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I am designing a circuit for SIM900 and PIC MCU. I am facing few problem on power supply for them. I will be giving 12v to the circuit. Now PIC works on 5volts and SIM900 works on 3.2v - 4.8v. At first I thought of using two different voltage regulators. But Now i am using 7805 to convert 12v to 5v and then using a voltage divider to convert 5v to 4v. Below is the schematic:

enter image description here

Here input voltage is 12v and using 7805 to convert it to 5v which will be given to PIC MCU. This 5v is also used in voltage divider circuit to convert it to 4v for SIM900. I have used R1=100 and R2=400 for voltage divider circuit. Is this circuit safe to use. Does voltage divider circuits are good to use. Please help. Thanks.!

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Don't use voltage divider unless in these situations:

  • high impedance load

  • inaccurate voltage

  • almost fixed current consumption circuits

If you have a digital circuit with different current consumption it means you have a variable load then your output voltage will vary in a wide range. This will affect performance of your circuit.

For better efficiency I suggest use a two stage regulator, first stage is a switching regulator with efficiency up to 85%. This stage will deliver you a 5 volt regulated voltage. In second stage use a linear regulator to produce 3.3 voltage. In this scenario you will have a good efficiency behind a stable circuit for your digital circuits.

Always remember : power supply is the most important thing in a circuit for working stable and desirable.

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No.
If you connect something at your voltage divider that have input resistance less than infinite (ideally) you load the divider and change the voltage ratio.

Here you can see recommended circuit to power the Sim900.

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A voltage divider is not generally used for this purpose. The reason for this is the output voltage of a voltage divider is calculated by the resistors used. The load (in this case your SIM900) will present a resistance/impedance in parallel with R2, hence changing the output voltage.

An example of where you would use a voltage divider is when the load is high impedance/resistance, and therefore does not affect the output voltage.

An alternative for your application is using something like the LM317, which is an adjustable voltage regulator. Just use the calculation in the datasheet to achieve 4V output based on your 5V input voltage.

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  • \$\begingroup\$ A SIM900 can drain up to 2 A of current during transmit burst while an LM317 can only support max 1.5 A. \$\endgroup\$ – Bence Kaulics Jul 6 '16 at 10:19
  • \$\begingroup\$ Good spot thanks Bence - a regulator with a higher power must be used over the 317 to accommodate max power consumption of the SIM900. \$\endgroup\$ – Nick B Jul 6 '16 at 11:43
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Using a voltage divider to provide a power voltage is a bad idea.

The first thing to do is to step back and look at the overall problem. Many PICs can run from somewhere in the 3.2 to 4.8 volt range. Use one of them instead of some relic that requires 5 V. Now you can use a single buck converter that makes 4 V or so from the 12 V input.

If you really really needed separate supplies, then you have to start with the current requirements of the two supplies. If one of them is much higher than the other, then consider a buck switcher to make the high current supply and a linear regulator to make the low current supply.

If supplies need to source similar current, then use a buck switcher to make the 5 V supply, and then linearly regulate that down to make the 4 V or 3.3 V supply.

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