# current doubling rectifier circuit

Discussion in 'General Electronics Chat' started by keithg, May 11, 2009.

1. ### keithg Thread Starter Active Member

Jan 17, 2009
40
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I have an exercise machine turning a stepper motor with a full wave bridge rectifier that is putting out 30~40vdc and 200mh. The project is an experiment for our elementary students. I connected the two negative and two positive wires for the dc output. How can the voltage be halved and the current doubled. Joining both poles in this manner would seem to be in parallel. Fact is, I don't really know what I am talking about! Ohm's law must apply here but I don't know how to wire it. Any help would be appreciated.

May 16, 2005
5,072
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3. ### keithg Thread Starter Active Member

Jan 17, 2009
40
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How do I connect the four wires from the bipolar stepper generator to the transformer? Your book will address many of our questions. Thanks.

4. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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You will need a 2:1 transformer rated for 8VA or more.

Put the transformer between the "generator" and the bridge rectifier. Hook your stepper-motor leads to the transformer primary, just as you previously hooked them to the bridge rectifier. Connect the transformer secondary to the bridge rectifier input.

The following section of the book explains how transformers work: http://www.allaboutcircuits.com/vol_2/chpt_9/index.html

5. ### DonQ Active Member

May 6, 2009
320
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What is a mh, as in "200mh"? I can take a guess, but there are other things in your question that I can not.

Do you really mean a "stepper motor"? Why not an alternator, or even a generator? Maybe it is a stepper motor, but in that case "a full wave rectifier" is not going to be the standard 4-diode arrangement. Two full-wave bridges? A three phase rectifier?

"The two positive and two negative wires"?? From the stepper motor? The bridge? Bridges?

"Both poles"???? Like the poles of a switch or relay? What?!

I can guess a lot of things based on the current description, but I don't think that's going to help any.

And I've never heard connecting a transformer to the output of a back-driven stepper motor. How many phases is this transformer going to have? Never seen a 4-phase transformer. Maybe transformers?

Since you appear to be doing something fairly non-standard, maybe you need to draw a picture. # of wires from the stepper(?) motor. Connections to the bridge(s), where did you measure 200mh, etc.

A little more precision in the description will make a coherent response much more likely.

6. ### thingmaker3 Retired Moderator

May 16, 2005
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DonQ is correct. A pair of transformers will be needed - one for each of the bridge rectifiers.

7. ### DonQ Active Member

May 6, 2009
320
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Well, I think I can imagine the configuration now. But I would bet that the output is going to be substantially above 60Hz!

Gear ratio... number of steps per revolution... It adds up (actually multiplies) really fast. If it's too much higher than 60Hz, transformers might be a problem.

Depending on what's there, and how it is currently wired, there is still a small chance that it would be possible to do it with a rewiring job instead.

8. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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Since this is a demonstration or exercise for school children, we might be wise to ask "What lesson is being taught?" Depending on the lesson and the level of student, winding proprietary transformers might not be out of the question. On the other hand, if this is a biology class measuring human muscle output, the wiser course might be something else.

With higher frequencies a voltage doubler might be practical, depending on project specifics. Components for such would be easier to find.

9. ### DonQ Active Member

May 6, 2009
320
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Except, I think he is wanting a current doubler.

10. ### keithg Thread Starter Active Member

Jan 17, 2009
40
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Here is a crude diagram of the circuit http://i189.photobucket.com/albums/z215/keithgum/circuit.gif?t=1242163094
I hope this addresses your questions. Wouldn't doubling the current enable the stepper generator to power a wider variety of dc appliances? This circuit is important as an experiment for our students but also for a summer project in the mountains of northern Luzon where many villages are without electricity of any kind. Even something as minimal as a stepper charging a 12v battery would be very welcome. I am leaving the states next month and hope to iron out the glitches soon as possible. Thanks again.

11. ### keithg Thread Starter Active Member

Jan 17, 2009
40
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To answer your question concerning the lesson's objective: Florida is a test taking, test practicing, test based, test mad state. The test rules supreme. The truly daunting problems of out time are of no consequence, only the test matters. I am the art teacher with much more flexibility than the academic classroom so introducing alternative energy is not threatening. We need to master this circuit for wind turbine projects next school year.

