The Tech Blog

This is one of the simplest design choices you can make in a power supply, yet I rarely see folks implementing them in their projects. Today, let’s dive into the world of bleeder resistors, their importance, and how to choose the right one for your circuits.

A bleeder resistor is a resistor connected across the terminals of a capacitor or in the output of a power supply. Its function is to safely discharge the stored energy in the capacitor when the power supply is turned off. This ensures that there is no residual voltage left in the capacitor.

Why use a bleeder resistor? First, it’s about safety. Personally, I have experienced multiple “discharge shocks” when repairing boards with large capacitors. They pack a punch and it hurts. Hence for large capacitors, using bleeder resistors is mandatory. Second, it allows components to turn off completely. When a large capacitor holds charge, the rest of the circuit remains charged and isn’t effectively off when power is removed. This causes a delay before a proper reset. If you plug the power back in, your digital circuits might not have had the chance to turn off before power is reapplied. Hence, they tell you to turn OFF power and wait for X seconds before turning it ON again.

How to select one? My rule is to use a resistor that is not too low(because idle power loss will be higher) and not too high(Discharge time is longer). Firstly, figure out how fast you want to discharge the caps after powering them off. That value equals 5 times the RC time constant(5 is just to get the value to zero, Realistically you don’t need to unless you are in the high voltage domain. Check the table to figure out your sweet spot). Now you have your resistor value, this resistor needs to be power-rated using V^2/R based on the voltage held initially by your cap.

The design is that simple. So please next time use them in your design. It doesn’t cost much. Even a slow discharge circuitry is better than no discharge one.

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.