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Need help sizing a resistor to reduce voltage from 277V to 120v with a 200mw watt load at 120volts?
Wiki User
∙ 14y ago
Start with the load of 0.2 watts. The power rule says that watts equals volts times amps, or amps equals watts divided by volts, so 0.2 watts at 120 volts means 0.00167 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 0.00167 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 0.00167 amps is 94,200 ohms. The nearest standard 5% value to that is 91,000 Ohms.
Before leaving, though, check the power in the resistor. It is 157 volts times 0.00167 amps, or 0.26 watts, so you need at least a half watt resistor.
Wiki User
∙ 14y ago
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