|
Model Lights |
R. W. Stuart A Christmas string of lights is manufactured: 50 lamps in series to be plugged into a 115 vac house socket. Each lamp has a published voltage rating of 2.5 volts. At 115 vac each bulb is dropping 2.3 vac and at 120 vac each bulb is dropping 2.4 vac -slightly under the published rating of the bulbs. The cold resistance of a 50 lamp string is meaningless because the resistance will increase drastically as the filaments come up to heat. We can settle out at 2.35 vdc per lamp for our purpose. Vdc and vac are the same in resistive circuits. A standard receiver Nicad pack is nominally 4.8 vdc with a 400 mahr capacity which should be dedicated to control. Use your flight battery only for RC control and fly a second 4.8 volt battery for the lamps-- to avoid the chance of a lamp failure killing your control system. Put a switch or something in the circuit so the lamps may be turned on or off. Two lamps in series (connected so that the same current flows through each) have a combined voltage drop of 4.7 vdc (2.35 volts x 2) which pretty well matches the nominal 4.8 vdc from the extra 4.8 volt battery. Two lamps in series may then be connected to a 4.8 battery so you can evaluate the amount of light produced--- should be about right. If you want more light use two lamps at each station, Twenty lamps then become ten sets of two series lamps, each of the sets connected to the battery directly (parallel). Small wire (18 gauge) or so may be used to each lamp set of two lamps. Buss wire should be at least 18 gauge. Each set of two lamps draws 125 ma and all ten sets would draw 1250 ma. The 400 mahr rcvr type battery is practically useless under this loading and might hold up for 15 minutes or so. If you decide to use 20 lamps, 4.8 volt Nicads are available at reasonable cost up to 1300 mahr rating which would run the lamps for 50 minutes or so, but would weigh more than a 400 mahr battery. Series sets may be added or deleted as you see fit, but each added set requires more battery or shorter running time. Other lamps may be used, but a new relationship of voltage, resistance, layout, and expected results should be done as above. I have no way of evaluating the vibration resistance of any given brand of lamp, but with the series two and parallel sets probably only one set would go out at one time. The option of using the sockets is up to you. + Calculations: 50 series bulbs at 115 volts, 115 volts/50 = 2.3 volts per bulb; 120 volts/50 = 2.4 volts per bulb; and 125 volts/ 50 volts = 2.5 volts per bulb (published rate. We can settle out at 2.35 volts per bulb. Two bulbs in series have a circuit voltage drop of 4.7 volts-- 2.35 volts x 2= 4.7 volts. Each two bulbs in series becomes a set and each set is connected in parallel with the battery. 20 bulbs is 10 sets. If two sets ( 2 bulbs per set ) are connected as shown in the drawing, with a 0,1 ohm precision resistor in series ( not shown ) stuck in where the switch is shown, gives us a means to measure the circuit current. When running the drop across the 0.1 ohm resistor is 25 mv. E=IR and I= E/R or I= 0.025 v/ 0.100 ohms= 0.250 amp (250 ma ). Each set then requires 125 ma and this current flows through both of the series bulbs, so each bulb drops 2.35 volts at 125 ma. Using the above we can calculate the hot or operating resistance of each bulb: E=IR and R=E/R=2.35/0.125=/2.35=18.8 ohms. The cold measured resistance of the same lamps was 9.1 ohms, 19.6 ohms, 10.8 ohms, and 6.1 ohms-- Each set draws 125 mahr when on and each added set draws another 125 mahr. |
Return to Stuart
Return to Home Page