Tight Arse Stir Plate

[barls]

For the more adventurous, here is a simple PWM circuit using a 555 timer. <$5 worth of parts.

is this running on 12 volts mate.

 

[Screwtop]

Could have a couple of versions, the Tight Arse Stirplate, and the Slightly More Seriarse Stirplate :lol:



Disclaimer: No seriousness is meant or implied in this post, there has been far too much of this already.

 

[LethalCorpse]

Do the math - Voltage (12V) total series resistance (max 50K + typically 160) therefore Current and Power = 2 fifths of **** all

eh? Let's assume the fan is a purely resistive load (it's not, but it'll do for argument's sake). If the label says 12V 300mA, it's a 40ohm load. If you use a 500ohm, 0.5W pot to slow it down as SJW has, the responses are shown in the chart below.

Note that the system current drops off sharply, which is what I mean when I talk about non-linearity of control. If we assume the fan speed to be roughly proportional to system current, this means it'll be very twitchy at the top end, with a long tail - it takes a small turn to cut the speed to very low, where it will stay over most of the pot's range.

Note also that the power dissipated in the pot peaks at about 8%FSD, with 0.9W. This nearly doubles the rated power of the pot, but the pot's rating is across the entire range. If you're using 8% of it's length, you've only got 8% of its power rating, or 0.04W. You're therefore overpowering the pot by 22.5 times. The power is only within rating above about 340ohms, in which range the system current is pretty well constant.

Since the pot is probably well underrated, this isn't likely to cause immediate and catastrophic failure, but I'd say it's a fair way north of 2 fifths of **** all.

Is there any other math you'd like to suggest I do, Screwy?

 

[LethalCorpse]

Lethal,

I've seen the circuit diagram but have you got any pictures of how to wire it up. You could call it the "Lethal method". Ps excuse me if you already have as I haven't looked back over this thread for a few weeks.

cheers

There's a modified version on this page down the bottom, which gives nice clear board layout instructions.

is this running on 12 volts mate.

yep, sure is

 

[SJW]

:lol: Thats gold. LC, do you ever get the feeling like your Robinson Crusoe on this one?

 

[LethalCorpse]

More Don Quixote mate.

I'm not banging on about this to be pedantic, but since the questions keep coming I'm happy to oblige. I do have some modest body of experience and qualifications in this area, and I'm always keen to share what I know.

 

[schooey]

... and I think it's very gratefully recieved on the most part, LC. Like I said, I built mine almost 12 months ago to your previous diagram on the LM317 and it took 5 mins, cost me about $6 and worked a treat. It;s always very handy to know an electronics geek whizz.....

 

[Screwtop]

Is there any other math you'd like to suggest I do, Screwy?

No, that basically proves that it is very nearly 3 fifths of **** all. I'm certain that you have impressed the whole of the AHF community with your absolute and infinite knowledge of electonics theory.

Your nearly as much fun as darren :lol:

 

[goatherder]

I used a 1k 0.5W pot in series circuit and ran into exactly the troubles LC mentioned - the control was very touchy and it died pretty quickly.

I simply substituted a higher rated pot (500 Ohm 3W wire-wound). It's been running for 2 years with regular use. The 500 Ohm pot gives a pretty good range of control over the magnet. The controller option seems the 'correct' way to solve the problem but there is no reason a properly sized series pot won't do the job.

 

[LethalCorpse]

No, that basically proves that it is very nearly 3 fifths of **** all. I'm certain that you have impressed the whole of the AHF community with your absolute and infinite knowledge of electonics theory.

Your nearly as much fun as darren :lol:

It's neither absolute nor infinite, but within its bounds you'll find me more than willing to hold forth. Whoever the AHF community are, I'm sure they can appreciate that.

It's only beer at the end of the day. I think I've said a few times now, it's your brewery, you do what you like, you learn from your own mistakes. If I can perhaps prevent some of those mistakes by steering folks in the right direction, so much the better.

 

[LethalCorpse]

... and I think it's very gratefully recieved on the most part, LC. Like I said, I built mine almost 12 months ago to your previous diagram on the LM317 and it took 5 mins, cost me about $6 and worked a treat. It;s always very handy to know an electronics geek whizz.....

I'm happy enough with geek

 

[PostModern]

I used a 1k 0.5W pot in series circuit and ran into exactly the troubles LC mentioned - the control was very touchy and it died pretty quickly.

I simply substituted a higher rated pot (500 Ohm 3W wire-wound). It's been running for 2 years with regular use. The 500 Ohm pot gives a pretty good range of control over the magnet. The controller option seems the 'correct' way to solve the problem but there is no reason a properly sized series pot won't do the job.

What does a 3W pot cost compared to a LM317, a .5W pot and a couple of 5c resistors? When I built my 317 based controller, Jaycar, David Reid and so on had them, but for crazy amounts of money.

 

[SJW]

What does a 3W pot cost compared to a LM317, a .5W pot and a couple of 5c resistors? When I built my 317 based controller, Jaycar, David Reid and so on had them, but for crazy amounts of money.

I just got the std type of Pot from Jaycar (technically speaking) for $2.25. I went to DSE and they had the ones Goathearder is talking about, the 500ohm 3w wirewound jobs for $8.95. But whatever I got, I know its a 500 ohm one, there is good control over the full range of speeds and it is not getting hot at all.
I did thank LC, its just a fact of nature that some/most blokes in brewing get right into the nuts and bolts about some part of our craft and if something is your specialty than it would really piss u orf to see information going out there that is not 100% correct.

Steve

 

[hockadays]

Managed to finally get my stirplate going using the LM317 circuit and the caps for extra protection. Thanks Lethal for you help.

 

[SJW]

Looks great man

 

[LethalCorpse]

No probs, happy to be of service, sorry we couldn't diagnose your PWM version issues though.

You sure your spacers aren't too big?

 

[MCT]

Can I ask what sort of a vortex I am looking for? I've got my fan flat out and am getting a vertex that probably goes down 1/3 of the flask. Is that OK, or do you want it bottoming out?

 

[the_fuzz]

Can I ask what sort of a vortex I am looking for? I've got my fan flat out and am getting a vertex that probably goes down 1/3 of the flask. Is that OK, or do you want it bottoming out?

What size flask?

I have a 1 litre version and get about 1/2 way down - still works a charm. Obviously the vortex gets smaller as the yeast population grows

 

[MCT]

What size flask?

I have a 1 litre version and get about 1/2 way down - still works a charm. Obviously the vortex gets smaller as the yeast population grows

Hmm, that's gonna be my problem. I'm using a 1L flask with just water in atm as I'm testing it out.
Might have to get a more powerful fan.... or pump more than 12v into it.

Cheers for that mate.

 

[schooey]

Can I ask what sort of a vortex I am looking for? I've got my fan flat out and am getting a vertex that probably goes down 1/3 of the flask. Is that OK, or do you want it bottoming out?

If that is just with water, you may be in trouble when you start growing the yeast. The liquid may become viscous enough for the fan to run out of grunt and stop (I have had this happen with a 60 mm stir bar in 3L starter).

But AFAIK, as long as the yeast is being agitated, you don't need a big ass whirlpool.