Lpc2000

Rob Jansen wrote:

>>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.
>>    
>>
>
>And do not forget to add a diode accross the FET, some MOSFETs have a 
>diode built in but it may be too small, depending on your load.
>Solenoids and things like motors have huge coils - keep this in mind 
>when designing.
>  
>

I generally use a 2N7000 or 2N7002 FET for driving relays.  They can 
sink up to 100ma easily and have an internal clamping diode to protect 
the gate when the coil (de)energizes.

TomW




>Check your mosfet of choice for Rds and Vgs. Rds is the resistance of 
>the FET when fully open (saturated) under specific conditions.
>Vgs is the voltage you need to open the channel (this is a bit simple 
>but will do to get an idea).
>
>So a MOSFET with Vgs = 20V will not do for a 3.3V lpc21xx controller, 
>you may want to go for a different part.
>
>Nice parts are the IRF540 
>(http://www.irf.com/product-info/datasheets/data/irf540.pdf), Vgs = 
>2-4V, Rds = 0.077 ohm.
>Id (continuous drain current) is 28 Amps (at Vgs = 10V, so it will be 
>less when driven by the lpc21xx directly).
>
>For smaller currents check out the IRF7403 (it's an SMD package ...) Vgs 
>= 1V, Rds = 0.022 ohm, Id = 8.5A.
>It's big enough for most of my applications and I tend to use this one 
>instead of a standard NPN transistor when driving small loads (up to a 2 
>amps).
>
>I get these part from http://www.reichelt.de (German ...) and the 540 
>costs 0.52, in SMD version 0.70 and the 7403 is 0.42 - all prices in Euro.
>
>I prefer the SMD parts. Since I design, etch and drill my own PCBs for 
>home projects I prefer SMD.
>Creating SMD layouts and transfering this to photo sensitive PCB 
>material is easy to do (I use HPs overhead film).
>Drilling holes is not only time consuming but I also tend to break the 
>(fairly expensive) drills from time to time ...
>
>Regards,
>
>    Rob
>
>  
>
>>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.
>>    
>>
>
>except from the nice smoke effects that is :-)
>
>
>
> 
>Yahoo! Groups Links
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-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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