How to drive an electric fan?

[Fred]

Pin 1: + 5.5 Volt generated by a MC7805 for testing
Pin 2: Fan's ground wire
Pin 3: Ground

See below, but you should have a current limiter on the input.

As suggested I tied running a 100K then 200K resister between pin 1 and the 5V supply but the MOSFET would not power up.

You might want to re-read. 100k from gate to ground, 100 to 1000 ohm (0.1k to 1k) from gate to 5v.

As is the fan runs at 9.4 Amps draw (rated as 12 A). When the MOSFET is added the current draw drops to 8 Amps, understand its normal for a loss but is this to high? The fan speed is noticeably slower.

Measure the voltage across pin 2 and 3, if there is a large drop, compare current with voltage with RDS on and see if they match. If they do, then it's OK. If they don't, then something is wrong. What is RDS?

I wrote most of this 13.5 hours ago, but fell asleep, woke up having missed doing something i was supposed to, rushed to other machine, and forgot about it.

Sorry for the delay.

Fred.

[Fred]

There is an ozzy company that sells a box which is just a heatsink with something like that inside too.

I can't find the ozzy one right now, but I came across it from a link on this forum, possibly from you oden.

Found it! http://www.dtec.net.au/PowerMAX%20-%20% ... 0Relay.htm

[oden]

Thanks for finding that! interesting product. If I can not get it working soon then that will be the answer.

Will make those changes you suggested in the next few days.

[oden]

Been reading abit about MOSFET in order to understand whats going on http://www.electronics-tutorials.ws/tra ... ran_7.html

When connecting 100K 2% 1/4 watt from Gate to Ground the MOSFET gets really hot and starts to conduct a limited amount of current, did not measure it but the fan was roughly half speed. Driving the gate high or low had no impact. And a resister between Gate and +5v of any value had no effect either.

From reading the attached article the 100K should only act as a pull down resister, so what could cause this behaviour?

[Dan]

sounds like you have a damaged/faulty MOSFET, or your pinout is incorrect, or the manufacturers datasheet is incorrect (doubtful).

can you post a photo clearly showing how it is wired please?

[Fred]

When connecting 100K 2% 1/4 watt from Gate to Ground the MOSFET gets really hot and starts to conduct a limited amount of current, did not measure it but the fan was roughly half speed.

Which gnd? Across the pins shouldn't do anything, same as it being off. Point of doing it is to ensure it stays off. Be suspicious of that one now. If it wasnt toast before, it could be post the fry up :-(

Driving the gate high or low had no impact. And a resister between Gate and +5v of any value had no effect either.

Either it's dead or you hooked it up wrong. If you have a spare, wire it up on the bench with a less demanding load like a small bulb and get it working, then increase the load. 12V through a 1k into gate should turn it hard on no matter the v drop in the lines.

From reading the attached article the 100K should only act as a pull down resister, so what could cause this behaviour?

Weird! Nothing normal, that's for sure. Photograph of your wiring setup?

Those fets will drive a moderate load without a heat sink and with almost no loss. Something is wrong.

Wait! You're not trying to switch 12v with it, are you? These thing only switch ground.

Fred.