BEMF Scavenger Cruda

[z.monkey]

Sittin' around work, bored again.  An idle mind is a dangerous thing.  So I decided to explore this Back EMF thing.  Also to make it challenging I am going to build a new widget with stuff laying around the shop.  Well, two shops, work and home.  But no buying parts, junk only...

So, I have a small audio transformer with a primary winding resistance of 65 Ohms.  That should be a good inductor to start with anyway.  Its a EI core, with a bobbin holding the windings.  Our starting circuit is a 4.5 volt AAA battery pack a switch and the transformer...

[z.monkey]

So when we close the switch the transformer develops a magnetic field.  When we open the switch the field collapses and the voltage produced by the collapsing field produces a voltage across the transformer.  This voltage will try and jump the gap of the switch and you can see a little spark there, even at 4.5 Volts.  So, we have identified what we are chasing, how do we catch it?

I add another switch on the ground side (DPST switch) but the voltage still wants to jump the gap...

[z.monkey]

So we need to build a little trap for that BEMF critter.  Them things is hard to catch...

These transient voltages happen really fast.  So were going to use a Schottky diode to trap it.  The diode will block the DC from the battery and only hold the BEMF in cap C1...

[z.monkey]

While we do get a little voltage on C1 with the last circuit as soon as we close the switch again the charge is sucked into the ground of the battery.  The BEMF aren't accumulating, so I am measuring the BEMF from one pulse each time.  We need to change the circuit to accumulate the BEMF pulses...

I am improve our BEMF trap with a second Schottky diode...

[z.monkey]

OK, now we got a trap, LOL!  My button pushing finger is getting tired...

Need to automate this thingy now.  I am going to use a DPDT relay to do the button pushing.  Then we gotta make a circuit to make the relay wiggle...

I found a LM393 dual comparator laying around.  We can use that to make a squarewave generator.  We can use a small MOSFET to drive the relay.  I found a 2N0702 that will work good.  Also found a sack of 1N5817s, Whoo Hoo...