MK484 Power Supply Design

The MK484 is a three-pin integrated circuit (IC) in a TO92 package, which requires a minimum of 1.1 V for stable operation. It has a typical rating of 1.4 V and an absolute maximum rating of 1.8 V. Therefore, when powering the radio using a battery you need to ensure that when the battery is new, the voltage is not higher than 1.8 V, and when the battery depletes, the voltage does not fall below 1.1 V. Ideally a zener diode could be used to provide voltage regulation, however, it adds to the cost of the build. I decided to design this by finding a suitable value for resistor Rp, which controls the voltage level to the IC.

With Rp at 15 kΩ, together with a new PP3 (9 V) battery, the voltage at pin 3 of the IC is at 0.9 V. This is when the IC produces the most powerful audio signal, which is almost at the threshold of breaking up. As the battery depletes, the voltage at pin 3 will also drop, and the IC works fine even when it drops as low as 0.6 V.

The interesting thing about this IC is that if you hook up your voltmeter to measure the voltage at pin 3, you will find that it changes as you tune through the band. With the volume completely down, and a new battery, I measured 1.05 V at one extreme end of the variable capacitor. As you turn the variable capacitor, the voltage gradually drops, and at the other extreme end, it is 0.8 V.

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