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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

  • 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

  • in reply to shabaz

    "Best Practices" are called that for a really good reason. As a tinker in the area of electronics I often ascribe some aura of brilliance and depth of knowledge to the designers of equipment that gets to market. This use of Tantalum Capacitors in an incorrect fashion to get the effect of (Zener Thermistors) really had me confused. After realizing that this improper use is actually causing damage to the component I am now realizing that the designer of this application was probably not that good or brilliant an engineer. The fact that the components did not catastrophically fail in this application was just pure luck. The plus is that the analysis of the application has forced me to experiment and learn a lot more about Tantalums than I previously knew. Thanks   for always lending your insights to my blogs, they are appreciated.

     

    John

  • in reply to jw0752

    Hi John,

     

    I don't know if it is best practice any more but I still do it, I try and pick one with twice the voltage rating, e.g. a 10V tant for 5V operation, but it does seem a risky design to use tant caps around motor or other inductor circuits if there is a chance that the voltage may spike higher. They are quite expensive anyway, so if possible I'll try to use some other cap type or combination of caps.

    Also there are other interesting guidelines for different types of caps as you know, e.g. for ceramic caps the higher voltage rating is also useful due to overall capacitance value, since this reduces as the DC applied voltage increases. The X7R variants are better in the respect, but are more expensive.

  • One final observation. Using a Tantalum capacitor as I have done in these experiments does damage it and makes it unusable in a normal power application. To verify this I tested one of the 33 uF caps that had been abused in this experiment. I tested it in a capacitor tester and found the capacitance to be normal. However when I hooked it up to the power supply and slowly turned up the power supply voltage towards its rated voltage of 16 volts it exploded at 14 volts.

     

    John

  • 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