Copy and past into a blank spreadsheet cell A1. It should populated the rest of the cells automatically. Input your transformer voltage in cell B2. Input voltage and current ratings for the bulb you are using in cells B3 and B4. Input voltage you wi***o operate lamp at in cell B5. Copy formulas from D6 through D13 into cells B6 through B13 and D15 through D16 into B15 through B16. (Get rid of the quote mark to change it from text to formula.)The correct value for a ballast resistor is calculated in cells B12 and B13. Enter the closest common value available in cell B14.
It is not necessary to use the exact value calculated in cell B12. In the example, the closest value found at Radio Shack is 150 ohms. In the case of wattage rating, more is better. Use a half-watt resistor to dissipate 0.1 watts. It will run cooler than a quarter watt resistor.
COMMA DELINEATED SPREADSHEET:
What Ballast Resistor for Lamp?,VALUES,UNITS,FORMULA
Voltage transformer (Vt),18.0,Volts,Given
Voltage lamp (Vl),18.0,Volts,Given
Current lamp (Il),0.0260,Amps,Given
Voltage desired (Vd),14.0,Volts,Given
Resistance lamp (Rl=Vl/Il), 692,Ohms,=B3/B4
Ratio (Ratio=Vd/Vl),0.778,,=B5/B3
Resistance cold (Rc=Rl/10),69.2,Ohms,=B6/10
Resistance desired [Rd=Ratio(Rl-Rc)+Rc],554,Ohms,=B7*(B6-B8)-B8
Current desired (Id=Vd/Rd),0.0253,Amps,=B5/B9
Voltage ballast [Vb=(Vt-Vd)],4.00,Volts,=B2-B5
Resistance ballast (Rb=Vb/Id),158,Ohms,=B11/B10
Watts ballast (Wb=VbId),0.101,Watts,=B11*B10
R = 100 120 150 180 220 270 330 390,150,Ohms,Closest common value
Voltage Lamp [VL=RdVt/(R+Rd)],14.2,Volts,B9*B2/(B14+B9)
deviation from Target dT=100(VL-Vd)/Vd,1
Wayne
