# Lightbulbs in series

Discussion in 'The Projects Forum' started by McZlik, Jun 23, 2012.

1. ### McZlik Thread Starter New Member

Jun 9, 2012
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Hey all,

As can be seen in the picture that I attached, I'm designing a circuit.
I now the basic stuff for this, but now I need some help to go further.

I need to connect 13 little low voltage light bulbs next to each other, each with an own switch to turn it on/off and 1 master switch to turn the whole thing off.

Now I want to know if what I draw is correct, or that I need some changes.
Details: the bulbs are 3.5V at 0.2A, with an E10 fitting.
Like this one.
The will have multiple colors, but I don't think it maters for these lights.

I'm wondering how I can calculate the source voltage I need, and if I need to add resistors to the circuit. (I think I do, but I have no idea)

To note out: I'm pretty new to electronics, and I really want to learn. Sow I'm asking not only for the solution, but also the way to get there

Greetz,

McZlik

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Last edited: Jun 23, 2012

Apr 16, 2011
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You will need to reduce the 24 Volts from the power supply to 3.5V somehow, otherwise the lamps will burn out almost instantly.

You COULD put a 100 Ohm 4 Watt resistor in series with each lamp IF you power supply can supply enough extra current (2.6 Amps), which I doubt. You then have 47 Watts of waste heat to get rid of.

I think it might be more practical to build a separate 3.3 Volt power to power your lamps, or change to 24 Volt lamps.

As for calculating the source voltage that you need, that is easy. 3.5V lamps require a 3.5V power supply. 3.3V supplies are more common and this is close enough.

Last edited: Jun 23, 2012
3. ### wayneh Expert

Sep 9, 2010
12,388
3,244
Yes, the biggest problem in your drawing is the mismatch of your power supply voltage to your load voltage. Adding resistors does solve it, technically, but is a poor solution because the majority of your circuit's voltage drop (proportional to power loss) is across that resistor. And this solution only works for constant current loads like your lightbulbs. For other, variable loads, it doesn't work.

A couple other things: Safety first. You should think about adding a fuse. And you should verify that the total current (13@0.2A=2.6A) is below the rating of SW1, and that the power and return (or ground, "-") wires are big enough. That's probably all fine with normal parts and wire, but it's wise to at least think about it and verify.

4. ### McZlik Thread Starter New Member

Jun 9, 2012
19
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Forgot to mention that the power supply is a stand-in.

Like I said in my post, I want to know how to calculate the needed voltage for the power
Sorry if it made a confusing setting.

So to rebuild my question, how much power do I need for my power supply, and if needed, what kind of resistors?

For the power I understand just 3.5V is enough?

Apr 16, 2011
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13 lamps at 0.2A each.

13 x 0.2 = 2.6A

So you need a 3.5V power supply that can supply 2.6 Amps or more.

Maybe go for 3.5 Volt 4A supply to allow for a margin of error

Hope I got the maths right

EDIT: having trouble reading! posts edited for 13 lamps, not 16. ...doh!

Last edited: Jun 23, 2012
6. ### wayneh Expert

Sep 9, 2010
12,388
3,244
Yup, that'd be ideal (no resistors). It should be rated for at least about 3A, IMHO. It's not good to run any component at its rating for long periods.

It's good to design for worst-case situations, which in your case is all bulbs lit. If this situation is rare in real use, you could consider backing off a bit, especially if the power supply is internally protected against overheating, over-current and such.

An old computer power supply would give you plenty of current at 5V, and would have all those safety features built in.

7. ### McZlik Thread Starter New Member

Jun 9, 2012
19
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Argh darn it, I keep forgetting mentioning things.... Sorry for that!

It needs to run on batteries. My guese 3 x 1.5V AA batteries aren't up for this, am I right?

Thanks so far for the help, it's really helping me out

Oh for the fuse, is there a way to calculate how strong it should be? (I know I should probably just search this on Google)

8. ### wayneh Expert

Sep 9, 2010
12,388
3,244
The AAs might work alright - I'd look for rechargeables. If all bulbs are frequently on, they won't last more than an hour or so and the bulbs may dim as more are turned on. It's possible to put another identical string of them in parallel with the first string so that you get more current, but I wouldn't do this unless you have a matched set.

The fuse should be sized to not blow in normal use (2.6A) but blow soon after that is exceeded. So you'll probably want a 3A fuse. The physical size and holder are optional. Just pick what you like. You don't need to worry about voltage rating - any fuse will be rated for MUCH higher voltages. You can choose whether you want slo-blo or regular. I guess I'd just pick whichever is cheaper.