12. ### DonQ Active Member

May 6, 2009
320
14
Only 4 wires total coming out of the stepper? If so, you're probably stuck with what you have. The transformer scheme might work, but I think you'll run into problems with the signal from the stepper being substantially above 60Hz. You could get or make high frequency transformers, but that's probably too far to go.

It might actually be reasonable to just hook the output directly to a 12V battery. I know this sounds bad, but at such a small current, with the relatively high winding resistance, this isn't really a problem. Instead of using the stepper as a voltage generator (based on its open-circuit voltage), this would put it into a current generator mode, with output voltage clamped to the voltage of the battery. The rest of the voltage will be dropped across the winding resistance. You might find that you generate more current than the 200mA(?) you appear to have measured before. Was this the limit before the voltage started to drop? How much current do you draw when you load it enough to drop the output to 12V? This is how much current you would generate using it as a current source.

If you read 200mA off of the stepper motor label, then these values may be different.

Another option is to just use an automotive alternator (not a generator). These are readily available, designed to provide high current at a relatively low RPM, and with the regulator will output 12V ( actually ~14.4VDC for charging a lead-acid battery). And the 3 phase output, with an internal bridge, gives a better voltage waveform. With a strong legs, you might even be able to get more than 10 Amps. If you do this, get as small an alternator as you can (20-25Amp?). Anything bigger than you need will just be extra weight.

13. ### keithg Thread Starter Active Member

Jan 17, 2009
40
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We have been recycling printers, photocopy machines, and scanners and have a near infinite supply of steppers of all kinds on hand. Would a unipolar stepper with 5 or 6 wires offer an alternative? An effective circuit with salvaged motors/parts is our goal.

14. ### DonQ Active Member

May 6, 2009
320
14
You're never going to get anything near what you would from a real alternator, but here's some things to look for in a stepper to use.

1) Bigger. (obvious) More chance of being able to provide more overall power.
2) Lower operating voltage. Lower voltage in equals lower voltage out when back-driven.
3) Lower coil resistance, generally the same as higher required driving current. More capable of producing higher current for an output.
4) Lower number of steps per revolution would also probably be a good thing, but a minor point.

More wires from the motors could give you the option of doing things like half-wave rectifying, or making split supplies. Some 6 wire motors may be 3-phase (more options) but they are probably split 2-phase (no help for you unless you want to try to modify it internally), or 8 wires (rare) would allow paralleling opposing windings.

Another way to get more current is to just parallel more motors. Identical motors, each with its own bridge rectifier, will just add currents. I would guess that the available input power (the push required on the pedals) is not the limiting factor at this point.

Using the 12V battery as a voltage clamp is still probably a good idea. Besides, pedal and wind both have the habit of not being continuous.

15. ### keithg Thread Starter Active Member

Jan 17, 2009
40
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I like the parallel motor idea because there is so little drag on the pedals with only one motor. Adding another will not be a problem. Also, I will try other steppers. Thanks again.

16. ### keithg Thread Starter Active Member

Jan 17, 2009
40
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Would this stepper be a better choice:

Specifications : PM55L-048-HP69
Nominal Voltage 24V
Resistance (Ohms) 30
Wires 5
Steps / Revolution 48
Step Size (degrees) 7.5
Current/Phase 800mA

17. ### DonQ Active Member

May 6, 2009
320
14
Best at this point to just try it. Maybe hook it to an electric power drill and measure the output.

18. ### keithg Thread Starter Active Member

Jan 17, 2009
40
0
I am testing the steppers as you have suggested. The alternator is a possibility if available used. At a junk yard, do I simply ask for a 20~25 amp alternator? Do I need to know the make/model car?

May 16, 2005
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20. ### DonQ Active Member

May 6, 2009
320
14
If you're getting a used alternator, probably just asking the guys at the junkyard is your best plan. There are so many options that I could only give you rough guidance.

Almost anything would work, but aim toward this:

Make/model almost doesn't much matter. You might just want to be sure that it is from a "negative ground" system (that will be %99.9 of what's available.)

Small engine = small alternator. Doesn't have to be 20-amp, but that will probably be the smallest you can find.

Some require an external regulator, some have regulators built-in. Built-in would be easier. Older alternators might be more likely to be external regulators (more hassle, and a couple extra wires, but still easily doable.)

Just tell the guys at the junkyard what you're doing, and that you're "budget limited". If it's not a busy time for them, they might be happy to help, and they know what they've got.

Last edited: May 15, 2009