Hi everyone,
First up, I’m a complete newbie with electronics. I’ve had a 2-day class through the Army and done some reading, but have no practical experience. That said, I could use some help with my first project. Please forgive (and correct!) me if the diagram is done wrong… though I have had to color code it for my own sanity when soldering the wires and it is missing resistors per my question below.
I’m a UH-60 pilot and I’ve come across a demilitarized caution/advisory panel from the helicopter. I would like to display the lights associated with four scenarios. The lamps have a published spec of 28 volts DC and 40 mA nominal. I found that I can only run two in series before I get noticeable dimming, so I’ve wired the parallel branches to have a maximum of two lights each. I briefly tried a 12V power supply to see if I could tone it down a bit, but I could only get one in series to be bright, and with the number of lamps to light I didn’t want to draw that much current through 21 parallel circuit branches under the worst case scenario.
My question comes in on what resistors to use in the circuit and where to place them. I know by Ohm’s law, 28 V / .040 A = 700 Ω on each branch. Each light has a resistance of 50 Ω as measured on my multimeter, so that’s a 600 or 650 Ω resistor on each branch. Or, since there’s a rotary switch, would it be better to just do a single resistor immediately after the switch on each main branch, applicable to their maximum currents? For example, my “red” branch at 120 mA would need 233 Ω resistance. What is the best place to put them? Would that even work right?
Secondly, what kind of resistors would I need- specifically, wattage. I’ve seen .25 to 1 –watt resistors. With the formula V*V / R, I keep seeing that I need over 1 watt. On the above example of the “red” branch, 28V * 28V / 233Ω = 3.36 watts. Is this the correct method for determining my resistor requirement? If not, what type of calculation should I be making so I don’t set anything on fire?
I appreciate any help!