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The light bulb

By on July 30, 2014
series light bulb trick










light bulb

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. And… they are going to disappear soon. So they deserve a little bit of attention before they become things of the past.

Light bulbs are banned from many countries around the world, in which they cannot be legally sold anymore. Their crime: they are not energy efficient and there are now much better light sources available. Fair enough, we must support the environmental movement.

But hey… we all use the famous “light bulb trick” when we are testing a power supply that we just repaired. The “light bulb trick” is one of Jestine Yong’s many great advices given to electronic repairers!

Light bulbs also can be fun and educative. 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. No problem, they grabbed their multi meter and, in no time, here was the result: around 73 (rounded for easy calculation).

light bulb trick

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, just apply the Ohms law:

P = V2/R = (240 x 240)/73 = 789 W   ?????

Wow, what is wrong?

Some students would measure again, then recalculate and get the same result, why? Interestingly enough, very few would come up with an explanation…

This was a great time to introduce the concept of COLD resistance vs. HOT resistance. Indeed 73 Ω is the cold resistance, at ambient temperature. When the lamp is hot, its resistance is much higher. Can we find out how much higher?

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:

light bulb tricks

I = 255 mA

V = 245 V

Then R = V/I = 245 / 0.255 = 961 Ω which is about 13 times the cold resistance!

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). It limits the current in case of a short circuit and gives us an immediate visual feedback.

Small light bulbs are also used as an efficient gain regulator in Wien bridge oscillators. 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…

Here is a Wien bridge oscillator using a light bulb as gain regulator.


Back 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:

α = 0.0045 = Tungsten temperature coefficient

The relation is: ΔR = R0 x α x ΔT   Then: ΔT = ΔR / (R0 x α)

ΔR is the difference of resistance: 961 – 73 = 888 Ω

R0 is the coldresistance: 73 Ω

So the calculation is very simple:

ΔT = 888 / (73 x 0.0045) = 2703 ˚C

If the COLD temperature was, let say 20 ˚C, the filament temperature is 2723 ˚C… Now wonder they get so hot! 

Inrush current

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. In our example the current would start around 3.3 A (240 V / 73 Ω), to stabilize at 255 mA a short time later. This means that the initial current will be 13 times higher when we turn on the switch. This is why light bulbs often fail when we switch on the light (they go bang in such a nice way!)…

This inrush current can be a serious nuisance. I remember a project, a very long time ago, when the LEDs were not bright enough. I had to design an alarm panel on which 100 small lamps would be blinking in case of a serious alarm situation. We could not find a standard power supply that would stand the inrush current. We had to preheat the lamps with a small current (just below the glowing point), so the current peak would not be so high and the lamps would last longer. Well… story of the past with solutions of the past! Today’s LEDs are so good.

How long does this current peak last? Some literature states that the current will reach its nominal value after 0.1 s or less. This is something we can demonstrate easily. For this we will use a small 12 V lamp, a DC power supply, one resistor and an oscilloscope. Do not try this with a 240V or 120 V or any AC mains voltage lamp! You might damage your oscilloscope or/and get an electric shock. Furthermore, AC is not suitable for this demonstration…

Our small lamp has the following characteristics:

Cold resistance (measured): 1.8 Ω (Inrush current would be 12 V / 1.8 Ω = 6.66 A)

Hot current (measured): 0.56 A

Hot resistance: 12 V/0.56 A = 21.42 Ω.  (12 x the cold resistance)

To measure the current with an oscilloscope we insert a 1 Ω resistor with the lamp and measure the voltage drop across the resistor, so 1 V = 1 A. For sure the 1 Ω resistance will interfere with the measurement but this is just for demonstration and we are mainly interested in the timing.

series light bulb trickseries light bulb tricks

Bingo, we can see a nice peak of current. Its amplitude is limited by the 1 Ω resistor and by the power supply’s internal resistance, hence lower than calculated. However we can see that the current reaches its nominal value after 75 ms. Less than 0.1 s, this is what we wanted to find out.

Sure enough, the light bulb is a non-linear device. It is a device that does not obey the Ohms law to the letter. Would this be another reason why it is banned? Just kidding…

series light bulbtricks

We, electronic repairers and electronic enthusiasts, will keep a small stock of light bulbs in our workshop. Not for lighting purposes, but for the useful things we still can do with them.

We will miss the light bulb!





This article was contributed by Gerald Musy from Penang, Malaysia.


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You may also read his previous article below:



  1. Neil McCourt

    July 30, 2014 at 5:38 pm

    Thanks great article

  2. Igor Levin

    July 30, 2014 at 6:59 pm

    Very useful and correct article!

  3. Humphrey Kimathi

    July 30, 2014 at 7:09 pm

    Well Put.. Gerald

  4. beh

    July 30, 2014 at 7:21 pm

    Hi Mr Musy
    thanks for article and very clear explanations.

    • Gerald

      August 1, 2014 at 11:11 am

      Thanks Beh. Always reading your articles and comments with interest as well.


