I’m making a stirrer to, er, stir my yeast starters for brewing. Something like this.

In the box there’s a 12v PC case fan with a couple of magnets araldited to the fan ‘hub’. The whole thing’s powered by a generic plug-in mains adaptor. I’ve got this circuit diagram, but also a couple of questions.

Where should I put an LED power indicator?
Would it be a good idea to protect it with a fuse in case the fan gets jammed or something? Where would the fuse go, and what amperage (or how can I calculate it short of jamming it myself and measuring the fault current)? I imagine the ac adaptor has an internal fuse but don’t want to be responsible for burning my block down…

Does the fan not turn too quickly for that? (edit:I’m being a muppet, that’s why you are using the pot of course – slaps head)

For the LED just a wee led and a 1 k resistor across the supply to the fan should do it

For the fuse I’m not sure I would bother. I would jam the fan and see how much current it takes but I wouldn’t think it would be much. Most likely problem is heat dissipation in the LM317. You don’t say what the input supply voltage is so it is difficult to estimate.

I confess in your shoes I’d have been tempted just to buy a 12V DC wall wart supply and connect it straight to the fan without all the gubbins or maybe just with a small cap across the fan terminals

Leffeboy, I’ve got a few ‘junk’ power supplies but the one I was going to use is 12vdc 1.5A. It’s from an old USB Hard Disk.

Yeah, with hindsight I think there are easier ways. I’ve just seen one online powered off a couple of rechargeable AA batteries. But anyway, the box is all drilled now.

So an LED and 1kohm resistor parallel to the fan terminals then?

So an LED and 1kohm resistor parallel to the fan terminals then?

Yep. I’m guessing that the end drive voltage will be around 10V so that should give you 8mA or so. Won’t be super bright but will work. Lower the resistor a bit if it isn’t bright enough. Alternatively put it across the 12V DC supply and the brightness won’t change as you move the pot. I didn’t realise that you were going to regulate the speed – i though you were just aiming for 12V out

I’d like to regulate speed as I’m not sure how fast it needs to go. Youtube has videos of these things doing 2000rpm with big vortexes, but I don’t think I need anything like that.

wow – i’m amazed you can drive a magnet at 2000rpm. Liking it though.

I’d guess the fan wouldn’t cope with the resistance of the liquid, especially at start up.

http://www.instructables.com/id/Variable-speed-magnetic-stirrer-from-a-PC-power-su/