# Mos Fet And Power Regulator

Discussion in 'General Electronics Chat' started by goldeneye123454321, Jan 14, 2009.

1. ### goldeneye123454321 Thread Starter Member

Oct 1, 2008
19
0
Hey, I decided to use a MOS FET to help control the voltage to a fan. And I am using a 7805(I Think) to keep the input voltage at a steady 5VDC to a POT. The Pot controls the voltage to the MOS FET, which in turn variably controls the fan. The 7805 gets very hot when I pass in 15VDC even though it is capable of handling 7 -30 V. Why is it getting so hot to the point where I burn my self and the thermal protection turns on? I hope this makes any sense. If you know what I am trying to say, is this a good or efficient way to variable control the fan? Any advice and help would be highly appreciated.

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Can you post a schematic of how you have things wired up?

What is the part number of the MOSFET? Include that on your schematic.

3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
We need to know more about the circuit. How much power does the fan draw, and how are you using the pot to control it? A schematic would help.

Part of the regulator's problem is that it has so much voltage presented to it. 10 of those 15 volts is lost as heat according to the power formula P = I*E. If your fan needs 500 ma current, for instance, the power dissipated by the 7805 is .5 * 10, or 5 watts. You would have to add a heat sink to properly cool the regulator.

4. ### italo New Member

Nov 20, 2005
205
1
What is the pot value 10Ω? disconnect all of it except the pot and see if your heating problem disapear.

5. ### goldeneye123454321 Thread Starter Member

Oct 1, 2008
19
0
i uploaded the diagram. The fan runs at 12VDC and draws 0.18A. The MOSFET number is 7805. Im using the mosfet to provide the 10K pot a steady 5V. Im using the pot to control the gate of the power fet. The power fet doesnt seam to heat up, but the mosfet gets very hot and shuts down. I hope this helps.
Another question, is this a logical way to reduce the input voltage by using a mosfet? or is there a much better, more efficient way to do this?

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6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
7805 is the the type number of a three terminal voltage regulator. See my above as to why it's getting hot.

The fan's 180 millivolts should not be too much of a load. But unless you're FET is logic level, the voltage out of the pot won't put it into full conduction. Most FET's need 10 volts on the gate relative to the source to full turn on.

7. ### goldeneye123454321 Thread Starter Member

Oct 1, 2008
19
0
ok, thanks. that fixed my problem.