Gas station without pumps

2019 August 14

Beginning design of a cat drinking fountain

Filed under: Uncategorized — gasstationwithoutpumps @ 22:11
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One of our cats likes to drinking from running water (a bathroom sink on a trickle setting), so my wife challenged me to make a drinking fountain for the cat that recirculates water in a water dish.  This project will be mainly physical design (3D printing, gluing things together) with a little electronics to control the pump.

I started by buying a very cheap pump from American Science and Surplus: an ET 23 series pump that they are getting rid of for only $2.50.  Somewhat surprisingly, there is a data sheet available for this pump from the manufacturer: http://www.et-pump.com/brushless_23.html, but (not so surprisingly) the specs are different from what American Science and Surplus claims. The manufacturer says that the pump is submersible and can be run at 5V to 12V, while American Science and Surplus says it is not submersible and runs on 4V to 6V.  Because the pump electronics are fully potted, I tend to believe the manufacturer on this one.

The pump uses a brushless motor with two sets of windings (and only 2 transistors to power them) and seems to start pumping at about 4V.  To characterize the pump, I used my Analog Discovery 2 to sweep the power-supply voltage from 0V to 10V, measuring the current through a 0.5Ω resistor.  The results were interesting:

At low voltage, the current seems to be exponential with voltage, as would be expected from having a diode in the circuit—the nonideality of 5.6 is consistent with about 3 silicon diode drops. Above 4V, the motor behaves about like a 26Ω resistor, though with a lot of noise. The turn-on and turn-off behavior between 2V and 4V is interesting—the pump takes a lot of power at these voltages. All these measurements were taken with the pump running dry—it likely behaves differently when pumping water.

The “noise” in the I-vs-V curve is not random noise—it is fluctuation in the amount of current taken as the circuitry for the brushless motor switches between the two sets of coils. If we set the power supply to a constant 5V across the motor in series with the 0.5Ω resistor, we can observe the voltage and the current for the motor:

The two coils seem to take slightly different peak currents when the switch for them is turned on, but both spikes are about 2.8 times the average current. The frequency is around 643 Hz, which implies a speed of around 19300RPM.

I tried controlling the pump with one of the PWM LED controllers that I made for the desk lamps. With a 6V power supply, I need about 60% duty cycle to start the motor, but then can turn it down to about 15–20% duty cycle.  With an 8V power supply, I need about 40% to start and with a 10V supply about 30% to start. All these were crude measurements by turning a potentiometer until the motor started, but they are consistent with about a 3.3V average starting voltage and ability to keep running down to almost 1V.  If the pump stalls at low voltage, one has to bring it up to about 3.5V to turn it back on.

The motor runs even with fairly slow, low-duty cycle PWM. The current spikes at the beginning of each cycle are large.

The PWM control seems to work even with a PWM frequency as low as 270Hz, which is somewhat surprising.  There does not seem to be much in the way of voltage spiking, even with no capacitor or flyback diode added.  There is a short-lived initial current spike of about 3.5A (staying above 2A for about 4µs), which probably comes from charging capacitor C3 in the motor, which is across the power lines after the diode D1 (which seems to be there to prevent reversed power supply).  The 11µC spike is consistent with C3 being about a 1 µF capacitor (or maybe 2.2µF), which seems plausible.  I’m not sure why the current drops to 0 before the motor voltage drops more than about a volt.

I bought some cheap plastic bowls from a thrift store, and my next task is going to be to design a 3D-printed base to hold the pump and the electronics under the bowl and a clip to hold a ¼” ID vinyl tube over the bowl.  The pump is not self-priming, so I need to drill a hole in the bottom of the bowl and glue on the pump to make a gravity feed to do the priming.

3 Comments »

  1. How to make a running water cat dish project fun. Or how to over think making a running water cat dish.

    Comment by gflint — 2019 August 15 @ 15:18 | Reply

    • Definitely more in the “fun” category. I did not need to measure the current or look at the electrical waveforms, but I enjoy that sort of thing. I now have to start on the mechanical design (the support base, the hose support, and the nozzle), which is a little more difficult for me. I have a vague idea how the pieces should be designed to go together, but I’ll probably have to go through a few iterations before it actually works.

      Comment by gasstationwithoutpumps — 2019 August 15 @ 17:39 | Reply

  2. […] Beginning design of a cat drinking fountain I […]

    Pingback by Cat drinking fountain completed | Gas station without pumps — 2019 September 1 @ 13:38 | Reply


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