The Power

Here I'm referring to the power required to drive the pump. After experimenting I found that the pump is only effective (with water or insulin) when driven by 240V. Not practical when you want something to be portable and driven from a battery of the sort you get in a mobile phone. So I needed a way to boost the power, just for the short time that the pump needed driving, so just a short 240V pulse. The obvious answer is to use the same approach as a camera photoflash. After all, many mobile phones contain these now and or course digital cameras, both of which are powered by small battery packs.

I settled on the Linear Technology LT3468. Click here for the datasheet. The basic charging circuit that I used is shown below.

I've modified it only slightly because I don't want 320V I want 240V to drive the micropump. So you just use basic GCSE physics to determine that you need two resistors in the series, 400K and 100K, and then take the power from across the 400K resistor. Using a simple formula where the output voltage of this circuit is split based on ratio off the resistors you get a voltage drop of about 256V. This is a little too high but you will see later why this is ok.

The main capacitor that gets charged is shown here on the right with the value of 100uF. This was a problem because most electrolyic capacitors are large and the higher the capacitance the larger they are. It also has to handle over 300 volts. After some testing I found that a 1uF capacitor would hold it's charge just long enough to power the micropump. At this capacitance capacitor are much smaller. You may know, the higher the capacitance the longer the capacitor can hold its charge before it leaks away. Luckily the micropump only needs 240V supplied for a very short time so the smaller cpacitor works. This is also the reason why charging the capacitor to 256V does not destroy the micropump. By the time the circuit has switched and delivered the voltage is has reduced to around the 240V that the micropump can withstand.

In the present capacitor is around 6mm in diameter and 10mm long. Click here for the datasheet. In the future I think I can reduce this further by combining a number of surface mount devices.

The battery I'm using is pictured below, click here for the data sheet. Note that this battery is little more than 2mm thick.