This means that the initial current will be 13 times higher when we turn on the switch. In our example the current would start around 3.3 A (240 V / 73 Ω), to stabilize at 255 mA a short time later. One thing to consider with light bulbs is that there is a large peak of current to take into consideration when we turn them on. If the COLD temperature was, let say 20 ˚C, the filament temperature is 2723 ˚C… Now wonder they get so hot! ΔR is the difference of resistance: 961 – 73 = 888 Ω The relation is: ΔR = R 0 x α x ΔT Then: ΔT = ΔR / (R 0 x α) Α = 0.0045 = Tungsten temperature coefficient Here is a Wien bridge oscillator using a light bulb as gain regulator.īack to our 60 W bulb, did you ever wonder what the temperature of the filament is? Assuming that it is made of Tungsten, which has a temperature coefficient of:
![disk led light blub disk led light blub](https://www.asseenontvwebstore.com/v/vspfiles/assets/images/disk%20lights.jpg)
There are other methods available, including a feedback using a FET transistor, but many designers argue that the bulb produces a much better result with less distortion… Small light bulbs are also used as an efficient gain regulator in Wien bridge oscillators. It limits the current in case of a short circuit and gives us an immediate visual feedback. The light bulb is a great PTC (Positive Temperature Coefficient) resistor and this is why it is so efficient in limiting the current when we first test a power supply after repair (the light bulb trick). Then R = V/I = 245 / 0.255 = 961 Ω which is about 13 times the cold resistance! We could calculate it using the 60 W and 240 V as printed on the bulb but I prefer to measure the current and voltage when the lamp is powered then calculate the resistance: When the lamp is hot, its resistance is much higher. Indeed 73 Ω is the cold resistance, at ambient temperature. This was a great time to introduce the concept of COLD resistance vs. Some students would measure again, then recalculate and get the same result, why? Interestingly enough, very few would come up with an explanation… Then I asked them if we could, based on this measurement, re-calculate the power (just for verification!) knowing that this lamp is rated 240 V. No problem, they grabbed their multi meter and, in no time, here was the result: around 73 Ω (rounded for easy calculation). When I was teaching basic Electricity/Electronics I used to give my students a 60 W light bulb and asked them to measure the resistance. Light bulbs also can be fun and educative. The “light bulb trick” is one of Jestine Yong’s many great advices given to electronic repairers! Fair enough, we must support the environmental movement.īut hey… we all use the famous “light bulb trick” when we are testing a power supply that we just repaired. Their crime: they are not energy efficient and there are now much better light sources available. Light bulbs are banned from many countries around the world, in which they cannot be legally sold anymore.
![disk led light blub disk led light blub](https://runspree.com/wp-content/uploads/2020/05/led-solar-powered-disc-lights-1.jpg)
So they deserve a little bit of attention before they become things of the past. How come, an article about something as trivial as a light bulb? Well, I like light bulbs! They were part of my childhood and they have been serving us nicely for a very long time.