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I have an application were I do not have the room for a dimmer switch but need to reduce the lumens of a 120v 25w incandescent bulb just a bit. There used to be a product called the Bulb Saver Button which was placed in the light bulb socket to reduce power consumption and increase the life of the bulb. It also reduced the brightness of the bulb. This would work for me but this product is no longer available. I'm wondering if I simply put, in line, a Current Limiter Power Thermistor, would it do the trick? I found on ebay this 10D-15 Inrush Current Limiter Power Thermistors 10ohms 5 AMP. Would this work like the Bulb Saver Buttons?

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You could use a common diode in series with the bulb, which will reduce its brightness significantly. The advantage of a diode is that the amount of power wasted in the diode is much less than if you used a resistor to do the same thing.

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No, this Thermistor would change almost nothing.

For your bulb when directly on the mains, the current is: 25 W / 120 V = 0.21 A And that means the bulb will have a resistance of 120 V / 0.21 A = 580 ohms.

Adding a 10 Ohms thermistor to that is insignificant as you'd get 590 ohms. Then the thermistor would heat up and get an even lower resistance. So in total, nothing significant changes. That thermistor is used in power supplies which need to charge large capacitors at startup. It will do nothing for your lightbulb.

Better ideas:

  • use a bulb with a lower power.
  • use two bulbs in series

A bit more "nasty" but it works: use a diode in series with the bulb. Since mains is AC, the diode will block one direction of the current. Your bulb will be somewhat flickery but also significantly less bright. Use a 1N4007 diode, these are common and cheap.

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  • \$\begingroup\$ I wonder what a ""Bulb Saver Button" uses \$\endgroup\$ – Rob Niosi Mar 20 at 18:20
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If you do use the thermistor it might dim a bit, but you will waste the power as heat and end up with a very hot thermistor. The thermistor only acts to limit inrush current (which does save the life of incandescent) and is not intended for use as a dimmer. You'd be better off using a series resistor.

A better way would be to use a solid state dimmer switch which varies the power by use timing, these are available everywhere. Or get a 20W or 15W bulb.

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  • \$\begingroup\$ Thanks for all this great information. I will try a diode. I wonder what a ""Bulb Saver Button" uses. \$\endgroup\$ – Rob Niosi Mar 20 at 18:19

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