The Tech Blog

Most folks who frequently fly in aeroplanes will usually zone out during the safety instructions demonstration (Although technically you are not supposed to) by the flight attendants. This week when I was taking a flight, I thought of taking the time to actually properly listen to the safety instructions for old times sake. Something that they said caught my attention this time. Whenever they talk about oxygen masks falling from the roof of the cabin, they tell you to tug and pull the mask. It opened up a lot of questions for me. Let’s discuss the basics first.

An oxygen mask gets deployed whenever there is a loss of cabin pressure, meaning there is a leak of outside air into the cabin maybe through a broken/opened window/door or the door seal malfunctioned. Once the pressure drop is detected, it triggers a control signal to all the seat roofs to switch a relay/latch to open up the panel to drop the oxygen masks. Now in case the sensor doesn’t trigger it automatically, the pilot can toggle a switch in the cabin to open it as well.

Now I had always thought there was a massive oxygen cylinder feeding oxygen to the cabin in emergencies. I couldn’t be more wrong! What you have is a small canister in every seat row that mainly contains Sodium Chlorate which decomposes to give you oxygen instantaneously. The tug/pull of the mask they talk about is to trigger a firing pin on the canister, to create a small explosion internally to start the reaction to release oxygen. So remember to do that next time.

The canister holds enough oxygen to supply to passengers for around 15 minutes only. This is the maximum time needed for an aeroplane to descend to a reasonable atmospheric height where oxygen can be taken in from outside and everyone can breathe normally. So why can’t you just have oxygen cylinders then? It weighs a heck of a lot more to store oxygen in a pressurized cylinder than to store chemicals needed to create it on the fly(pun intended).

Look around and keep an open mind. There are so many things we can learn from just being curious and asking the right questions.

Last week a client was working on some high-voltage PCBs and I thought it’s a nice time to address those here as well. How do you design for High-voltage PCBs? Let’s at least discuss a major aspect of design, for that you need to understand two key components, Creepage and Clearance.

Clearance is the minimum spacing between 2 items in a PCB through air or Line of Sight. These could be track-to-track spacing, track-to-components, or component-to-components. Now creepage is the spacing between 2 items along the surface of the PCB. Check images for clarification. These differ in cases where there is a slot on the PCBs between 2 items, Clearance distance will be the straight line path between them, but creepage would be all the way around the slot. So it will be much higher.

Now for high-voltage PCBs, these terms are important because high-voltage sections can always arc over from one section to another if the distance is too small. Hence you must give some sort of clearance or slots between. It depends on the environmental conditions(Humidity, dust), Altitude(Air pressure reduces with height so does the breakdown voltage of air) where your PCB is used, and the coatings you provide on the PCBs(Conformal or Soldermasks). The standards which govern these are mentioned in the guideline IPC 2221B document. Check the image for a table that tells you the minimum spacing needed between conductors for different use cases. For cases above 500V, multiply the voltage difference after 500V with the multiplication factor and add it to the row above.

Use the table from IEC 60950-1 Device Safety standard for Creepage values. It contains a table for minimum creepage distances for different voltages and degrees of pollution the PCB might be subject to.

Now next time you do high voltage designs keep the distances in mind. What are your favourite high-voltage design tips?