Advice needed for wiring a segmented lighthouse lamp

[Mimmo89]

Hello,
new to the forum and not totally sure in which category I'm meant to post this, so feel free to move it around as appropriate (if that's a thing).

I've got a new project in my mind (haven't we all) and I need some help with wiring; I know this is a woodworking community, but I'm sure there are plenty of very handy people that will have an answer for it.
It might be a pretty basic problem, but my electrical knowledge is slightly below basic.

I want to turn a segmented lighthouse at the lathe and put one of those recessed spotlights at the top of the body. The spotlight will be wired to a switch and then to a plug and that's easy enough.
I then want to add some smaller LED bulbs inside the body of the lighthouse to mimic light coming out of windows that I will be drilling in the body.
- Is it just as easy as to wire the smaller LED bulbs in parallel?
- Will I need to use resistors (and what Ohms?) if I want to use smaller 4.5V bulbs I have left over from a previous project? Or do you suggest using a regular bulb? The 4.5V bulbs would give a much dimmer light, which is what I'm aiming for.

Hope it makes sense?

Thanks in advance for your help.
Dom.

 

[Spectric]

LEDs require a resistor to limit the current otherwise they will be blown. The color of an LED will have an influence on the forward current needed and the formula for the resistor is

R = (Applied voltage - Forward voltage) / Forward Current

 

[Mimmo89]

Thank you for your reply, it sounds like I was on the right track.

So, bearing in mind the specs of my mini bulbs [ Wattage = 0.25W , Voltage = 4.5V , hence I = 0.25W / 4.5V = 0.056A ], it sounds like I'll need a Resistor = (220V - 4.5V) / 0.056A = 3848 O. Do you approximate up or down?

And the circuit would look like something like the attached image?

Is this circuit something common/safe? I really want to avoid burning my house down if I can help it!

 

[Fireburst]

I’m not sure if that will work. If I remember back in my school days I was told electricity will always take the path of least resistance but don’t quote me on that as I may be completely wrong.

 

[gcusick]

Are you intending to use a 240V spotlight at the top, powered directly from the mains? If that’s the case, connecting simple LEDs lower down will be difficult.

LEDs are devices (diodes) that conduct in one direction only; connected to AC, the diode will block half of each cycle. In the conducting half-cycle, the LED current (Iled) can be determined by a resistor with value (Vsupply - Vled)/Iled. Vled will typically be about 3V. In the non-conducting half cycle, the LED will see the full supply voltage: this is a problem, because the reverse voltage that LEDs can tolerate (known as Vrrm) is typically 5V. Exceeding this voltage (repetitively) will damage the LED. So connecting LEDs to the mains via a resistor is not a good idea!

I would suggest using a 12V spotlight lamp, and powering the whole lighthouse from a 12V DC wall-wart. Then you can connect the additional LEDs as a set of series-connected pairs, each supplied through a resistor.

HTH
Geoff

 

[porker]

There are several ways you can run LEDs from mains but the simplest way would be the advise above and drive the whole lot from a 12V transformer including your spotlight. You can then drive the LEDs from the 12v with a current limiting resistor.

If you go down the mains route you can get round the LED reverse voltage problem by putting a diode in the opposite direction across your LED. The problem with this approach is that you have a high voltage across the current limiting resistor (mains roughly) so you are dissipating a lot of current (around 2.5W) which needs a biggish resistor and is wasteful.
Using a capacitor in series would work best with this arrangement but this is getting into deeper electronics.

 

[MorrisWoodman12]

Have you considered getting a plug in power supply? They are readily available at many online retailers for less than £15 or so. I would recommend RS or CPC/Farnell as you will get a properly approved device not something from a questionable source. This way all your safety issues are under control and you don't have to muck around with lethal mains. It all depends upon the power of your spotlight as you will need at least that plus a smaller amount for the LEDs. Allow a margin so it's not working flat out.
Don't forget the power rating of any current limiting resistors. Calculate (supply voltage - forward voltage) x current, allow a 50% margin and look for a resistor with that or larger power rating.
Have fun.
Martin

 

[Mimmo89]

I knew it wouldn't be as straightforward as I thought! Thanks for the help gcusick and porker. And fireburst and spectric too.

