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I have built an automatic watering system using an Arduino board and a boost converter for powering the 5V DC pump (1-3W), but I have a problem. Each time the pump is activated, the DC-DC converter (bough from here) shows that the voltage drops to 0V, although the pump still works and I'm not sure if this is normal or not. I am powering it by a 2.6 Ah 18650 LCO battery and it should be enough power in it to drive the pump.

The question is... why does it drop to 0V when the pump is turned on?

And moreover, if I try to connect also the Arduino board to the same power, the voltage drops from 5V (which I set manually) to 4.8V and I can't change it anymore in the booster.

Is there something wrong with the switching or is it a normal behavior of a DC-DC Boost Converter? If not, what can be the cause for this drop?

Please see the diagram of my system and the components below :

enter image description here

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  • \$\begingroup\$ You have provided a wiring diagram (and the Fritzing diagrams press all the wrong buttons on some of the regulars on this site). We require a schematic diagram to understand the schema of the circuit. For example, which pin is which on the transistor - or is it a MOSFET? There's an easy-to-use tool built into the editor toolbar. Are you really running a 100 W DC-DC converter from a pair of AA cells? \$\endgroup\$ – Transistor Sep 23 '17 at 14:52
  • \$\begingroup\$ I would gladly do that, but how do I include Arduino and the DC-DC converter in a diagram? I don't have a background in electronics, so I don't really know how to make a schematic diagram with the above components. \$\endgroup\$ – Physther Sep 23 '17 at 14:56
  • \$\begingroup\$ There's a custom component down near the end. Double-click it to add and name the required number of pins. 'R' to rotate it. 'H' and 'V' to flip horizontally or vertically. Make a start on it and we'll clean it up. Use the GND symbol (at the top) for anything connected to 0 V or GND. (It saves a load of cross-over wires.) \$\endgroup\$ – Transistor Sep 23 '17 at 14:57
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    \$\begingroup\$ Motors have this nasty behavior of consuming peaks of current during startup. I don't know if your converter can cope with those peaks \$\endgroup\$ – Claudio Avi Chami Sep 23 '17 at 16:26
  • \$\begingroup\$ If the motor runs, the DC-DC converter must be producing some voltage. Measure the output voltage with a separate voltmeter, rather than trusting the display on the DC-DC converter. \$\endgroup\$ – Peter Bennett Sep 23 '17 at 17:42
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Each time the pump is activated, the DC-DC converter (bough from here) shows that the voltage drops to 0V, although the pump still works and I'm not sure if this is normal or not.

Since you say that the pump still works (and is therefore still receiving power from that Ebay boost converter module) then the display on that module must be misleading you. The output from the boost converter cannot be both zero (as shown by its display) and non-zero (as shown by the pump operating) at the same time!

Unfortunately it is impossible to diagnose the reason for this behaviour with certainty as neither you, nor the readers here, have the schematic for that module, the firmware for its display MCU (see below) and the ability to make all the necessary measurements.

This situation of "why does the module from Ebay / AliExpress behave like that?" is the price paid for using those cheap modules. :-( As I have mentioned on other answers, since buyers can't realistically get the necessary support from an Ebay / AliExpress vendor, then buyers have to either reverse-engineer the module themselves and provide their own support - or else accept that they will never know why strange things happen. :-(

The question is... why does it drop to 0V when the pump is turned on?

The output doesn't drop to 0 V, because if it did then the pump could not operate - but as you have pointed out, the pump does operate even when the display on the boost converter module shows 0 V.

Using an oscilloscope (to be able to see the peaks) or a multimeter (which can only show slow changes) would allow you to see more about what the output voltage is actually doing, to avoid being misled by the display on the boost converter module.

According to the images in that Ebay listing, the 7-segment display on that boost converter module is driven by an STM8S MCU (similar to many stand-alone 7-segment voltage displays also available on Ebay). We don't know whether that could get confused or behave incorrectly, with certain types of loads... Perhaps the current peak when starting that pump is a problem for that boost converter module?

There are two components missing from the diagram supplied, which might have an impact on the behaviour you describe:

  • Missing flyback diode across the motor (add one, to prevent damage to the MOSFET - although it might already be too late to avoid damage to the MOSFET which is already being used).
  • Missing bulk smoothing capacitor (e.g. hundreds of uF) close to the motor. That will help to reduce the peak current load on the boost converter.

Also:

  • The IRF520N MOSFET (Infineon datasheet, Vishay datasheet - I don't know which brand you have) is not the most suitable for this job. It is not a "logic-level" MOSFET and at a 5V Vgs, it is not fully saturated (i.e. it is operating in the linear region) and will only be able to pass a limited current (and will heat up more while it's doing that). The datasheet shows optimal switching at around 10V Vgs, which you cannot achieve from the output of an Arduino.

  • I would also suggest adding a pull-down resistor between the MOSFET Gate and Gnd, so that it isn't floating before your Arduino sets that pin as an output. A small Gate drive resistor might also help, especially when driving MOSFETs with large Gate capacitances directly from an MCU.

These topics have been handled before in other questions and so can be researched there.

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