Electronic – A simple voltage divider

There are two situations where a voltage divider can provide a relatively constant voltage to a load. First, if the load draws much less current than the current that flows through the voltage divider when no load is connected. Second, if the current drawn by the load is constant at the desired voltage. Motors and relays usually draw relatively large currents and so they don't fit in the first category. The current drawn by a motor can vary greatly depending on its load torque so a motor doesn't fit well in the second category either. The current drawn by a relay is also variable as it is switched on and off. Therefore, using a voltage divider is usually not a good choice for things like motors and relays.

The important question, which you haven't told us, how much difference there is between the available voltage and the desired load voltage. If the available voltage is much higher than the specified load voltage there is the possibility that the full supply voltage will be supplied for a short time and destroy your relay or motor. (For an inductive load the initial current when you switch it on must be zero, so the full supply voltage could appear on the load until current starts flowing.) In this situation you can't use PWM either, because the PWM pulses apply the full voltage.

If the difference between your supply voltage and the voltage needed for the motor or relay is small then you might get away with adding a series resistance but it's not a very good design. The best thing to do is provide a regulated voltage supply at the appropriate voltage for the load. Your choice of voltage regulator will depend on current and voltage requirements for the loads.

This one can be solved by inspection. Notice that the 5V source is in series with the 12V source but oriented in the opposite direction. So, the voltage across both resistors is just 7V. You could use the voltage divider equation again to find the voltage from A to B, but since the resistors have the same value the voltage across each of them must also be the same. So the voltage across A and B is 3.5V, with A being the more positive point. Now the voltage between C and A is just the sum of the voltages from C to B and then from B to A.