I guess I'll start looking around for 12V spotlight then! (Although from a quick search I'm under the impression they won't be as bright?)

So you suggest wiring the eventual extra LEDs in series with each other? May I ask why? Since I'm at it I might as well learn something rather than just following instructions.
Would the circuit look something like this?

 

[trvlr]

Safest and easiest way... as already suggested above... use a plug in power sourde to drive the LEDs. Much as one does for under kitchen cabinets etc... Far safer and easier to replace should the need arise anon...

 

[Mimmo89]

Have you considered getting a plug in power supply? They are readily available at many online retailers for less than £15 or so. I would recommend RS or CPC/Farnell as you will get a properly approved device not something from a questionable source. This way all your safety issues are under control and you don't have to muck around with lethal mains. It all depends upon the power of your spotlight as you will need at least that plus a smaller amount for the LEDs. Allow a margin so it's not working flat out.
Don't forget the power rating of any current limiting resistors. Calculate (supply voltage - forward voltage) x current, allow a 50% margin and look for a resistor with that or larger power rating.
Have fun.
Martin

Hi Martin, thanks for your reply too!

Would your power supply be just an alternative for the 12V DC wall-wart gcusick suggest before you?

If I were to use a 12V spotlight and 3x 4.5V LEDs, would you recommend using a 25-26V DC power supply?

Many thanks for the input.

Dom.

 

[MorrisWoodman12]

Dom. Hi!
If you have a 12V spotlight why not use a 12V power supply? The bulb can go directly across the supply. Is your LEDs are 4.5V then two in series gives 9V and if at 56mA (0.056A) gives a resistor of about 53 Ohms or 56 Ohms nearest preferred value (NPV). A 1/4W should suffice. For any single LED then 132 Ohms (150R NPV) at 1/2W but use a 1W to give margin.
Yes the power supplies I recommend are also known as wall-warts. They have many names.
Hope this helps - it helps me get my brain ticking over.

 

[Mimmo89]

That's great, thanks for clarifying!

Sorry if I'm being a pain but could you please also confirm that this would be the circuit I should go for? Just want to make sure I'm getting everything right.

 

[MorrisWoodman12]

In essence yes. You can't wire all three 4.5V LEDs in series as this would need 3 x 4.5V = 13.5V to work. So either each LED should be wired in series with a 150R / 1W resistor across the supply. Alternatively two LEDs can be wired in series and in series with a 56R 1/4W resistor across the supply.
I hope it all makes sense.

 

[imageel]

That's great, thanks for clarifying!

Sorry if I'm being a pain but could you please also confirm that this would be the circuit I should go for? Just want to make sure I'm getting everything right.

Can you clarify what LED's you are intending to use - maybe a link to the website/item you have bought ?
LED's are constant current devices so need a limiting resistor in series with a fixed voltage supply to limit the current drawn to within design limits, typically the voltage across a conventional single diode LED is less than 2V.
Once you have the spec for the LED and a similar one for the DC wall-wart you are using then we can calculate the value of the resistor...

 

[Mimmo89]

In essence yes. You can't wire all three 4.5V LEDs in series as this would need 3 x 4.5V = 13.5V to work. So either each LED should be wired in series with a 150R / 1W resistor across the supply. Alternatively two LEDs can be wired in series and in series with a 56R 1/4W resistor across the supply.
I hope it all makes sense.

I see I get it now, when earlier they suggested to wire the LEDs in series they meant in series with their respective resistors! Makes sense now.
Thanks a lot, very helpful.

