Lpc2000

Robert Adsett wrote:

>At 02:45 PM 11/2/05 -0500, Tom Walsh wrote:
>  
>
>>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.
>>    
>>
>
>
>Well the original post did say they had a 2 Ohm coil.  That in turn would 
>suggest a multiamp drive was needed.  That diode in the 2n7000 won't 
>protect against inductive spikes though, it's in the wrong part of the 
>circuit.  You are probably getting avalanche breakdown and I don't think 
>I've seen a data sheet for either of these parts with an avalanche rating.
>
>  
>
OW, missed that one!  Well, bigger FET and larger diode(s) then...  
Perhaps an IRF530, lots of power in that, somewhere around 6 AMPs, 
IIRC.  It could also be switched from 5volt logic.

Trouble with transistors is that they are current devices, they magnify 
current based on the beta (gain) of the device.  You don't suffer that 
problem with an FET as it controls current with a voltage on the GATE 
(low current draw on the control voltage).  Definately would go with a 
MOSFET or BI-FET.

TomW


-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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