I’m new to electronics and am trying to ground myself in basic concepts. On my test circuit board I have a 5.08V power supply, a 200 ohm resistor and a 1.5V incandescent lamp in series. The voltage drop over the resistor measures 5.05V and the voltage drop over the lamp measures .02V. The current measures .02A. Why doesn’t the lamp have more voltage available to it? Thanks!

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    \$\begingroup\$ The answer is your lamp has way less than 200R resistance, but the math still does not add up. \$\endgroup\$ – Trevor_G Mar 15 '18 at 15:26
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    \$\begingroup\$ how are you measuring the voltage drop across the resistor? .... what device are you using? ... are you sure that the resistor is 200 ohms? \$\endgroup\$ – jsotola Mar 15 '18 at 15:28
  • \$\begingroup\$ I'm using a multimeter to measure the voltage drops and amps. \$\endgroup\$ – Annette Gates Mar 16 '18 at 19:36

The cold resistance of your lamp is only around 1 Ohm. As such most of the voltage is being dropped over the resistor.


simulate this circuit – Schematic created using CircuitLab

If you want to drive the lamp from 5v you need to measure the hot current through the lamp at 1.5V so you can calculate the hot resistance. Knowing that you can select the right size of resistor. Be aware it will be small and will need to be high wattage.

  • \$\begingroup\$ Thank you. That makes perfect sense, and the schematic really helps. Could you tell me how I know what wattage I need for resistors in different circuits? \$\endgroup\$ – Annette Gates Mar 16 '18 at 19:34
  • \$\begingroup\$ @AnnetteGates Usw watts = current * volts. \$\endgroup\$ – Trevor_G Mar 16 '18 at 20:39

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