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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

Parents

  • Tonight I will investigate whether the Tantalum capacitor has any characteristics related to temperature when it is hooked up in a circuit backward to normal polarity. I my previous post I was using the term reverse biased to describe this configuration. I have decided that I was incorrect in using this term in this manner. I should have said that the capacitor was hooked up reverse polarity. Our previous experiment had established that the Tantalum capacitor hooked up reverse polarity in series with a small motor acted very similar to a Zener diode.

     

    Tonight I am going to do an experiment to see what effect heat has on the Zener characteristic of the Tantalum Capacitor. Here is the schematic for my experiment:

     

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    Here are a few pictures of the actual test setup on my bench:

     

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    The Plan is to take readings of the current in mA and the Voltage across the motor as I move the temperature slowly from room temperature to 100 degrees C. The current at each reading will then be multiplied by the corresponding voltage reading and a resultant Wattage to motor will be derived. The Supply is set at 10 VDC. I will not list all the readings but they are available if anyone is interested. The results revealed that the Tantalum Capacitor's Zener effect had a negative temperature coefficient. As the temperature of the Tantalum rose the voltage across it decreased and the current through it increased.

     

    Temperature                         mA                         Voltage (Across Motor)                    Derived Wattage (mW)

     

    Room 28C                              113                         5.89                                                  670

     

    50C                                        172                         7.03                                                  1210

     

    95C                                        260                         7.89                                                  2050

     

    As we can see over the range of temperatures from 28 degrees C to 95 degrees C the power delivered to the small motor has tripled. This is a good indication that the original designer knew the properties of a Tantalum Capacitor hooked up in reverse polarity could be used to control the power to a small fan motor, increasing it if the temperature rose and decreasing it if the temperature cooled down.

     

    John

Comment

  • Tonight I will investigate whether the Tantalum capacitor has any characteristics related to temperature when it is hooked up in a circuit backward to normal polarity. I my previous post I was using the term reverse biased to describe this configuration. I have decided that I was incorrect in using this term in this manner. I should have said that the capacitor was hooked up reverse polarity. Our previous experiment had established that the Tantalum capacitor hooked up reverse polarity in series with a small motor acted very similar to a Zener diode.

     

    Tonight I am going to do an experiment to see what effect heat has on the Zener characteristic of the Tantalum Capacitor. Here is the schematic for my experiment:

     

    image

     

    Here are a few pictures of the actual test setup on my bench:

     

    image

     

    image

     

     

     

    image

     

    image

     

    The Plan is to take readings of the current in mA and the Voltage across the motor as I move the temperature slowly from room temperature to 100 degrees C. The current at each reading will then be multiplied by the corresponding voltage reading and a resultant Wattage to motor will be derived. The Supply is set at 10 VDC. I will not list all the readings but they are available if anyone is interested. The results revealed that the Tantalum Capacitor's Zener effect had a negative temperature coefficient. As the temperature of the Tantalum rose the voltage across it decreased and the current through it increased.

     

    Temperature                         mA                         Voltage (Across Motor)                    Derived Wattage (mW)

     

    Room 28C                              113                         5.89                                                  670

     

    50C                                        172                         7.03                                                  1210

     

    95C                                        260                         7.89                                                  2050

     

    As we can see over the range of temperatures from 28 degrees C to 95 degrees C the power delivered to the small motor has tripled. This is a good indication that the original designer knew the properties of a Tantalum Capacitor hooked up in reverse polarity could be used to control the power to a small fan motor, increasing it if the temperature rose and decreasing it if the temperature cooled down.

     

    John

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  • in reply to jw0752

    Great test John.

     

    The circuit is clearly exploiting the Tantalum response to temperature.

     

    You have to admit, it does a reasonable job by doubling as a temperature sensor for the circuit.

     

    I would not recommend this solution for long term projects, but it seems to work as a quick and dirty circuit.

     

    DAB