NERD ALERT Pure Electronics Question

[MEXICOWANDERER]

I want to use 78xx voltage regulators in my 36 volt lighting. To power integral with heatsink 1 watt fans. Clarification: I purchased new surplus PC obsolete heatsinks with ball bearing fans for less than the cost of a new fan.

OK. Limited space for additional electronics. Fifty watt LED chips I drive at max safe current.

7812 voltage regulators have a max input voltage limit of say 30 volts. They cannot handle 36 volts.

So I came up with a MOSFET, Bipolar 100K resistor circuit that zener's incoming voltage to 18. Far less Delta T burden on the 78xx.

With the tiny fan only load imposed on the 78xx, and MOSFET (which passes full 78xx current)...

Do you think the TO220 packages of the 78xx and MOSFET will demand heat sinks? Vertical clearance above the PCB will be limited (but enough ventilation) The MOSFETS I selected but not ordered are rated 30 amps @ 50vdc.

The heatsinks I use are black anodized and need a slow boat from China. It sure would be nice to save several dollars per light and gain a lot of headroom. The push down airflow of the heat sink fan could aid fan power supply cooling.

Yes they do make DC power supplies that "could" do this. Check incoming voltage limit, price and size. I am building an 11 lamp array.

Remember this is for a very low current fan.

[wnjj]

After rereading, the MOSFET dropping 18V may be the issue, as Gordon says.

[wnjj]

A quick google search found some other nerds asking about power dissipation for a MOSFET in a TO220 case. They came up with ~62 degrees C per watt. So if your fan current times the 6V you're dropping from 18 to 12 is a watt or less it'll probably be similarly ok for your 78xx.

[GordonThree]

How low is "low current"? Even 100ma at a Vdrop of 18 volts is 1.8 watts, which is definitely in the requires a heat sink category.

Is there any way you can run three fans in series off the 36v supply, to burn up some of that drop? It would also eliminate the need for the 7812 all together.