# GSM Dialer/Gate Opener Current Consumption Question

Hi there,
How many milliamps does it need from the alarm board to run?

(no name supplied)

GSM Dialer Current Graph

As can be seen from the graph the current consumption drops with increasing supply voltage.  The is due to the use of an efficient Switch Mode Power Supply (SMPS) buck regulator on board which converts a high voltage at low current to the low voltage and high current required by the GSM transmitter chipset.  The GSM system requires up to 2 Amps at 3.6v.  However, due to the SMPS at 28v the current requirements during transmission are as low as 60mA average.  This can also be explained due to the 12.5% duty cycle used during GSM TDMA transmissions.  GSM transmits only in short powerful bursts with a 1/8th duty cycle.

The system’s electrical energy requirement every second (by definition this is know as Power) is the same whether the PCB is supplied by 12v, 18v or 28v.  Power consumption can be calculated for DC loads using the following formula:

Power  = Voltage x Current  normally written  P = VI

The inverse current plot is expected when we re-arrange the formula as follows:

I = P/V

As the power requirement, P, is constant no matter what voltage we supply we see as the voltage, V, is increased I, the current, must be reduced.

GSM Dialer Power Consumption

The above graph showing power consumption at various voltages shows the most efficient supply voltage is approximately 12v particularly when idling.

Please note these figures were obtained on the 1800MHz band (EE network) with a low RSSI of 10.  They were obtained during the higher power ringing phase and would drop during talk-time.  Transmission at 900MHz (O2 network) may give higher current consumption due to the higher power output of the transmitter but may also be lower if the signal strength is superior.
Here is the measured current consumption at various DC voltages:

28V
20mA idle
60mA during call (peak 70mA with relay on)
18v
26mA idle
95mA call (115ma Relay)
14v
30mA Idle
110mA (145mA Relay)
12v
34mA
127mA (160mA Relay)

Regards,

Michael

The GSM Alarm Dialer and SMS sender is available from the Advent Controls website using the following link: