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  • Author Author: jw0752
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Unusual Way to Wire a small DC Motor

jw0752

Despite 60 years of playing with electronics this is the first time I have seen a DC motor wired in this fashion.  At first I did not expect it to work on DC current but to my surprise it took off and ran perfectly. Furthermore it did not matter if the polarity was forward or reversed. The motor continued to spin happily along. Just for the fun I also applied an AC current and once again the motor continued to whirl away. Here are some pictures of the motor wiring:

 

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Just to clarify how the motor is wired I also produced this small schematic:

 

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On first appearances I assumed the motor would start to turn and then stop when the small tantalum capacitor became charged. I did not initially understand why this did not happen. I am hoping some of my friends on the Forum with more experience and knowledge will confirm my speculation that the counter EMF in the motor actually discharges the capacitor allowing it to accept a second charge and continue to maintain the magnetic fields that spin the motor. I suspect that the low ESR of the Tantalum caps makes their charge and discharge rapid enough to facilitate this process. I do not know why the designer of the piece of equipment where this small cooling fan was used chose to use this particular circuit. It was something new to me and it caught my attention. Now I just want to understand it.

 

John

  • Very interesting design.

     

    The use of the diodes and capacitors is ingenious, simple, and very cost effective.

     

    The three components, don't forget the motor coil, all work in harmony regardless of AC or DC input.

     

    Each part creates a simple circuit to support motor operation based upon the exchange of current between each component.

     

    In some ways it is like a resonant circuit with the LC components with the two diodes providing the energy from the drive voltage in either direction.

     

    It should work fine with low current and low load conditions.

     

    DAB

  • in reply to michaelwylie

    They don't like lightning up them either.

     

    We had units out in the field that would suffer more indirect strikes (raised ground potentials, etc).

    We learnt to relace all the tants as eventually they became heat and smoke generators .... as well as stink makers.

     

     

    Mark

  • in reply to michaelwylie

    Hi Michael,

    I suspect the explosion has come from heat failure or from pressure generated by boiling the electrolyte. If one keeps the current below the destruction level there are apparently some interesting and this case useful properties to be had. I would never have considered using a polarized capacitor in this fashion but then I am still trying to learn the basics.

    John

  • in reply to jw0752

    Every polarized capacitor I've reverse biased has exploded. How can this be safe?

  • SOLUTION FOUND!

     

    I took   suggestion to try to set up the circuit using a different motor and other components. I was not able to get the motor to turn. I ran some tests on the original motor and found it to be very sensitive. I could get it to spin just using my Ohmmeter probes. I hooked it up and started looking at the circuit with the oscilloscope. There was no sign of any sort of inductive kickback. All the readings that counted were pure DC. Some ripple was noted in the 5 mV area but this was just motor noise. I broke the circuit down and started to look more closely at the diodes and the capacitors. It was noted that one of the two Tantalum Capacitors is always hooked up reverse biased. I set up this small circuit and collected data on the voltage across the Tantalum capacitor as the voltage was raised from 1 volt to 15 volts. Here is the circuit that I used for the test.

     

    image

     

    Input Voltage                    Voltage Across Reverse Biased Tantalum

     

         1V                                   1V

         2V                                   1.7V

         3V                                   2.1V

         4V                                   2.3V

         5V                                   2.6V

         6V                                   2.8V

         7V                                   3V

         8V                                   3V

         9V                                   3V

         10V                                 3V

         11V                                 3V

         12V                                3.2V

         13V                                3.2V

         14V                                3.2V

         15V                                3.2V

     

    So apparently at less than some critical voltage a reverse biased Tantalum acts like a Zener Diode. Since the little motor was able to run on very low current the reverse bias current through the Zener like Tantalum was enough to make it run as intended. If you go back to my original schematic and substitute 3 volt Zener diodes in place of the Tantalum capacitors it makes sense that the motor would spin as soon as the Zener break down voltage was exceeded.

     

    I still have a question for anyone who can help though. Why would an engineer have used Tantalums in such a manner? Why not use Zeners if that was what was intended? If it is of any help in answering this question there is a real probability that the original equipment was powered with AC current. The fan was in a small enclosure tasked with cooling a very bright halogen bulb. I have just had an idea and I returned to the test bench and heated the Tantalum. Heat did cause the Zener voltage to drop. Could this have been a way to incorporate a thermostat on the fan to increase its capacity as the unit heated up?

     

    At least we can now cross out my original analysis and deal with a situation that makes a little more sense.

     

    John