I have a 28V 8A power supply driving 3 2A stepper motors. I'd like to add a 12V .1A fan and a 12V.130A solenoid to this.
I found I can use a 7.5K and a 10K resistor to form a voltage divider to turn the 28V to the 12V I need, but I'm unclear how to size the resistors. Do I use I = 28 / 17500 = 1.6mA so W = 45 mW thus 1/4 watt resistors will be fine, or do I use I = .230A so W = 6.5?
You cannot use a voltage divider circuit to feed power to a device. They are only suitable for providing signal levels.
You can however use a simple series resistor to drop the voltage based upon the current drawn but it's a very poor system of reducing voltage since any excess current during start-up of the device will drop the voltage below the desired operating voltage.
You'd be far better using a voltage regulator, preferably a switched one to regulate the 28volts down to 12. A single regulator will be able to handle the total requirement of 0.23 amps. However you need to exercise care with the solenoid since it will produce a sizeable back emf kick when it is de-energised. A reverse biased diode across its coil will suffice.
However, in answer to your original question, here are the resistor requirements.
For the fan, which requires 12 volts at 0.1 amp you need to "lose" 16 volts at 0.1 amps so you need a 160ohm resistor. Wattage requirement is 1.6 watts.
For the solenoid, you need a 123ohm resistor and wattage is 2 watts.
For the fan, which requires 12 volts at 0.1 amp you need to "lose" 16 volts at 0.1 amps so you need a 160ohm resistor. Wattage requirement is 1.6 watts.
When using a potential divider the rule of thumb is to have ten times the current flowing through the resistors as you want through the load. However for powering something this means you have to waste a large amount of current, so the technique is useless as jackrae said.
Thank you both for the info. I'll look into voltage regulator.