BTW, have you considered using LEDs instead? They'd be just as bright with 1/10th the current.

Apr 16, 2011
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AAs are fine, but two rather than three, and I agree with wayneh that battery life will be short.

D cells are bigger, but more cost effective - again use two.

I also agree with wayneh that LEDs would be more efficient, but series resistor will be vital for LEDs.

I do not see the need for a fuse, but it won't hurt to fit one.

10. ### McZlik Thread Starter New Member

Jun 9, 2012
19
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Thanks for all the help, I attached my new design.

About the LEDs, yeah I know, but it's a project for a friend how wants the bulbs for the project he's building. Sow not my choice

Let me know what you think about the circuit =]

About the wires, should it be good to just choose wires in thickness comparable to audio speaker wires? I don't know what the names are.

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11. ### McZlik Thread Starter New Member

Jun 9, 2012
19
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A quick update from my friend, he's willing to go to two D cell batteries, shouldn't make any difference right?

Apr 16, 2011
250
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Two D cells is good

Pretty much any wires that you find lying around can handle 2.6A, unless they are really thin. Audio speaker wires will be more than adequate.

.... wish I had a friend

13. ### McZlik Thread Starter New Member

Jun 9, 2012
19
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Doh, dude should make up his mind....

As of the problem that a 2 D-cell battery holder can't be found here, we're going back to the AA cells.

But then again, only holders for 2 or 4 cells.
With 2 cells, how long should it be able to run?

Apr 16, 2011
250
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Guessing a bit.... not long.

If all lamps are lit, then maybe an hour using expensive lithium batteries.

The less lamps that are lit, the longer the life.

The cheaper the batteries, the shorter the life.

Sounds expensive, whatever way, even if you used D cells. Is there an option to use a wall-wart?

15. ### McZlik Thread Starter New Member

Jun 9, 2012
19
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Haha, that's true.

Luckly it's a build to only be lit for a few minutes at a time

He's a big steampunk fan, sow it's just for showing off, not for decorating

16. ### wayneh Expert

Sep 9, 2010
12,388
3,244
The individual bulbs can just use hookup wire, something like 22 gauge I think. The main lines that could potentially carry all the combined current could also be 22 gauge if distances are short (less than, say 6"). Speaker wire or zip cord (power cord) is fine too, but probably overkill unless you have long runs more than a few feet.

Rechargeable D cells are usually just AA cells in a bigger package. I wouldn't bother.

17. ### cork_ie Member

Oct 8, 2011
348
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Your post says lightbulbs in series, but your circuit shows the bulbs are in parallel.
Your voltage supply is 24Volt - you will also need to consider how good the regulation is in this supply ie output voltage V's load.

If you want to use them in parallel then you will need to reduce the voltage supply to 3.5Volt or put a resistor in series with each bulb which will be very wasteful in power.
If they are standard incandescent bulbs what you could do is construct a PWM circuit between the 24V supply and the bulbs and limit the max duty cycle to about 15% this would also give you the advantage of being able to dim the bulbs using a potentiometer incorporated in your PWM controller.

18. ### McZlik Thread Starter New Member

Jun 9, 2012
19
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Doh! I didn't notice my fault of the name, ups

You're correct about that, should be called in parallel.

Anyways, if you look at the latest design(or read the whole thread, no insult tended), we have dropped the supply to 3V, 2x AA batteries.

A PWM also would have been a nice solution, but would make the project to expensive though. And a PWM would be overkill for the purpose.
Thanks for the interesting idea though! Maybe a good idea for one of my own projects sometimes

19. ### cork_ie Member

Oct 8, 2011
348
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Ok no problem, I had actually posted much earlier on but for some reason my post did not appear on the thread until about 4 hours later!!. This is the second time this has happened in so many days and makes my replies look as if didn't readthe thread before posting.

You can use an old flash light as a D Cell Holder & on/off switch.

a PWM circuit for this kind of set-up would cost peanuts - less than \$3-00

555 timer + 3 or 4 resistors + 2 capacitors + pot + power transistor.

20. ### wayneh Expert

Sep 9, 2010
12,388
3,244
Not a bad point. This would allow you to use a 6V or a 12V battery (or pack), solving the problem of limited battery time. Many circuits wouldn't like the PWM but a lightbulb is perfectly happy.