Circuit Protection Measurements in PSLab Hardware
Pocket Science Lab by FOSSASIA is a versatile analytical tool which can be used by a variety of user. They span from school kids to professional engineers who need a measurement device to ease up their task. In electronics and electrical fields, circuits do get damaged a lot. There is no perfect method to keep the circuit damage proof but the only thing we can do it minimize the damaging causes as much as possible. PSLab is no exception.
Since PSLab has an audience involving school children, it is quite possible for a short circuit to happen in devices while using. A short circuit is simply a non-resistive path between a supply pin and a ground connection. When a short circuit happens, there is a huge amount of electrons flowing through a small path which will eventually melts if there is no safety mechanism to stop the rapid electron flow.
PSLab contains a plenty of sensitive integrated circuits to support its wide variety of features. They will easily burn if a short happens and the user will have to dispose the device as it will no longer be functional. That is the case if there is no safety feature embedded in the printed circuit board to fight against damages of that nature. We have included a special type of fuses to overcome damages occur due to short circuits.
A fuse is a passive component which allows the flow of current up to a threshold value. When the current increases, the fuse blows and the flow is disrupted. A normal fuse will require replacement. But in PSLab, we have included a special type of fuse known as a polyfuse. This type of fuse does not require replacement. In the following figure it is shown as “F1”
Figure 1: Position of fuse in PSLab V4 PCB
The speciality about poly fuses is that they will recover its current carrying capabilities once the short circuit is open. The auxiliary name “Resettable Fuse” will make sense as it will reset to its original state once the circuit is safe.
The working principle behind a polyfuse mainly depends on its resistance. When the current flow through a polyfuse is at the rated values, it will have a minimum resistance between the input and output terminals.
Figure 2: A poly fuse
As the current flow increases, the particles inside the fuse will start moving fast. The slow motion of these particles are the ones keeping a smooth current flow across the fuse. When the particles are moving rapidly, it will oppose the current flow through them.
Imagine a situation where a short circuit happens. There will be sudden rapid flow of electrons. These electrons will collide with the particles inside polyfuse which were at a calm and steady motion. Now their motion is disturbed and the movements will be random and fast. This will cause the fuse to heat up to a high temperature. As the graph in figure 3 illustrates, it’s resistance highly increases with the temperature.
Figure 3: Temperature vs Current graph of a poly fuse
The rapid electron flow will have no opening to go as the fuse path is now heavily resistive. When the current flow is stopped, PSLab will turn off. All these things happen so quickly that the short circuit would not damage any internal components anymore. Once the user sees that the PSLab is off and he solves the cause for the short circuit, the fuse will cool down which results in a lesser resistance. Now the current flow will come back to normal making the PSLab device working again.
Polyfuse – Mouser: https://www.mouser.com/datasheet/2/240/Littelfuse_PTC_LoRho_Datasheet.pdf-365270.pdf
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