# MOSFETs, I don't understand something about them

#### lukamila82

Joined Jun 4, 2018
2
So today I woke up in the morning and got the idea to make a bicycle phone charger and I wanted to use a MOSFET and an LM7805 voltage regulator.
The generator I have would be turned on by the wheel and it would generate a voltage between 1 and 7 volts. If the voltage is over 5 volts it would get regulated by the LM7805 but if it is under 5V, it can't be regulated because the LM7805 can't step up voltages so I thought that I might use a logic level MOSFET as a switch. Basically, if the voltage is under 5V, the MOSFET won't allow it to go trough, but if it is above 5V, it could go trough.

I can't seem to understand how to wire up a MOSFET to do that, but I have tried something (the picture below).

Please help me and sorry if I make noob mistakes, I'm fourteen and I am just learning basics of electronics for now.

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#### ericgibbs

Joined Jan 29, 2010
12,942
hi 82,
Welcome to AAC.
When using a 7805 5v regulator, the input voltage must be at least 2.5V higher than 5v, so it would require a 7.5v input.
There are Low voltage drop out regulators which require only ~0.1V ie: 5v input.
E

#### kubeek

Joined Sep 20, 2005
5,753
Hi and welcome to the forum. What you need is a comparator, that will measure the input voltage and decide to turn the mosfet on above the 5V (or better 7V, because the LM7805 recommends at least 2V difference between input and output to regulate properly).

See this circuit made in ltspice.

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#### lukamila82

Joined Jun 4, 2018
2
Thank you very much for your replies!

#### crutschow

Joined Mar 14, 2008
27,232
Use a low-dropout regulator such as this.
It has a minimum voltage drop of 500mV.
If the input voltage is 5.5V or above it will regulate the output voltage to 5V.
Below 5.5V the output will be about 500mv below the input.

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