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Hi.
I have a 40mA current but I need it to be 700-1000mA. What transistor should I use?
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Hi.
I have a 40mA current but I need it to be 700-1000mA. What transistor should I use?

NPN or PNP? Maximum voltage being switched?
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NPN or PNP are both fine. The more popular the transistor is, the better, since I have a better chance for finding it in a store.
Maximum voltage should not go higher than 7V. The input voltage is 5 and it's fine if it stays the same, but a little bit higher doesn't kill either.
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I will use two transistor, connect them in a Darlington configuration circuit. The transistor are 2N3904 ( max 200 mA ) and TIP31 ( max 2 A ) and don't forget a heatsink for the TIP31. and a base resistor for the first transistor and another one for the second transistor. Or buy a Darlington transistor Ic of 2 A or more.
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Are you sinking or sourcing current?
For those currents, I would suggest low Rds, Logic level MOSFETs.
P-channel for Sourcing current.
N-Channel for Sinking current.
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My favorite choice (if you can find it) - ZTX851, NPN, high gain, high current (5A), low saturation voltage, in tiny e-line package...
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Ugh, that part has awful #s:
IC=0.1A, IB=5mA*
IC=1A, IB=50mA*
IC=2A, IB=50mA*
IC=5A, IB=200mA*

Arduino only puts out 40mA, this part could only control ~0.7A.
And it costs $1.09.

This 48 cent MOSFET Logic Level (can control with 0/5V gate) can control all the current, and will run cooler.
http://search.digikey.com/us/en/products/NTD4960N-1G/NTD4960N-1GOS-ND/2409565
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@CrossRoads

I agree with you about ZTX851 about high IB needed to switch high IC. It need an another transistor in a Darlington config for this transistor to operated. I also notice the switching and bandwith is pretty good. Maybe that is why this transistor is expensive.

And I also agree with you for the OP to use a MOSFET Logic Level.
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