OK, honestly, I HAVE to ask. Am I just getting REALLY damn old or does $200 for ANYTHING, particularly a simple hobby-type grow like this, sound absolutely RIDICULOUS? Am I the only one? Anyone? Bueller?

$400 is NOT cheap unless it is a car.

$200 is NOT cheap unless it is a major appliance.

$100 is NOT cheap unless it is a 27" television.

$50 is NOT cheap unless whatever it is is going to perform sexual favours all night long.

I am completely amazed at how people here seem to think that whipping out $200-500 for individual items to grow a few plants is somehow economical. If I am growing for medicinal purposes, it isn't making me any money, and EACH of those items at that price would buy me a couple months worth of stuff at the rate I need. Seriously - who has this kind of money to just pull out and spend? That's groceries for a family of 4 for a week or two.

Now, that said, your fan probably does not run on 110V and that is why the transformer is on it. If the transformer had been elsewhere in the case, the odds are you would have already had need of a fire extinguisher unless you were REAL lucky and it just fried and died fast leaving that wonderful ozone-burnt-circuit smell.

I'm all for MacGyvering things if you know what you are doing, and I do stuff like that all the time. But, that's a shock hazard if you touch certain parts on the transformer, and throwing a bug on it won't tell you squat. Microwaves have ground plugs for a reason - to stop the electricity from going through YOU and instead into the ground in case of a fault. A bug on the transformer or fan motor would not be providing a path to ground like you would when you pick it up, so its survival means nothing. That fan attached to metal and not using a 3-prong plug to ground it gives me visions of you coming back here one day telling us you now must use speech recognition software to write your posts because you fried your fingers when a fault developed in that transformer. Mmmm. Crispy, burnt flesh smells. That's subtle. I'm sure that it won't get anyone's attention.

IF you are really determined to use it, and I am not saying it is a good idea, you need to make a box out of a non-conducting material, AND you need a 3-prong plug with a wire connected to a ground location on that assembly (such as where the bolts connected it to the MW chassis - possibly - hard to tell from the pics).

It is only 50cfm (but I have a small space), but a small bathroom exhaust fan only costs $20 and it is HELLA safer to wire up to a plug (although I'd still use 3 prongs to be safe). It is also square which means it is easy to rig up any number of different things for filtering (like using the air filter you are thinking about, or cutting up an odour-controlling furnace filter or blowing it through a box of aquarium charcoal or SOMETHING)...

I'm not going to say I am an expert, but I used to work as an electrical contractor's assistant, I've rebuilt dozens of electrical motors (including fans) and power supplies, and I have a lot of experience working with both AC and DC-powered items.

All I can say if you won't listen to anyone is that I hope you have neither kids nor pets - at least none that you value.