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Soft Start for Flashlights Schematic

Soft Start for Flashlights

Application:

A project with only 2 parts, but is great for addressing an everyday situation that is irritating at best and dangerous at worst. This circuit protects the bulb in flashlights (FL) from high switch-on current to make the bulb last longer.

Description:

For a standard incandescent flashlight (FL), this is a easy little modification make your FL bulbs last longer. High powered FLs typically run their bulbs hot to get a brighter light from them. They also have a much lower on-resistance when cold, so that when you turn them on, the bulb passes a much higher current than it was designed for. This is why the most common time for a bulb failure is when turning it on.

The transistor and resistor limit the current while turning on the circuit and protect the bulb from an initial high current turn on. A simple resistor in series with the bulb might be a tempting option, but there are a couple problems with that approach. Just adding a resistor would reduce the voltage available to the bulb, and aid longevity, but that would reduce the brightness. The resistor would also be wasting energy getting hot instead of using that energy for light. This solution is better in that it limits current at startup and wastes very little energy when in use and when off.

In this application, it might be easier to insert the batteries in the FL“backwards” so the circuit connections and parts have the best fit in the body of the FL. FL design was stagnant for decades, but now there are many new technologies available, and in some cases, it can even be easy to bring some of them to an older one you already have. In addition to this circuit, you could also take advantage of newer LED and battery technology to really increase the brightness, “on” time, and lamp life of your old FL.

Schematic of flashlight soft starter circuit

Soft Start for Flashlights Schematic

Parts List

RefDes Name Value Part Number Digi-Key Part Number Description
R1 RESISTOR 47M HMC1206JT47M0 HMC1206JT47M0CT-ND RES 47M OHM 1/4W 5% 1206
S1 SPST   SUPPLIED N/A OPEN
LS1 LAMP   SUPPLIED N/A INCANDESCENT
Q1 N-CH   BUZ11_NR4941 BUZ11_NR4941-ND MOSFET N-CH 50V 30A TO-220AB

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