# What exactly the difference between DC and AC light bulb

Discussion in 'General Electronics Chat' started by dante_clericuzzio, Jul 11, 2016.

1. ### dante_clericuzzio Thread Starter Member

Mar 28, 2016
68
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What exactly the difference between DC and AC light bulb? and how it is made? This is because when i tested 2 volt DC light bulb using 200 volt AC it won't light up but with 2 pieces of AA batteries its just light up like that...so i am curious what is the real difference...and secondly why LED can light both using AC and DC..

2. ### crutschow Expert

Mar 14, 2008
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3,362
What type of DC and AC light bulb?
I can't imagine how it could survive the application of 200Vac and still work when you applied 3Vdc.

3. ### dante_clericuzzio Thread Starter Member

Mar 28, 2016
68
2

Just maybe i think its the frequency is too low

Jul 18, 2013
10,833
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Are you are talking filament lamp.
If you test a 2v lamp on 200v it is now open circuit!
Although they will work on either voltage as long as it is rated for the particular lamp There is a subtle difference, using DC results in more electrons being boiled off on the negative end eroding the filament.
Max.

5. ### wayneh Expert

Sep 9, 2010
12,374
3,226
That bulb did not survive 200V. The voltage likely came down in response to the load of the bulb. What did you use to supply power to it?

An incandescent bulb like that one will light with current passing in either direction. An LED can only pass current in one direction, so it will be half as bright on AC as on DC, all else equal.

6. ### dante_clericuzzio Thread Starter Member

Mar 28, 2016
68
2
I am using Synchronous motor to try lighting up the bulb...when i turn the motor it produces 200++ volts AC but it won't light up that bulb (2.5 volt @0.3amp) i have....but it can light up AC bulb rated 200 volt @ 3 watts...that makes me confuse

7. ### wayneh Expert

Sep 9, 2010
12,374
3,226
A 200V, 3W bulb has a current of 3/200 = 0.015A or 15mA and a resistance of 13,333Ω. Your motor is able to drive current into that load.

The flashlight bulb has a resistance of 0.3A/2.5V = 0.12Ω. It would be even lower than that until it heats up. Your generator can't handle the load. You'd find very little voltage across the bulb.

Your problem is impedance mismatch. Like trying to pedal your bike up a hill in the wrong gear. If your motor was wound with lower gauge – thicker – wire, it would generate less voltage but would be more able to power a low impedance load.

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8. ### dante_clericuzzio Thread Starter Member

Mar 28, 2016
68
2
If i add resistor let say 666 ohm in between to increase the resistance will the bulb light up? do you think

9. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
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V=IR
V/I=R
How did you get I/V=R?

10. ### DickCappels Moderator

Aug 21, 2008
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667
That's good but incandescent light bulbs are also thermistors.

Be aware that some kinds of filaments sometimes mend themselves when subjected to mechanical shock while voltage is applied. I discovered this while life testing a lot of incandescent lamps. That might possibly account for why the lamp did not work at 200 VAC and then worked again at 2V.

Another possibility is that your 200 VAC is coming through a high impedance so it looked like 200 VAC on your voltmeter bu might have dropped to close to zero (relatively speaking) when the load was applied.

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11. ### dante_clericuzzio Thread Starter Member

Mar 28, 2016
68
2
But when load of AC bulb rated 200 VAC @3 watts it light up brightly that is one heck of confusion...why 2.5 volt / 0.3 Amp doesn't work

Last edited: Jul 12, 2016
12. ### wayneh Expert

Sep 9, 2010
12,374
3,226
Sloppy mistake. It doesn't change the point I was making but I should have known better. I saw that very low ohms number and that should have triggered a double check. Instead I just accepted it and moved on.

13. ### GopherT AAC Fanatic!

Nov 23, 2012
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Those are reciprocal ohms, that way one can say, large load and have a large number, or a small load and have a small number.