Self accelerating reed switch magnet spinner.

[synchro1]

@TK,


       Improved bearings, coupled with increased storage capacity and charge time would probably speed the rotor up somewhat along with the drop in current consumption. A stronger watt bulb might do the trick too. Try connecting a stock Lights of America type 120 volt LED bulb as load!

[MileHigh]

TK:

If you are tempted you could have another go with your op-amps for your current sensing.  I am assuming this would cost a fraction of what differential probes would cost.

You may have some "signal" type transformers or something that will work fine.  Preferably a 1:1 ratio but even that's not that critical.

You do the dual 9-volt battery setup with a single inverting op-amp and you build this:

[current sensing resistor] -> [inverting op-amp] -> [signal transformer] -> scope channel.

The op-amp is converting the input voltage to a low-impedance output voltage.  That means that when the op-amp drives the primary of the signal transformer, within limits it can supply increasing current when it outputs a DC voltage.  In other words it can energize the inductor in the transformer.  That generates a DC voltage on the transformer output.  It may sound funny to state "the basics" but the important thing to realize that this gives you the ability to pass an uncorrupted waveform from the primary to the secondary down to a fairly low frequency.  In other words, this will give you a "poor man's differential probe" that should work find above a nominal frequency.  The frequency might be quite low, perhaps 10-15 Hz.  Likewise, the larger the transformer the more "headroom" you have for coupling low frequencies.  You know those transformers that might be one-inch cubed?  Something like that, substantial.  I am guessing smaller might work too.  You don't want to get too large because we are going to make the assumption that the smaller the transformer, the better the high frequency response.

Simple test:  You breadboard your inverting amplifier and feed it with a square wave from your signal generator.  The output goes to your signal transformer, and the secondary of the signal transformer goes to your scope probe.  Suppose you start with a 200 Hz square wave.  Your scope channel should show you a very faithful, isolated replication of that wave form.  Then start lowering the frequency.  If you are lucky, let's say you get down to 10 Hz before you start to notice that the transformer is "choking" and failing to replicate the low and high "DC" levels.  If we assume that this is all true, then if the pulse motor is pulsing at say 30 Hz, then the the secondary waveform on the transformer should be an exact replica of the waveform on the primary.   So for five dollars or less, you should be able to enjoy all the benefits of a differential probe using an op-amp, with the understanding that as long as your pulse frequency is above a certain threshold (and you will know that threshold frequency) then you will see a nice clean waveform on your scope display.

When you think about it, it's a nice little potential project.  Two 9-volt batteries, current-in and current-out terminals, an inverting amplifier, perhaps a trimpot arrangement for your input resistance, a trimpot arrangement for your feedback resistance, a trimpot to zero the op-amp output when there is no signal, and the signal transformer, and isolated ground and signal terminals for your scope probe.  You can adjust the gain of the amp whenever you want, and away you go.

Again, there are two major assumptions here: 1) with the op-amp driving the transformer input you will get faithful signal propagation above a certain low pulse frequency, and 2) the op-amp and signal transformer bandwidth will be high enough to give you a quite faithful waveform reproduction.  Perhaps when you compare the original signal and the transformer-coupled signal on your scope for typical pulse motor current waveforms, you will barely notice the difference.

There is also a limitation.  We are assuming that most current waveforms in a pulse motor are of the form <nothing><pulse><nothing><pulse>.  In other words there is no DC bias in the waveform.  If there was a DC bias in the waveform, that would mean there was some constant current flow.  Naturally you can't scope a DC bias.  The op-amp output will induce a constantly rising current waveform in the primary of the transformer.  Eventually the op-amp will crap out or the transformer will get saturated and start to heat up and you will lose all of the signal on the secondary.  But for fun, you could buffer the main op-amp output into a second amplifier.  You could connect a pair of LEDs to the second op-amp output.  So those LEDs could show you "above ground" and "below ground" signal activity - in other words, the direction of the current flow and the approximate magnitude of the current flow.  Just a neato "LED scope" to let you know if something is alive and pulsing without even having to hook up your scope.  If you tuned it right, you might be able to decently judge current flow and direction just by the apparent brightness of the LEDs.

If you don't have a high-bandwidth signal passing transformer then you could probably find something excellent on DigiKey.

So, isolated signal probing is just an op-amp away!   If PW is watching, he could "polish" my comments and give you the real deal also.

It would be a fun and easy project to do.  As you might imagine the identical concept could be done for an isolated voltage probe also.  I think the key is having the right transformer.

MileHigh

P.S.:  There may be an issue that would have to be investigated.  I am wondering what would happen if you say feed a square wave into the input that varies between say zero and five volts (like a current waveform).  Every positive pulse in the square wave will induce current to flow in the transformer primary.  When the op-amp output goes to zero volts the primary of the transformer will still be conducting current.  The op-amp output is an active ground at this time so the transformer primary inductor sees that as a short circuit and current keeps flowing.  So that could be problem, every positive pulse will induce more more and more current to flow into the primary until it saturates.  So you may have to add an AC coupling stage before the main inverting op-amp.  Well, it could be more of a challenge than I thought.  There might even be something in the app notes that would be much better....

[TinselKoala]

https://www.sparkfun.com/products/8882

[MileHigh]

TK:

You just blew me away with that amazing link.  80 KHz bandwidth and it's fully isolated!  Just requires 5 volts!  It's quite amazing and just gives you all the crap I discussed in one tiny board!

It's great but at the same time, for some and I for sure for you, there is the satisfaction and fun of building something.  But it's still damn amazing.  I also suspect that you can get much higher bandwidth with the op-amp and signal transformer.

MileHigh

[MileHigh]

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