Lpc2000

Solenoids require a lot of current to "pull-in" the plunger. After the 
plunger has been pulled-in a smaller amount of current is all that is 
required to hold it. Some solenoids may have two windings, the primary 
high-current pull-in winding, and the secondary low-current hold 
winding. These windings may be switched over internally by the plunger 
action. If your solenoid has only a single winding you will find that 
the large amount of current flowing will produce heating effects. It is 
usually advisable for this reason, and also for reduced current 
consumption that the winding be switched to a lower current either by 
means of reduced voltage or by the PWM method. The PWM could be 
implemented in sw as it only needs to be a low frequency.

If a bipolar transistor is used to drive the solenoid then it will 
usually require a significant amount of base drive current to achieve 
saturation, much more than a logic pin can usually provide. The device 
of choice for driving solenoids has to be the MOSFET due to the fact 
that it requires practically no current to drive, it features an 
integrated clamp diode, and they are available in special configurations 
such as autoprotected.

Although it is true that you don't need a resistor in the gate drive 
circuit it is good practice to include one in the event of a MOSFET 
failure for whatever reason and there is a breakdown from drain to gate 
(yep!). In which event the unprotected logic device will release it's 
magic smoke and forever cease to function.

2N7000s are unsuitable for anything but small signal or very low-current 
(IMO, don't use them). The IRF7403 that was mentioned is a good 
candidate for medium power drive. Personally, if I have room I allow for 
standard TO-220 through-hole parts. My favorite is the IRF3205, they are 
cheap and being rated at 110A are hard to destroy, the pcb tracks 
vaporise first.

I find that due to the excellent clamp action of the integrated diode 
that an external coil diode is not (never) required. Also, but not very 
importantly, the breakdown voltage of the integrated diode allows the 
magnetic field to collapse faster increasing the response time of the 
solenoid release. If for instance you were driving a relay and needed 
faster release, you would not use a normal diode across the coil. You 
could either feed the power via a series fast-zener (etc) or depend upon 
the integrated diode. Which by the way means that the coil needs to dump 
it's current in reverse through the power supply so it is advisable to 
have a diode (preferably a transzorb) device across the supply rails or 
you could find those voltage spikes appearing at rather sensitive places.

Summary: use low RDSon low Vgs mosget with series resistor (10K-100K), 
along with a clamp diode across the supply.

*Peter*




Nice Guy wrote:
> I know this is kinda off topic, but I'm having problems controlling a
> solenoid.  I want to use a gpio pin to simply turn the solenoid on and
> off.  I used a NPN transistor in series with the solenoid to control
> it, but the solenoid only has around 2 ohms resistance and the
> internal resistance of the transistor must be much higher because most
> of the voltage is droped across the transistor instead of the
> solenoid.  Could someone recomend a better solution to this problem or
> suggest a good place on the web to find info.