A few days ago I got some sealed lead-acid 12V batteries rated at 7Ah and so today I decided to give it a shot. I checked the resting voltage of the battery I was going to use and it was sitting at 12.80V. I tried to measure the amps coming out of it but I nearly burned up my wires in trying to do so. I think the meter was reading around 15A or so before I realized what was happening to my wires.

So anyway, I hooked it up to my setup (with 1374 ohms) and got lots of noise out of my coil and strong repulsion on the rotor. The amp reading was negative. Something strange was going on here...

So I pulled off the resistors and stuck what I thought was 680 ohm resistor in and this time it spun right up. The current draw was around 540 mA (!) while the RPM was low. Well, it turns out that I mistook a black band for brown and so it was really 68 ohms on the trigger. A bit on the low side.

I then put an honest-to-God 680 ohm resistor in place and it showed resonance right away. No chance of starting the rotor there. I substituted a 390 ohm and this time no resonance in the coil although the current draw was around 390 mA with an RPM somewhere in the middle range of what I've seen with this setup.

With around 490 ohms the RPM did go up while the draw went to around 360 mA. I had to stop testing, though, because I noticed that the resistors were getting extremely hot! Now these are only rated at 1/4W, but still, there shouldn't be that much juice flowing on the trigger side, should there? I mean, we're talking mV/mA range here on the trigger side. At any rate, I'm off to get some beefier resistors so I don't have to worry about them burning up.

On a side note, I figured it might be useful to power this thing from a stock wall-wart PSU. I found one rated for 9VDC at 1A, but it turns out these labels lie. I measured 12.83V coming out (hey, it looks just like a fresh battery! ) at around 5A or so. So after my experiments with the lead-acid battery came to a screeching halt, I decided to try to power the current setup (at 490 ohms) with the PSU. It spun up to high RPM with only around a 270 mA draw.

Now I'm really confused here. I guess I shouldn't be too surprised that the battery, with it's higher amps, should be pushing more current through this circuit than the PSU which is putting out about three times less amps. But it seems like the current draw should be the same, no?

On another side note, there's a good lesson here about resistor selection. It seems that the optimal resistance depends heavily on the voltage being supplied on the front end. I checked the voltage on the 12V battery (made up of 1.5V cells) and it was around 10.5V. Boy, do I feel foolish. (/me smacks forehead)