Unusual Way to Wire a small DC Motor

RE: Unusual Way to Wire a small DC Motor

jw0752 — Sat, 06 May 2017 05:37:24 GMT

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

RE: Unusual Way to Wire a small DC Motor

jw0752 — Sat, 06 May 2017 04:00:10 GMT

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

RE: Unusual Way to Wire a small DC Motor

DAB — Fri, 05 May 2017 19:37:24 GMT

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

RE: Unusual Way to Wire a small DC Motor

jw0752 — Fri, 05 May 2017 05:21:10 GMT

SOLUTION FOUND!

I took [mention:b0bc65b9ecdc4307bd967592f00e340a:e9ed411860ed4f2ba0265705b8793d05] 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.

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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

RE: Unusual Way to Wire a small DC Motor

dougw — Fri, 05 May 2017 05:10:56 GMT

Strange....

How is this for speculation:

Maybe if you think of the commutating motor as an inductor that is flipping back and forth between its terminals....initially current flows through the inductor to charge a capacitor.

Wen it rotates it commutates which reverses the connection, but the current does not want to stop flowing through the inductor.

This causes the side of the inductor that was receiving current to want more current and thus it goes low, discharging the capacitor that it is newly connected to.

The exit side goes high to keep the current flowing, which charges the other capacitor - which could boost the high side voltage beyond the rail to start the next cycle.

Maybe this is a way to limit motor current or motor speed with higher efficiency than using a resistor?

It also seems to ensure the motor always turns in the same direction, regardless of voltage polarity, even AC, which is important for a fan.

RE: Unusual Way to Wire a small DC Motor

shabaz — Fri, 05 May 2017 02:17:37 GMT

Interesting circuit! I too have never seen this. I'm tempted to try it on a motor, although it is late here. Sleepiness may override curiosity for once : )

This is my guess, it could be wrong:

Maybe it is worth picturing this like a bridge rectifier, i.e. diamond shape, with the two diodes at the top half of the bridge rectifier, and the two capacitors being the bottom half, and the motor therefore occupies the usual DC output position.

When power is applied, (lets say +ve at the top, -ve at the bottom for now) then the right-most capacitor won't play any role in the voltage at the right side motor junction; the voltage will be high regardless of the charge state of that capacitor. On the left side, if we assume that the left capacitor is discharged then the motor gets a burst of power, enough to turn it. But, as it starts to turn, the current consumption drops because that left capacitor is charging. But, motor is spinning, and since the right side of the motor is held at a voltage, the back EMF causes the left side of the motor to drop to a very low voltage, to allow that left side capacitor to discharge. At some point the back EMF drops (due to slowed motor) which causes current to pass through the motor again, charging that left side capacitor again.

Anyway, maybe I'm completely wrong : )

RE: Unusual Way to Wire a small DC Motor

kas.lewis — Fri, 05 May 2017 01:43:18 GMT

Hello John,

I've seen this arrangement (without the diode) on hobby motors. More specifically I used to see it on RC car motors. I never fully understood why but just assumed the cap help[ed to remove noise or power ripples from the power lines.

If someone else could explain this better I would love to have that understanding as well.

Kas