  5. Henry

    July 30, 2014 at 7:23 pm

    Very good article on a humble light bulb topic. I always enjoy your various write-ups with a scientific approach to the subject. Good read, good info, please continue with more in the future. You and other contributors make this website interesting. It helps many of us to learn and expand our knowledge, or simply to maintain interest in electronics repair. I would say articles with some theory on operation of various electronic components presented in a simple and easy to understand way will be beneficial to many. Thank you.

  6. Luciano

    July 30, 2014 at 7:52 pm

    Hi Mr. Gerald Musy,
    What a wonderful and informative article! Well analysed! i want to read it again and again. I like your articles. Keep it up.

  7. Silvio Regis Silva

    July 30, 2014 at 7:58 pm

    Very good article! After the disappearance of Filament Lamps still have a halogen lamp, which can be used as Lamp in Series, but the powers are higher.

  8. Andrea Del Corso

    July 30, 2014 at 8:03 pm

    Thanks for this article!Very good !

  9. serge

    July 30, 2014 at 8:40 pm

    Good done. Useful and interesting. Theory and practice are in one bottle.

  10. carlos guilherme

    July 30, 2014 at 9:49 pm

    Great article and as you said:
    "We will miss the light bulb!"

  11. yogesh panchal

    July 30, 2014 at 11:12 pm

    Mr.Gerald Musy,

    Thanks for refreshing the things .

  12. Philip odhiambo

    July 30, 2014 at 11:58 pm

    Thanks. so wonderful. I almost argue with my fellow repairman that these light bulbs are outdated, but i now have a stand.

  13. kamal kumar

    July 31, 2014 at 12:37 am

    light bulb is good articl

  14. Andre Gopee

    July 31, 2014 at 1:08 am

    Wow! what a great article... it is truly and interesting article because it teaches you how much respect you must have for this little light bulb... Also your explanation show us how to calculate the wattage of a bulb if you do not know the watts. Very interesting and informative. Thanks.

  15. Graeme Partridge

    July 31, 2014 at 2:21 am

    Enlightening article. The humble light bulb, we tend to forget about the characteristic of this so called simple device, I used the light bulb trick back in the day not only as a replacement for the fuse for testing for shorts, but as a PSU load.

    I'am currently experimenting with solar power and invertors, so this is a good reminder of the current surge when first switched on.

    Great article, thank you.


    • Gerald

      August 1, 2014 at 6:42 am

      Hi Graeme,

      Thank you for your comment. You are right, I remember someone wanted to use an array of 100 W bulbs in parallel to simulate a 1000 W load. He had a big surprise when he switched them on... Better use power resistors.

      If I need a 1000 W load I borrow my wife's domestic iron 🙂


  16. Andrew Ramos

    July 31, 2014 at 3:12 am

    Great article! No wonder I like this repair blog a lot!

  17. Tom Dunaj

    July 31, 2014 at 3:33 am

    Thanks for the very interesting article!

  18. Merlin Marquardt

    July 31, 2014 at 4:03 am

    Very nice article on the light bulb. Still not sure though that I understand the "light bulb trick".

    • Gerald

      August 1, 2014 at 11:34 am

      Hi Merlin,
      Thanks for your comments. I have seen your query about the "light bulb trick" a couple of times. So I will try to summarize it in a few sentences:

      - The light bulb is connected in series with your load (power supply under repair). Basically replacing the fuse.

      - If your load is a short cut the whole voltage will be applied to the bulb which will light up immediately. The bulb will also limit the current to your load (shorted) to its nominal value (0.41 A if the bulb is 240 V 100 W). Avoiding the big bang!

      - If there is no short, the current will not be enough to glow the bulb. The bulb resistance will remains low, just like if your load would be connected directly to the mains.

      Hope this helps.

  19. Robert

    July 31, 2014 at 4:22 am

    Thanks Gerald for another excellent article. What they don't tell people is that the new light bulbs are much more toxic to the environment! Instead of saying goodbye to the filament bulbs, we should be protesting and raising a noise about the mercury and poisons that will be going into our environment. When one of the new bulbs break, it should be removed by a HazMat crew!

    • Gerald

      August 1, 2014 at 6:26 am

      Hi Robert,

      Thank you for your comment. You are right regarding the CFLs. Also they generated electrical interferences in my oscilloscope so I banned them from my workshop 🙂 Using LEDs instead.

      I can predict that they will be disappear as soon as the LED prices will become competitive (probably soon).


      • Robert

        August 1, 2014 at 8:27 am

        I agree.

  20. moshe

    July 31, 2014 at 4:52 am

    great article!!
    and a fascinating insight into the humble light bulb.

    any chance of posting this as a downloadable pdf as it would be a fantastic piece for young students.


  21. Imraz

    July 31, 2014 at 5:12 am

    A very nicely explained article on the outgoing light bulb. Surely I will miss the light bulb but no choice have to go with the flow of technology. Thanks for the article Mr Gerald.

