The main circuitry on the GSM Gate Opener Board requires a voltage of about 4.1 to 4.2 volts. Obviously for a low power product such as this the most convenient source of power will be the gate control board power supply which can vary between manufacturers from 12VDC to 24VDC. So we have on the PCB an efficient DC to DC converter system. This takes voltage from up to 24V and regulates it down to 4.1v with 80% efficiency.
The switch mode regulator is an efficient way of reducing the voltage in the circuit. It does this by transferring energy stored in a coil of wire in to a capacitor wasting very little energy in the process. The alternative to this type of regulator is a linear regulator which simply dissipates excess energy as heat. At 24V this kind of regulator would be less than 16% efficient which is why we designed in a switched mode regulator. Not only does it save energy it also keep the board running considerable cooler than a linear regulated system – increasing reliability.
The system is based around the Diodes Inc. AP5004. This device operates at over 300kHz. The higher the operating frequency the smaller the coil of wire ‘L1’ can be. Thus we chose this high frequency chip to keep the size of the board as small as possible. As always with engineered systems there is a compromise to be made and in this case we needed to ensure the power supply can supply the surge current of 2A seen during transmission burst. This limited how small we could design the coil.
Although the board requires up to 2A during transmission the average current in to the board is always less then 500mA. This is due partly to the fact the voltage from the gate supply is reduced to the GSM module – as the voltage is reduced in a switched mode power supply the current available is increase. The main reason however is that GSM systems only transmit in small bursts of about 0.6ms (0.0006 seconds) every 4.6ms (0.0046s). This means that although the system requires 2 amps during transmission it is only transmitting for a maximum of 12.5% of the time.
You may be familiar with the sound lower frequency GSM phones make when placed by a speaker – this is the sound of the burst coming through the speakers every 4.6ms!
The +VE input to the board features a reverse polarity protection diode so the board cannot be damaged by incorrectly connecting the power. The board is not suitable for 24VAC supplies and we recommend a separate power supply in this case. We will shortly be introducing a AC-DC rectifier board for AC gate opener control boards.