and that still didn't do the trick. Thanks alot.