Self accelerating reed switch magnet spinner.

[TinselKoala]

Yah, gotta get me one of them thangs.

But you are right, I could build what you suggest with parts on hand, for nothing. Probably will go for the store-bought solution though, until I discover it can't work right for some reason.

Meanwhile, some unpowered rotor rundown data is coming up, with and without propeller load, CW and CCW:

http://youtu.be/_zOwadZH6T0
(still uploading)

(boring video, but confirmation of 90 second rundown from 2000 RPM)

[TinselKoala]

TK:

About the apparent lack of RPM change in your clip.  Note that you are tapping into the coil discharge while the main agent causing the push on the rotor is the energizing of the coil.  So in theory tapping into the coil discharge is soaking up the "left over" energy and the pushing on the rotor has already been taken care of.

MileHigh

Yes, I believe you are right. The spike and the subsequent ringdown are energy loss mechanisms, normally resulting in the neon lighting up and the heating of the coil and the mosfet. (Some would have you believe that it goes back into the battery; I have one neon to the battery positive, just in case...) So siphoning it off to power an external load is "just" scavenging energy that would be "wasted" as heat and stress otherwise. Free energy, imho, since it's not useful until you go get it and use it, but it costs nothing _more_ than what you are already spending.

[MileHigh]

TK:

I think I fixed my problem.  You know how when a scope is AC coupled you can see how the AC coupling reacts to a DC step input, and it takes several seconds for the trace on the scope to "settle."  It's just a fairly long time-constant AC coupling action we are seeing there and we have to do the same thing with op-amps.

New pipeline:

[current sensing resistor] -> [inverting op-amp] -> [AC coupling with long time constant] -> [unity gain buffer with offset compensation] -> [signal transformer] -> scope channel.

Note this is still a one-chip solution.

We need to do an AC coupling with a long time constant so that it allows low pulse frequency signals to propagate without noticeably distorting them.  We only need one zero-offset point so the unity gain buffer is the best place to do it.  That way the signal transformer primary will have no current flowing through it when the current sensing resistor senses no current.

Let's pick a time constant of five seconds.  That's longer than we typically see for the AC coupling for a scope.

C = 1000 uF,  R = 5 Kohm gives you a time constant of five seconds.  As you can see you could easily make the time constant much larger if you want.

The output from the inverting amplifier will connect to one side of a 1000 uF non-polarized capacitor.  The other side of the capacitor connects to the non-inverting input of the unity gain buffer.   There is also a 5 Kohm resistor connected between ground and the non-inverting input of the unity gain buffer.  So the 5 Kohm resistor is the "DC bias bleed-off resistor."

With this configuration it will be just like an AC-coupled scope trace but with a much longer time constant.  This will ensure that low frequency signals are properly coupled and imperceptibly distorted and the signal coupling transformer primary does not get saturated with constantly increasing current gong in the same direction because all the DC bias gets removed from the signal.

If you notice both the AC coupling and the op-amp driving the signal transformer are designed to be "friendly" to low frequency signals so that you can look at an undistorted current pulse waveform that might be pulsing at 10 Hz.  At the same time any average DC bias will be removed from the signal after about 25 seconds due to the RC coupling circuit.

Again, this is a design without any finesse in designing op-amp circuits but it should give you want you want:  isolated current sensing so you can use an ordinary scope probe anywhere in the circuit.

MileHigh

[MileHigh]

TK:

Note: I am not factoring in the primary winding resistance here.  So this is a work in progress.  Hopefully sill worth reading.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

The number crunching for the signal transformer primary inductance:

You can see the robust little output transistor set from your TL082 in the attached schematic.  You can also see 15 millimperes current output is the limit for the chip at -9 volts.  (It's better on the positive side.)

So that means the output impedance is "zero" at up to 15 milliamperes of current output.  So that means that the op amp can energize the primary with any DC voltage until the output current hits 15 milliamperes.

This causes an interesting revelation.  We want to energize the primary of the transformer with lower voltages, because lower voltages give the op-amp more "breathing room" in terms of time while running with a zero output impedance.

So, let's say that inverting amplifier is low gain, and the maximum voltage presented to the signal transformer primary is +/-0.25 volts.  This is not an issue because you just adjust your scope gain for the display.

i = 1/L integral v dt

We know that v is a constant, so you can take it out of the integral.

i = v/L integral dt

Therefore i = vt/L

Hence, L = vt/i

So, now we just have to plug in the conditions we want!!!

Let's say that we want it to take 1/2 seconds before the op-amp craps out when it tries to put a 0.25-volt DC signal into the primary of the signal transformer.  The current after 1/2 second reaches its maximum of 15 milliamperes.

Notice in this case the signal seen on the secondary of the transformer is fully integral - an exact copy of the input signal.  There is no drooping due to a time constant.  That's because the time constant is "infinity" - the op-amp will hold it's voltage like it has a zero output impedance, and L/R becomes infinity because R is zero.

L = ((0.25 x 0.5)/0.015)

L = 8.33 Henries.

Well, that sounds pretty large after all that crunching.  Perhaps I will try to factor in the primary wire resistance later on.

MileHigh

[synchro1]

TK:

About the apparent lack of RPM change in your clip.  Note that you are tapping into the coil discharge while the main agent causing the push on the rotor is the energizing of the coil.  So in theory tapping into the coil discharge is soaking up the "left over" energy and the pushing on the rotor has already been taken care of.

MileHigh

@Tinselkoala,

Lenz delay is caused by a phase shift that speeds the rotor up. Increasing the load beyond your "left over" energy can produce it. I suggested attaching a 120 volt LED, which has sophisticated circuitry in the base. The back spike power will not illuminate it to full brightness, but draw an additional amount that should cause a lag and result in the speed up effect. I got this kind of reaction with my Bedini SSG'S, and think it's worth a try!


The bonus feature of your MHOP circuit might include a possible continuing increase of acceleration with the simultainious readjustment of dwell.