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...
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...
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...
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...
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...
So, as I am sitting there, soldering together the squarewave generator, I'm thinking why only hit the inductor in one direction? We gotta a double throw relay, so lets smack it around both ways. I'm an alternating current kind of guy anyway. So I use my tricky wiring super powers to make the inductor current go both ways...
Also made a Schottky Bridge Rectifier to smooth out the bumps...
It occurred to me after I had posted all of this that I am using the secondary on the audio transformer. The high impedance winding. The low impedance winding is probably too much for my tired AAA battery pack...
So, after a day of playing with my BEMF trap the AAA battery pack is worn down to around 4 volts. The reconfigured circuit will pass DC and the BEMF into the trap cap (C1). If I run the circuit without the inductor I get the battery voltage minus a couple of diode drops in the trap cap. But when I use the inductor the trap cap voltage will swing up to 21 volts. It does not maintain this voltage with a load. I put an LED with a 220 Ohm resistor on the output the voltage on the output goes down to around 3.5 Volts. So yes we can trap and store BEMF, but there is not much current there. Maybe I should use a better inductor...
I'm thinking maybe I'll give a whirl with my Magnetized Composite Planar Transformer...
Gotta wait 'till I gets home tho...
Quote from: z.monkey on April 15, 2010, 12:16:51 PM
So, as I am sitting there, soldering together the squarewave generator, I'm thinking why only hit the inductor in one direction? We gotta a double throw relay, so lets smack it around both ways. I'm an alternating current kind of guy anyway. So I use my tricky wiring super powers to make the inductor current go both ways...
Also made a Schottky Bridge Rectifier to smooth out the bumps...
Hi Z.monkey,
I think it is very good idea to bring up this topic, especially the changing of the current direction in the coil that has a ferromagnetic core. Please continue as you wish and when you have some time, have a look at this link here and go through on it because I find Nali2001's double MOSFET switch very innovative in his Circuit003.jpg: http://www.overunity.com/index.php?topic=4300.msg214471#msg214471
(in the Circuit003.jpg I think there is a gnd symbol missing from the right hand side MOSFET)
Keep it up, very educational and much needed job you do.
rgds, Gyula
Howdy Gyulasun,
Yeah, I agree this issue needs to be addressed. It is a very specific phenomena, it can be measured, and in the right circumstances it is the right design. Like in the instance where you want to maximize your flyback (BEMF), like high voltage transformers. Automotive ignitions and electron guns in TVs are where this works good, with the right inductor. But I don't think you can call it free energy. The output impulse is still less power than the input, even though the output voltage goes way up...
I am cautious about using MOSFETs to try this because they don't turn all the way off. In a relay you have a break before make condition on the switches that can't be duplicated with transistors. Like in my low voltage example, if I used transistors, I probably would not get 21 volts open circuit. It would be lower voltage, still there would be voltage gain, but not as much as using a relay. In order to get the BEMF to flow as efficiently as it can you have to disconnect the inductor. A transistor just can't do that. There is always going to be a small leakage current preventing the inductor from rapidly discharging. The Schottky Diode bridge gives the BEMF a low impedance path to follow, so it doesn't try and jump the relay switch gap. I do think we can maximize the concept to make circuits more efficient, and to a large extent semiconductor manufacturers know this and incorporate these ideas into their designs. Mo on that later...
So, thanks for you interest...
Lets make it mo betterz...
So, here I hooked up the scope on both sides of the inductor to give it a looksee. Its ticking at 23.8 Hertz. With a load I am getting 4.1 volt pulses in the inductor. The battery voltage right now is 3.89 Volts. Notice that there are some anomalous spikes in the waveform. This is the relay contacts bouncing, which is actually helping the circuit produce BEMF. I'll explain in a minute...
In this shot I am going to zoom in on one pulse to take a look at the switch bouncing. This is the drawback to using mechanical contacts. Any time you have mechanical contacts there will be high frequency bouncing, this is why there is a debounce circuit. That one is big in digital stuff because sensitive logic will produce a stream of bounces with a mechanical switch. If you are doing something like counting pulses its a big problem. Here in this circuit is is providing lots of transient pulses which we are rectifying down to DC that we can use...
Then here in this shot I remove the load and adjust the scope for higher voltages. We can see when the circuit is unloaded the voltages shoot up to 21 volts...