I'm attempting to design a circuit to accept signals from four different solenoids, and to then combine them all into a single, DC signal. So I think I need some sort of 4-phase bridge rectifier, but my voltages are on the order of a few mV.

Most bridge rectifiers have forward voltages that are too high for this application, and I can't seem to find any diodes with sufficiently small forward voltages for me to build my own rectifier.

How should I approach this problem? Oh and I can't use active circuit elements (op amps, transistors, etc) to amplify voltage.

 

A precision rectifier and a mixer circuit might do what you want. Tell us more about these signals.

 

I'm thinking a precision rectifier (an op-amp circuit) or 4 of them, and a summing junction (another op-amp circuit. Oh, and you can't get there from here without using active elements. There is no such thing as a millivolt level passive rectifier.

 

A few questions; A solenoid is a coil that activates something mechanical,( a lever, a shaft or the contacts in case of a relay) You apply a voltage, AC or DC and it operates. It does not produce signals (unless you talk about back emf etc.) Unless your definition of a solenoid is not the same as mine.
Are these "signals" simultaneous or at different times. I am thinking that with a diode inline and a resistor, you could feed them into a simple and/or gate in cmos and get what you want.
Just a new perspective on the problem

 

The four signals are generated by a small magnet oscillating at anywhere from 5-50 Hz within an inductive coil; I have four such magnet/coil pairs on the wind energy harvesting device. Each coil's induced signal has a maximum voltage of about about 0.1 V. The resistance of my inductive coils (which I mistakenly(?) called solenoids) is about 13.5 Ohms.

So each magnet/coil effectively acts as a generator, and the magnet oscillations are induced by the wind.

I need a way to combine these signals. Is there a way to do this without using active circuit elements? The wind energy harvester will be in remote locations, so I won't have access to external power for the active elements.

 

You may want to consider using a solar/battery system for your active components. You NEED to get the power from somewhere..

I also have the feeling that a lot of power is being wasted using vibration in such a manner for power production.

 

Is this experimental? There is not much you can do with a total of 400mV Given that you can "add" these outputs somehow. The resistance in the connecting cables/wires will eat up your little output. Harnessing wind at that level is not worth the trouble. If this is for experimental purposes, then maybe connect it up to a battery powered recorder.
A bit more input might help.

 

I'm working on a modification of this: http://www.popularmechanics.com/technology/industry/4224763.html

The "power conditioning unit" at 0:59 is what's giving me trouble.

How would a battery powered recorder benefit the system?

 

It looks like a buck-puck.

 

Could you possibly use a step-up transformer to get the voltage you need to overcome the forward voltage of the diodes and then rectify that? Or will 400mv not be enough to induce a magnetic field?

Im thinking if you got a little ferrite core and some really thin wire you could make a little torroid transformer that would be nice and compact. I tore apart a few dead atx power supplies and there's lots of little inductors in those things and sometimes even toroidal transformers. I have a bunch but I don't know how to find out the values or ratings of them.

What's the generator for and what can you really do with 400mv?

OH! That got me thinking. If a transformer's no good maybe you could set up a gear ratio to drive the magnet faster than the prop? Like on a bicycle! Would that generate a higher voltage or just a higher frequency? I've heard that's how the big wind turbines work.

BTW (I'm still a noob too so correct me if I'm wrong) unless those 4 different magnets are at different positions on the shaft - it probably isn't 4 phase ac. If they are in the same position on the shaft then it will give you single phase ac (right?). Also if you wind or hook up those coils backwards they will give you a signal out of phase with the others. That will drastically reduce your voltage also. Make sure everything's in phase (wound in one direction and connecting the same way). If that's not the case then maybe you could rewind or make bigger coils with more turns which should give you a higher voltage output.

 

If you were experimenting, a battery powered recorder could log (in memory or on paper) your various outputs.
These instruments are sensitive enough to record your small voltages and frequencies. From there you could determine what wind speed is necessary to produce anything useable.
That was my thinking anyway.
Regards

 

That's a great idea and will be very helpful for our field tests. Fortunately, we also have a wind tunnel available to us.

In other news, increasing the wire gauge allowed me to pack many more turns onto my coil spools, and I'm now attaining outputs of 1-2 V, which easily light my indication LEDs. More importantly, these voltages are easily rectified.

Thanks to everyone for your help.