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.

Reference:

Polyfuse – Mouser: https://www.mouser.com/datasheet/2/240/Littelfuse_PTC_LoRho_Datasheet.pdf-365270.pdf

How to get a cost effective PCB for Production

Designing a PCB for a DIY project involves in making up the schematics which then turned into a PCB layout. Components used in these PCBs will be mostly “Through Hole” which are commonly available in the market. Once the PCB is printed in either screen printing techniques or using photo resistive dry films, making alterations to the component mounting pads and connections will be somewhat possible.

When dealing with a professional PCB design, there are many properties we need to consider. DIY PCBs will simply be single sided in most cases. A professional printed circuit board will most likely to have more than one layer. The PCB for PSLab device has 4 layers. Adding more layers to a PCB design makes it easier to draw connections. But on the other hand, the cost will increase exponentially. The designer must try to optimize the design to have less layers as much as possible. The following table shows the estimated cost for printing for 10 PSLab devices if the device had that many layers.

One Layer Two Layers Four Layers Six Layers
$4.90 $4.90 $49.90 $305.92

Once the layer levels increase from 2, the other layers will be inner layers. The effective area of inner layers will be reduced if the designer adds more through hole components or vias which connects a connection from a one layer with a connection with another layer. The components used will then be limited to surface mount components.

Surface Mount components (SMD) are expensive compared to their Through Hole (TH) counterpart. But the smaller size of SMD makes it easier to place many components in a smaller area than to Through Hole components. Soldering and assembling Through Hole components can be done manually using hand soldering techniques. SMD components need special tools and soldering equipments to assemble and solder them. Much more precision is required when SMD components are soldered. Hence automated assembly is used in industry where robot arms are used to place components and reflow soldering techniques to solder the SMD components. This emphasizes that the number of SMD components used in the PCB will increase the assembly cost as well as the component cost but it will greatly reduce the size of the PCB.

SMD components comes in different packages. Passive components such as resistors, capacitors will come in 0.25 mm upto 7.4mm dimensions. PSLab device uses 0805/2012 sized package which is easier to find in the market and big enough to pick and assemble by hand. The packaging refers to its dimensions. 0805 reads as 0.08 inches long and 0.05 inches wide.

Finding the components in the market is the next challenging task. We can easily purchase components from an online store but the price will be pretty high. If the design can spare some space, it will be wise to have alternative pads for a Through Hole component for the SMD component as Through Hole components can be found much easier than SMD components in a local store.

The following image is taken from Sparkfun which illustrates different common IC packages. Selecting the correct footprint for the SMD IC and vise versa is very important. It is a good practice to check the stores for the availability and prices for the components before finalizing the PCB design with footprints and sending it to printing. We may find some ICs are not available for immediate purchase as the stocks ran out but a different package of the same IC is available. Then the designer can alter the foot print to the packaging and use the more common packaging type in the design.

Considering all the factors above, a cost effective PCB can be designed and manufactured once the design is optimized to have the minimum number of layers with components with the minimum cost for both assembly and components.

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