 

[Mimmo89]

Can you clarify what LED's you are intending to use - maybe a link to the website/item you have bought ?
LED's are constant current devices so need a limiting resistor in series with a fixed voltage supply to limit the current drawn to within design limits, typically the voltage across a conventional single diode LED is less than 2V.
Once you have the spec for the LED and a similar one for the DC wall-wart you are using then we can calculate the value of the resistor...

Hi Imageel,
Here is the link to the LEDs
https://amzn.eu/d/0Gq2NDh

And these are the specs
Wattage = 0.25W , Voltage = 4.5V
Many thanks.

 

[gcusick]

Hi Imageel,
Here is the link to the LEDs
https://amzn.eu/d/0Gq2NDh

And these are the specs
Wattage = 0.25W , Voltage = 4.5V
Many thanks.

Those LEDs include a resistor in the package. If you buy the 12v version, then you can simply wire them in parallel with the 12V spotlight.

 

[ajs]

I'm not quite clear what you mean. By a "segmented lighthouse" do you mean a Fresnel lens? If you want to retain that bare in mind they are designed for a small single light source rather than something distributed as for a number of LEDs.

Controlling LEDs is relatively straightforward if you pay attention to their operating characteristics - they are markedly different to a resistor or an incandescent bulb. Essentially they will not conduct at all until the voltage reaches a certain threshold, at which point the current they allow through increases almost vertically with even a small further increase in voltage. It follows that slightly undervoltage, they won't work, but slightly overvoltage they will self-destruct as they exceed the limit of their power handling.

Instead you need to look at how much current you are feeding them instead of the voltage which is what the resistor limits - if the voltage is slightly off the current throught the resistor varies by a small amount and doesn't allow the LED to run dangerously out of control. There are three basic methods of wiring whch I've drawn out in pictures 1-3 below. 1 is the simple circuit with a single LED and a single resistor to keep it in check.

If you want more than one LED then if you have enough voltage you can daisy-chain LEDs with a single resistor, the value of which will need adjustment for best results but that can increase efficiency as power can be used for lighting that is otherwise wasted in the resistor. The other basic arrangement is parallel as in 3, where each LED is on its own branch an protected by its own resistor. That uses additional current instead of voltage so it can work with a low voltage as for 1 but needs a power supply with the grunt to drive it - for LEDs which are generally low current to begin with that is unlikely to be an issue for a practical power supply.

The combination to avoid is shown in 4 with a single resistor driving LEDs in parallel, it may seem that limits the current to the right level but in practice the slightest imbalance in the LEDs will cause the majority of the current to go through one of them, which will then heat up and make the imbalance worse, leading to the same overload as for no resistor at all.

In practice if you are using large numbers of LEDs you will probably want to combine the approaches as in 5. You can have as many LEDs in series as the voltage allows coupled with as many branches in parallel as the current allows. I'm hesitant to give numbers here since white LEDs, especially high powered ones, are not highly standardised devices. If you show us the power supply and specific LEDs you intend to use we can advise from there along with suitable resistance values.

 

[Scruples]

Use a readily available power supply from a toy charger, say 6-12V @ 500mA. Use that supply for both coloured window lights and the top bright lantern light. There's always a possiblilty, if you're ambitious is to look an sequencial control of several LEDs to simulate a revolving light.
There are many sources of information online to guide you through the components you'd need.

 

[Mimmo89]

I'm not quite clear what you mean. By a "segmented lighthouse" do you mean a Fresnel lens? If you want to retain that bare in mind they are designed for a small single light source rather than something distributed as for a number of LEDs.

No, segmenting is a wood turning technique to turn, normally, bowls and vases out of "bricks" of wood. I wanted to make the body of a lighthouse with this technique, I attached an image so that you can see what I mean. (I thought I was coming up with an original idea but obviously someone else thought about it before me!)
Thanks everyone for your help I think I've got the answer I needed now, I just need to use a 12V spotlight and 12V power supply.