Lpc2000

At 04:31 AM 11/2/05 +0000, 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.

Actually transistors don't really have a resistance drop then have a 
voltage drop.  If you are driving it into saturation the drop will be from 
0.5V to 1V or even higher depending on the current.  Speaking of current 
the low resistance suggests a high current coil (or a shorted coil). 
Driving that coil with 3V would give a better than 1Amp draw, can your 
power supply support that?  Is the transistor large enough to sink that 
much current?  Maybe you just fused the transistor.

For that kind of drive I'd go for a MOSFET myself.  They are resistance 
devices.  You might be able to drive them from the I/O pin.  If you use a 
logic level FET and drive them at 5V you certainly will be able to.

First I'd check what the drive for that solenoid is supposed to be though, 
that's an awfully low resistance.  I rather suspect it's shorted.  Have you 
got a part number/data sheet link?

Hmm, there is another possibility.  I've run into this in the past on some 
contactors that have four quick connect tabs on the coils.  The first 
instinct is to connect across the two quick connects on the top or bottom 
of the device.  Unfortunately those are connected to the same side of the 
coil and you can end up trying to drive a lot of current to no effect.  Any 
possibility you are not actually measuring across the coil but rather 
somewhere else?

Robert


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