I wouldn't do that. You might have noticed that some of my posts are longish. It is better, IMHO, to provide too much information rather than to leave something out because you don't think it is important. If folks don't want to read a long post, well, they won't. Perhaps a few zillion electrons get terribly inconvenienced with longer posts, but that's their problem.
Ok then. Hey I had an eventful day today. Went to the electronics store, did some reading, and talked to a friend about series vs parallel circuits. Also, received
an industrially produced magnet in the FedEx. All these things together gave me much to think about!
First, the new electromagnet. It's made to run on 24 V, and its resistance measures at 87.7 Ω. I put it on my 16 V power supply and read the current, which was a nice, predictable 0.17 A. Its effect in the fluid is less than ideal, probably because of the electromagnet's short, fat shape. But honestly, the effect is pronounced enough, and it's a compromise I can live with. It can only get better when I give it the 24 volts it wants. I read
here (Wikipedia) yesterday something that suggested that windings with a long core have magnetic fields which extend out from their ends. I'm going to assume that the converse is also true (i.e., that short squatty cores have more contained fields), because I'm not getting the spikes in the fluid with this store-bought magnet as I was with my own. However, it pulls less than 1/5 of an Amp, so that's of course a huge advantage.
< * the forum breaths a collective sigh of relief * >
Nevertheless, I'm undecided. I believe what you guys are suggesting is that if I'm going to build my own magnets, I start with the power supply requirements, field strength requirements, and build the parts to suit. Thinking about this, I started to wonder what it would take for me to make a magnet of my own that would give me what I see now in the one I like, but would play nice with a PSU the same way the store-bought one does. What if I had a 16 V / 4.5 A PSU
like this one? What would I have to do to get 9 electromagnets working nicely on it? And what would I have to do with those electromagnets to get the same result (in the fluid) I'm already seeing?
Well the first thing is, each magnet would need to draw
(4.5 A) / (9 electromagnets) = 0.5 A
If I wired them in parallel, each would always be guaranteed the same voltage, and their amperage would simply be added up when evaluating against the PSU's current rating, correct? This is an important point, so if I'm wrong, please let me know.
My best electromagnet was drawing 5.5 A and had ~ 262 turns. The strength of the field in Ampere Turns was thus
(5.5 A) x (262 Turns) = 1441 AT
I'm not sure shooting for the same AT will give me the same result, but let's assume it would and run some calcs. If anyone has a comment on this assumption, I would be glad to hear it!
To get the same AT from 0.5 A, I would need
(1441 AT) / (0.5 A) = 2882 turns
Wow. But still...
If I'm wrapping around a 0.375" diameter core, I get
(0.375 in) / (12 in / ft) x (pi) = 0.098 ft. per turn
To get 2882 turns, that would be
(2882 turns) x (.098 ft) = 282 ft.
282 ft is the length of wire for the turns on my given core size, but what gauge wire should I use? This only comes down to tailoring it to draw the current we used above. What resistance do I need to hit 0.5A on 16V?
(16 V) / (0.5 A) = 32 Ω
Looking it up in the calculator I see that
31 AWG gives me 37 Ω at this length (and that 30 AWG is only 29 Ω, so 31 AWG is the way to go)
What would this magnet look like? According to
Wikipedia's wire gauge table (matches against my measured results with the 24 AWG wire, so I trust it) 31 AWG gets 112 turns per inch. That's
(2882 turns) / (112 turns / inch) = 25.73 inches
If I made the core 6.5" long I would have to traverse it four times. And then I'd have to do that whole operation nine times (once for each magnet). Is it worth the effort? Or should I just settle for the store-bought mags and call it a day?
I see that there is 31 AWG wire available on eBay for a decent price... and I am tempted to favor the hand-wound approach because the exposed wire would look really good in a wood and plexi enclosure... and I'm pretty happy to know how to calculate all this stuff now!
+++
@duke37 I see you just posted info on this same subject. Can you look over these calcs and tell me if you see anything fishy? PWM on the Arduino is the next subject I'll take up, but I'd like to make sure my thinking here is straight before moving on. I'm going to bed now, though, and I'll check in the morning. 2 am is late enough for me! Thanks for your help!