Wednesday, May 30, 2018

Guatemala, Day 10

Greetings again from Quetzaltenango. I spent my first weekend down here trying to get some work done as well as exploring the town a bit. I found a laundromat that charged Q20 (about $3USD) to do my laundry (washed and folded!), and I got invited to a family gathering by my host family. We drove out to Tio Tito's house to celebrate his birthday. The family was highly amused by my weak Spanish skills and so we laughed alot and bonded over a tipple or two. Families are fun.

More relevant to this blog, we've been busy working with our students to strengthen their basic engineering and fix-it skills. On Monday I gave a lecture on the fundamentals of motors - just about everything we'll be tackling in a hospital will have a motor of some sort in it. And yesterday (Tuesday) the lecture was about ventilators and oxygen concentrators. We talked about way of testing whether a ventilator is pushing out the correct volume an pressure. To measure volume, you can use the ventilator to inflate a balloon, and then put the balloon in a bucket of water to see how many inches the water level goes up. From there you can calculate the volume of displaced water, which is more or less the volume of air. To measure air pressure you can connect the ventilator to a clear tube with a u-bend that's filled with water. The more pressure, the more the column of water will be displaced.

The transformer and light bulb.
Our labs have been fun, too. On Monday we had the students solder a cable into a transformer that we're going to be using to build a power supply later this week. The transformers are supposed to convert 115V input to 24V output, but we measured outputs closer to 27V. After some research, we decided that this difference was because we were running the transformers without loads. Loads draw current, which will tend to lower the output voltage a bit. To test this hypothesis, we endeavored to try loading our transformers. The transformers are rated for 2.5A, so we needed a lot of load. We found some ceramic-encased high-power resistors in our toolkits and tried putting two of them in parallel. That drew a measly 200mA and barely affected the output voltage. Then we had the idea to take a light bulb out from the ceiling fixture and try putting that in the circuit as well. The bulb drew an additional 300mA and we succeeded in getting the output down to about 26V. I'm guessing we'd need to draw something closer to the full 2.5A rating to get the actual 24V output. We had fun though - the students got a kick out of seeing the lightbulb used as a tool.

Battery pack, switch (red), and LED (clear, at left)
Yesterday we had the students wire up a flashlight using a very simple circuit of batteries, a switch, and an LED. The point of the lab is to get some more experience soldering and trying to think through basic issues such as how should I glue this all together to make it feel like a useful tool. We also had the students determine whether a current limiting resistor was needed. The two AA batteries gave 3V total, and the forward voltage rating for our LED was about 3.6V. Because we're operating below the diode "on" voltage, we knew there was a good chance that we could use the diode resistance to limit current. We tested this by wiring up the circuit with a potentiometer in series and then slowly decreasing the resistance while simultaneously measuring the current. Sure enough, even when the resistor went to zero, the diode wasn't drawing more that its max recommended current of 20mA, and that sucker was *bright*.

We also spent some time learning about fuses. Every lab group took a 1A fuse and shorted it across a pair of AA batteries. That created more than enough current to get the fuse to blow - we looked at the fuses before and after and you can see the little fuse wire inside gets physically burned away. Blown fuses are a constant source of headache when repairing medical equipment!

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