# N Channel MOSFET As Switch.

Discussion in 'General Electronics Chat' started by moorejohn90, Sep 24, 2012.

1. ### moorejohn90 Thread Starter Member

Dec 9, 2011
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Hi, I have asked many question about MOSFET but I haven't got the right answer. At first I want to ask my knowledge about BJT. In BJT, we have to use correct resistor value to connect base of transistor for getting desire collector current.
Example - Required Load Current - 1A
- Transistor's gain 100
- Voltage that are going to supply in to the base of transistor is (5V)
- Supply current to load is 12 V
- For getting base current - V = IR
5-0.7 = 0.010 R
R = 430 ohm
- According to above calculation, I have to use 430 ohm resistor to connect in series with base of transistor for getting 1A collector current.
As for MOSFET, how do I control the transistor for getting correct (desired) current to drive the load?
Thanks.

Jun 22, 2012
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3. ### moorejohn90 Thread Starter Member

Dec 9, 2011
57
0
Hi Dodgydave, yep I know, MOSFET doesn't have to use resistor to control load current(Drain current) but have to use as pull down resistor.
I mean, I am not going to use resistor for controlling base current.
I want to know, how to control the value of load current (Drain current) that are not going to fry the load (not exceed their max current value).
Thank.

4. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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The transistor will only pass current depending on the resistance of the load and voltage of the supply, so for example if your load is 100 ohms and has a 10volt supply , it will draw 100mA, Mosfets behave like switches they are on or off,

so just use Ohms law to find your Drain current.

5. ### moorejohn90 Thread Starter Member

Dec 9, 2011
57
0
Hi Dodgydave, Your reply is very very useful to me. You gave me the answer that I have being searched for long time. So, both of the transistor(MOSFET) and load will not happen the bad thing, if the load doesn't draw the current more than the max current of MOSFET.
Thanks.

6. ### moorejohn90 Thread Starter Member

Dec 9, 2011
57
0
Hi, by the way, so Can I use MOSFET to switch a LED to motors with no resistor (on the side of load)?

7. ### Dodgydave AAC Fanatic!

Jun 22, 2012
7,605
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leds need a load resistor , motors dont.