[jklinck]

The fan won't start when I plug it in. If I give the fan a little spin it will turn on and keep going. The stir bar rattles on the outside of the flask and when I get it to the middle it stops the fan dead in it's tracks. There is enough spacing between the fan magnet and the top of the box, so that isn't the issue.

This is a homemade stir plate with these items:
12v ac/cd converter
12v computer fan
rare earth magnet (the half moon kind like you find in a computer hard drive)
25 ohm rheostat
1 inch stir bar


The rare earth magnet is glued to the middle of the fan.

[a10t2]

Take the potentiometer/rheostat out of the circuit. Does it run at 12 V?

Is the transformer actually putting out 12 V?

[jklinck]

Take the potentiometer/rheostat out of the circuit. Does it run at 12 V?

Is the transformer actually putting out 12 V?

I don't know much anything about electrical stuff so I assume the adapter is putting out 12v, but I don't have any way of testing it. When I took out the rheostat and wired the adapter directly to the fan I still had the same issues I mentioned before.

[a10t2]

I assume the adapter is putting out 12v, but I don't have any way of testing it.

Sounds like you need to buy/borrow either a DMM (http://www.radioshack.com/product/index.jsp?productId=4214667), or a second power supply for testing.

[punatic]

Use a car battery?

Good advice about buying a multi-meter.  They're worth their weight in gold.  You can pick up a cheapy at Harbor Freight for $3.  Not real fancy, but they work pretty good for what they do.  And you won't cry when you break it, like you would with a Fluke.

[Slowbrew]

One of the coils in your fan motor maybe burned out too.  The motor won't start properly but will spin if you give it a nudge and a bad fan will be hard to control the speed on too.

As stated earlier you need to be able verify your power supply is kicking out 12V.  If it is, make sure the rheostat is working correctly.  If those both check out then its probably a bad fan.

A cheap DMM will answer the first 2 questions.

Paul

[dzlater]

I had the same problem with the fan not spinning on its own.
Do you have the magnet directly mounted to the fan?
I put a plastic spacer, (lid from a film canister) between the fan and the magnet.
Works great.

[dhacker]

A powerful rare earth magnet . . (such as from a salvaged hard drive) that is mounted too close to the coil of the motor can interrupt the inductive/ magnetic field generated (and necessary) for the motor's operation. Try moving the magnet away from the center of the motor's hub . . you then may have to use another magnet opposite that magnet or do something to balance the whole thing.