Inside Air Pump: Aquarium Air Pump Mod

Is your aquarium air pump too loud? Read this article that may inspire you how to electronically scilence your valuable equipement.

Recently, a 2-year old son of my friend got a present: a gold fish and a bowl. Yes, the very INFAMOUS bowl and a very famous goldfish. I was immediatly distressed with the scene, although the fish seemed in good health after a whole month, thanks to literally daily water changes. But can you imagine a little gold fish daily excercise jumping from a 2-3 liters bowl into a 0,2 liter glass fort and back durring the water changing procedure? Neither can I, but such is life.

So, I decided to help my friend a little in a right way. I had a 10 liter (~3 gal) quarantine aquarium laying around, got a second-hand air-pump over an auction site for a bargain and a brand new sponge filter. Perfect items for such a small setup!

Below is a look of little goldfish's new home. It is still work in progress (hey, let's my friend do a part :), but some pebbles should be added first thing. I have noticed an immediate change in fish's behaviour. It is now swimming very happily and fast like never before, and it likes to hide under the rocks!

Now, this section of my blog will become a bit technical, not a usual thing for an ordinary aquarium blog, but bare with me, there is a simple advice at the end :)

The only trouble was: the pump was way too strong and loud for a small aquarium like this. Although the model was exactly the same I personaly have for years, probably it was a different production batch. The circuit was the same, but electrical values of the internal coil (electromagnet) were different.

The generic air pump is a very simple device: it has an input transformer (electro-magnet to be precise) which converts your AC Main's electrical energy (Power = Voltage x Current) into an electromagnetic field with it's high inductive coil. This magnetic field then attracts and repells an iron arm that is attached to the pump's housing in an elastic way, which in turn drives a diaphragm that pumps the air. The air is then directed through a small metallic cylinder you can commonly see on the outside of the pump. And that's a whole story about it!

Of course, some air pumps have an additional electronic circuits to control the voltage/current in the electro-magnet, so you can precisely adjust it's strenght and the amount of air that's gonna be pushed. Unfortunately, not all models have that feature, but it's usually not necessary, anyway. However, there is a simple solution if you happen to have a similar trouble!

Common pump model has a coil, a 2-position switch and an electronic diode. In one position, diode is simply bypassed, and the pump gets full power from AC Main's outlet. For example, in Europe this is 220 Volts x Current in Amperes. Current is determined by electromagnet's coil impedance (let's call it "electrical resistance" for clarity, although it's really called impedance). Now, in another position in series with the coil a simple diode is added (1N4007 usually). This diode only passes current in one direction, but blocks it's passing in another. You heard of a safety non return valves for air-pump protection, right? This valve passes the air from an air pump in one direction, but blocks it's flow in return path, so if you put air pump below aquarium height and turn the pump off, there is a strong possibility that the water from an aquarium may get back through the air hose and damage the pump! This valve is mechanical equivalent of above mentioned electronic diode. So, since the pump work on alternating current that changes it's direction 50 or 60 times per second (Hertz = frequency unit), with the diode our air-pump is gonna get only half the power.

Still, with this diode the pump was way too strong, so I needed something else. First thing that comes to my mind was to add a series resistor of 10000 to 20000 ohms and with 5 Watts rating. Why 5 Watts, when ordinary air-pump draws maximum 2-3 watts, you may ask? Simply, because resistors generate heat, which is not a good idea inside a small air-pump housing, so bigger resistor = less heat = good safety.

But I didn't like this idea at all, resistor's body heated up to a warm 35-40 degrees (C) after only couple of minutes, so I decided to use something else: a CAPACITOR!

Not any capacitor mind you, but a special X2 Class MKP types designed to withstand large voltage surges up to 2500 Volts on the Mains line. Remeber: safety first when you deal with high voltage and current sources. And water. This kind of capacitors are used on your computer's protection circuits and filters. Good news everyone: the values we need for this modification are very cheap :)

Capacitors have a resistance (again for clarity, but it's impedance in tech terms) to an alternating current and they do not heat up at all! Perfect component, don't you agree?

Now, I don't like to shoot in the dark, so I had to determine a precise value to limit the power around 1/8th of power with a diode. I have used my electronic knowledge to do this kind of magic, but also used a help from a free electronic simulation software SIMetrix to verify the results.

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Optimal value of capacitance was found to be 47 nF (nano-Farads). If you wish to experiment, you can get standard capacitors from 10 nF to 100 nF, depending on your air-pump model this values should work!

No, I won't bother the audience here with air-pump equivalent circuit, equations and scientific conclusions. For all those who are indeed interested and have some knowledge about complex analysis of AC circuits, here is a paper I've wrote: air-pump model complex analysis
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How it looks inside? Here are some pictures with the tools I have used (soldering iron was not pictured) - it was a 5 minute job literally! I have cut the brown wire (it was the simplest to cut, since this wire goes directly into electro-magnet) and just placed capacitor on each side = effectivly, this is called "in series with the rest of the circuit".

Final note: when you put a capacitor in series with the coil, switch in position with the diode won't work anymore! This is due to the fact that capacitors do not pass DC current and for a split of a second capacitor will charge to the very high voltage of around 300 V DC (or 155 V DC in USA) and then stop = air pump will not work. The only switch position with coil and Capacitor that normally works is the direct one (bypassing the diode). If you like to have original full power, remove (or bypass with a wire) the diode and in that position the pump will be stronger (useful for this).

The pump became quite and everyone was happy :)