I have an old piece of hifi equipment that has some indicator lamps on the LCD. The circuit that drives them is 28V AC. I have two 14V 115mA bulbs in series connected to this circuit.

Is there any way I can reduce the current per bulb from 115mA to ~70mA to try and reduce the brightness?

The obvious answer is to buy different bulbs, but that's fairly expensive since they're a niche old incandescent lamp from the late 80s. I've got plenty of resistors knocking around so if there's a quick fix I can do that would be preferable.

  • 2
    \$\begingroup\$ Putting a resistor in series should work then. Make sure it's rated for the power. \$\endgroup\$
    – Hearth
    Jun 12 '21 at 18:08
  • 1
    \$\begingroup\$ If you've got the equipment open already, you might think about changing the incandescent lamps to LEDs (perhaps 2 in anti-parallel for each bulb as it's AC) with a suitable resistor. They should have a longer life than incandescents, and they'll give off less of the heat that ages other components near them. \$\endgroup\$ Jun 12 '21 at 18:28
  • \$\begingroup\$ 14V bulbs from '80s are probably just some type of 12V automotive lamps. These are still mass-produced and sold at mass-product prices. Most of them also have 24/28V varietiy as well. White LEDs come with a blue or pinkish hue that most hi-fi people consider unpleasant. If you go for LEDs, you may like yellow ones better. \$\endgroup\$
    – fraxinus
    Jun 12 '21 at 19:15
  • \$\begingroup\$ Do you have the flexibility to put 3 bulbs in series instead? \$\endgroup\$
    – marcelm
    Jun 12 '21 at 20:30
  • \$\begingroup\$ I considered LEDs, but due to the way everything is wired I really don't know the best way to go about doing it on an AC circuit. I've also got little to no knowledge with regards to the necessary resistors for those, or anything. Colour isn't an issue, I know you can get good enough warm white ones now and it's for an LCD so it's tinted anyway. The bulbs are T1.25 grain of rice style bulbs, they're available but you can never buy less than 10 at a time so you're looking at £10-15 delivered from any source. \$\endgroup\$
    – Jademalo
    Jun 12 '21 at 23:53

14 V at 115 mA gives a resistance of 120 Ω. Adding a 120 Ω resistor in series would be a good first attempt.

The power of your bulbs is 14 × 0.115 = 1.6 W and will be less than that with the series resistor but design for 1.6 W and the resistors will run cooler. If you have 1/2 W resistors then use three 39 Ω in series or three 330 Ω in parallel.

  • \$\begingroup\$ Would the total draw from the transformer with 2 bulbs and a 120ohm resistor be higher or lower than it is currently? I'm slightly anxious to not overload the circuit. Thank you for the suggestion! \$\endgroup\$
    – Jademalo
    Jun 12 '21 at 23:55
  • \$\begingroup\$ Ignore me, I'm dumb. I keep thinking if I add an extra load then it would draw more current, but lower resistance is a higher load. Doh. Why exactly would 3 39ohm resistors in series work to not overload the 1/2w power issue if there's 1.6w running through all three of them? \$\endgroup\$
    – Jademalo
    Jun 13 '21 at 0:32
  • \$\begingroup\$ With the extra 120 ohms in series the voltage across the resistors will be about 28/3 = 9 V or so and power dissipated will be less than 1.6 W. I have to be a bit vague here as the bulbs will be running cool and the filaments' resistance will decrease. Try it out and feel the temperature by touch. You won't damage anything by adding series resistance. \$\endgroup\$
    – Transistor
    Jun 13 '21 at 7:06
  • \$\begingroup\$ Alright, I'll give it a shot then, thanks. Thanks for the clarification and detail! I've realised I was getting power disappated confused with power going through, thinking it was like a piece of wire not being able to handle a current load. \$\endgroup\$
    – Jademalo
    Jun 14 '21 at 2:30
  • \$\begingroup\$ Good. I forgot to point out that at reduced voltage the bulbs should last forever. \$\endgroup\$
    – Transistor
    Jun 14 '21 at 6:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.