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power MOSFET choice..

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TKS

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can any one over here help me whit the choice of a mosfet...

i have a device that is comparable whit a light a + and a -

it needs @ max 12 amps... ( i think i need 4..)

@ 18volts.. maximum (12volts)

are there optocouplers that are that big ???

a relais is ttto sloow i need the quickest way possible...to switch..

i'm talking about switching times in the order of ms..

soo a mosfet should work..

TKS

ohja the device is a car fuel injector..
 
I don't think you will find an opto-coupler that big, could be wrong though. Using a MOSFET shouldn't be a problem, it will easilly switch in ms if driven properly.

You could use an IRF640 that has a maximum current of 18A and can dissipate up to 125W. You will probably not be able to drive the MOSFET directly from the opto-coupler device so you will need a small driver circuit to drive the MOSFET.
 
mhh..

mhh...

and what could i use as a driver for that..???

a mosfet only require a pulse right..???

TKS
 
Yes, but for fast switching need discharging the source-gate cap.
CMOS inverters can also do this.
 

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Yes MOSFETS can be switched with a pulse, but they still need current to drive them. A MOSFET like the one we are talking about will have a relatively large parasitic capacitance, when you switch the MOSFET this needs to be charged up that takes current. The faster the switching the more current you need. The maximum output current from your opto-coupler will probably be quite small, depends on what you use have you decided yet?? So the current from the opto-coupler device wont be large enough to properly drive the MOSFET.

A driver circuit for a MOSFET just consistos of two or three smaller transistors used to amplify the current from the opto-coupler. I haven't got a schematic editor available at the moment so I can't give you a circuit. They are very simple maybe someone else can suggest one.

I looked at some opto-isolators some of them are designed specifically to drive MOSFETs directly, two of these are TLP250 and TLP251. They have sufficient output current to drive the MOSFET directly (you will need a resistor between the output of the opto-isolator and the gate of your MOSFET).

Does this clear it up a bit?
 
mhh..

well i already learn the less you decide the more poss there are to do something...but i also learn that there are many way of doing the same thing..

well i have 5volts and lets say 7miliamps..

(microcontroller) output...

soo if that one can drive a optocoupler and the optocoupler can drive the mosfet we are round..

when you have time / the tools can you give me then the exact sheme..???

TKS
 
OK using a TLP250 or 251 you will be able to directly drive your MOSFET use something like a 65ohm resistor for the gate resistor. The opto-isolators cost about £1.50 from RS www.rswww.com

Now for the interfacing of the microcontroller to the opto-isolator, the datasheets states that a forward current through the LED of 7mA is the minimum to switch. This is very close to the output you have given from your microcontroller, are you sure this is right? If you find the dataheet for your microcontroller it should give this, you also need to know if this is what the output can sink or source, this is important most output stages are better at one than the other, usually better at sinking. Which the output stage is the best at willl determine how you connect it to the opto-isolator.
 
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