  22. Paul Zafi M.

    July 31, 2014 at 6:02 am

    A fantastic material for many others like me,you are educating many technicians who barely can read and understand the practical aspect of electronics.Keep moving the new generation with the latest technology on electronic repairs.We are rely going to miss the light bulb. Thank you.

  23. Mark

    July 31, 2014 at 7:53 am

    Hey Gerald,
    Thanks for a well written article. Sometimes when trying to diagnose a fault, we forget about the rules and laws that lead us in the correct direction.
    It is always fun to have some useful facts and figures.
    Yes, I will be trying the experiment with the globe and oscilloscope.
    Now where did I put my globe?....................

    • Gerald

      August 1, 2014 at 6:33 am

      Hi Mark,
      Thank you for your comment. Be careful and use a low voltage bulb (6, 9 or 12 V) and a DC power supply or a battery so you and your oscilloscope will be safe.

      Happy experiment!


  24. J varela

    July 31, 2014 at 9:39 am

    Dear Jestine, now I'm in my country Honduras, when I knew your site I was working in Timor Leste. Even from here I remain true to your electronic articles.

  25. Joshua oloo

    July 31, 2014 at 3:10 pm

    Thanks Mr.Gerald for the Article.I've realised that light bulb trick is the way to go on matters of testing SMPS for shorts.Regards

  26. Ray Marais

    July 31, 2014 at 4:20 pm

    Good day
    How about someone designing a electronic replacement for the light bulb trick. Always good articles. Keep up the good work. How about making your articles printable as they were before.
    Have a good day

    • Robert

      August 1, 2014 at 8:31 am

      Mr. Yong does in his e-book, Repairing Switch Mode Power Supplies.

  27. nilesh tayade

    July 31, 2014 at 5:02 pm


  28. Salim Khan

    July 31, 2014 at 8:45 pm


  29. Bill

    July 31, 2014 at 10:48 pm

    Gerald, loved the article. Good tutorial on using ohm's law & power formula.
    In the second photo you show a multimeter and and a clamp-on ammeter hooked to a breakout box. Can you provided some details on that box?

    • Gerald

      August 1, 2014 at 11:08 am

      Hi Bill,

      Thank you for your comments and interest. I sent a description of the box with more pictures to Jestine. He will publish it on his blog.


  30. Gerald

    August 1, 2014 at 6:12 am

    Thank you to all of you for your kind and positive comments.


  31. shahid ahmad

    August 2, 2014 at 4:41 am

    HI sir,
    so nice of you sir the article is very good as always.continue shairing your articles sir we are learning alot from them.

  32. ahmed

    August 2, 2014 at 4:05 pm

    great article ..tank you a lot sir ..

    • ahmed

      August 2, 2014 at 4:42 pm

      please sire..i want from you more explain about use lamp in protect FET transistor and what the characteristics lamp can i use ??

  33. Taofiki ishola Akinleye

    August 2, 2014 at 7:01 pm

    Thanks very much.the article is well understood and very intresting.keep it up.

  34. Merlin Marquardt

    August 2, 2014 at 9:07 pm


    Thanks for your explanation of the "light bulb trick". I think I understand how it works now. Thanks again.


  35. Rachid

    August 3, 2014 at 6:46 pm

    A nice document. Only I still don t get the picture how this can be usefull to test repaired deviced.

  36. Capt36

    August 3, 2014 at 7:20 pm

    What a nice refresher course, in a well-written article!

    Keep up the great work!

  37. Milos

    August 4, 2014 at 3:52 am

    Great article.
    Well done.

  38. Keith

    August 4, 2014 at 4:32 pm

    Great article - so easy to understand - thank you 🙂

  39. Alain

    August 4, 2014 at 11:27 pm

    Very informative, there is so much information I can apply to my repairs in vacuum tube audio (that share in their use of tungsten )

    One interesting thing about the new energy saving bulbs is that when used in my inspection lamp anywhere near my occiloscope the display goes crazy as these bulbs radiate a high frequency field

    Changed light bulbs to the one in your article...problem solved.

  40. t-ramadoss

    August 5, 2014 at 2:49 am

    reading this article-i remembers my life in government technical institute-thank you-

  41. Humberto

    August 6, 2014 at 6:30 am

    Hi Sir. good article. By the way, the light bulb method is called ¨Test Lamp¨in my country, Cuba, and it´s used principally by electricians, not so much by electronics, now I´m trying to get one, but it´s almost imposible. Anyway, good explanations given for you, have a good day.

  42. Nimal

    August 20, 2014 at 4:05 pm

    Very informative article. Thank you for updating our knowledge on the simple light bulb.

  43. Antonio Marques

    August 23, 2014 at 8:34 pm

    Thank you very much. Greetings from Portugal in Europe.

  44. Raymundo Saura

    August 27, 2014 at 4:01 pm

    Thanks great article for me

  45. Gerald Millward

    January 20, 2016 at 8:17 am


    Thanks for a most entertaining (and informative) article.


  46. José Luis

    October 16, 2016 at 4:41 am

    Great article. I finally know why I never came out calculations bulbs. Nothing like hot or cold.


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