Casio MJ1200 Calculator Teardown

Pocket calculators are a classic example of something which has been around since the 1970s and has undergone optimised design over the years. These still do have a place in the world despite the touch-based software phone calculators out there. The tactile button press feedback is something that a heavy user will always appreciate.

Casio Calculator Teardown
Casio Calculator Teardown
Casio Calculator Teardown

The basic building blocks of a calculator will be your power source, a matrix keypad, an array of 7 segment LCDs and an ASIC(on a black blob) built for cost. The matrix keypad will be a standard row-column arrangement with multiplexing being used to read out the button presses. The button would consist of a carbon-coated unit with a zig-zag portion on the PCB which will effectively make a short with carbon when pressed. The 7 segment LCDs are monochrome and mostly reflective based. Since most calculators don’t carry a backlight they are quite low power to drive(One of the reasons why calculator batteries seem to last forever). The LCD segment is connected by a ribbon cable which is usually glued in place in a PCB(Not soldered) with carbon tracks. If you ever see certain segments in a calculator not lighting up, try heating the cable connector on the PCB end, it should mostly work.

The power section of these calculators usually contains an alkaline button cell battery(1.5V, Not rechargeable). Most of the calculators come with a solar cell array to act as a dual power source. This one had 4 solar cells with a total open voltage of approx 2.5V. It uses the classic power OR-ing circuit. Both battery and solar cells feed into the same PCB track with individual Schottky diodes(Parallel). So solar does contribute to the longevity of the calculator’s battery life. The Schottky diodes in the circuit prevent the reverse charging of the battery and have a low dropout of approx 0.15V. It’s the cheapest possible arrangement based on BOM costing for powering the device. That 0.15V drop is OK for low power circuits like these but won’t scale for larger circuits and hence most IC companies make a killing with power path management ICs these days (Well that’s a discussion for another day entirely)

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Back to Basics: Inductor Rated Current vs Saturation Current

Saturation Current

When choosing inductors in your design, one of the most confusing aspects is the rated current and saturation current values(there are other alias names like Irms, Itemp etc) mentioned in the datasheet. In older datasheets, these are rarely explained. What should you choose as the upper limit current rating for your inductors? Sometimes saturation currents are more than the rated current. It becomes a troublesome design choice till you understand what these actually are.

Rated current is the amount of current that can go through an inductor to heat it up by a fixed temperature value from the ambient(usually 40°C). Meaning if the rated current is 3.5A, once this amount of current is passed through the inductor and once a steady state is reached, the temperature of the inductor will be 65°C if the ambient temp is considered as 25°C.The saturation current mentioned in the datasheet is the current that when passed through the inductor, it becomes “saturated” and starts losing its inductance value by 20-30% of the original value.

So when choosing an inductor you can push the current values to maximum saturation current before inductor values start to drop(Ideally you shouldn’t but you can). You can pass more current than the rated value through an inductor, provided, you can give adequate cooling for your inductor. It’s given in a datasheet so that you don’t exceed the operating temp range of the inductor. If you are using the inductor in a use case where your ambient temp is 70°C, at rated current, the inductor temp will be 110°C which might be beyond the operating temp of the inductor packaging and it will stop working altogether.

So choose your inductors carefully. It’s not always about the inductance values alone.

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