What size resistor to drop 12vdc to 1.5vdc?

Answer 1

The voltage drop across a resistor is equal to the resistance times the current flowing through it. If you do not have a constant current load you cannot use a resistor and must use an active solution. Please read up on ohms law before continuing with your project.

Answer 2

Not possible only if you are going to have a constant current (amp), what you will need is a voltage divider, that is two resistors in serie between your positive and negative 12V+ -^^^^^-1.5-^^^^-0V but no values can be calculated before you know what your max. current is, an estimate R1=120k, R2=15k I agree with crismicro. If you know what the current draw of the load is going to be then its easy to calculate using the formula E / I = R where E= the voltage, I = current, and R= resistance. For example: lets say that you wanted to power a 1.5V light from 12V and you knew that the light required 100 milliamps (.1 Amps). We know that we want to have 1.5 volts to power the light which means that we have to drop 10.5 volts with the resistor. Using the formula gives us 10.5 V / .1 A = 105 Ohms

Answer 3

Start with the load of 2 watts. The power rule says that watts equals volts times amps, or amps equals watts divided by volts, so 2 watts at 120 volts means 0.0167 amps.

Now, look at Kirchoff's current law, which implies that the current in a series circuit is the same at all points. This means that the current in the resistor is also 0.0167 amps.

Now, look at Kirchoff's voltage law, which implies that the sum of the voltage drops in a series circuit adds up to the voltage source. This means that the voltage across the resistor is 157 volts.

Last, look at Ohm's law, which says that resistance is voltage divided by current. 157 volts divided by 0.0167 amps is 9,420 ohms. The nearest standard 5% value to that is 9,100 Ohms.

Before leaving, though, check the power in the resistor. It is 157 volts times 0.0167 amps, or 2.6 watts, so you need at least a five watt resistor.

Answer 4

The resistance in ohms is equal to the voltage drop (9) divided by the current in amps. If the current is constant, that works, but if the current is variable a resistor cannot be used.