Electronics Forums > Need to solve missing LED in Christmas light.

# Need to solve missing LED in Christmas light.

Sam Nickaby
Guest
Posts: n/a

 02-27-2006, 01:04 AM
James has a 60 LED Christmas Lights made by Phillips. Each LED takes
4-Volts . They run on 120-Volt AC.

Question: If James brother steals 10 LEDs from the 60 LEDs, James now
has 50 LEDs. What is the value of the resistor in Watts and resistance
that James should put in place of the missing LEDs in order to prevent the
LED from burning out?

Thanks

Tim Williams
Guest
Posts: n/a

 02-27-2006, 02:05 AM
Does the textbook mention the current drawn by the string?

You may be interested in Ohm's law, to finish this one.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

"Sam Nickaby" <(E-Mail Removed)> wrote in message
news:8esMf.40783\$(E-Mail Removed). com...
> James has a 60 LED Christmas Lights made by Phillips. Each LED takes
> 4-Volts . They run on 120-Volt AC.
>
> Question: If James brother steals 10 LEDs from the 60 LEDs, James now
> has 50 LEDs. What is the value of the resistor in Watts and resistance
> that James should put in place of the missing LEDs in order to prevent the
> LED from burning out?
>
> Thanks
>
>

John Larkin
Guest
Posts: n/a

 02-27-2006, 02:32 AM
On Mon, 27 Feb 2006 01:04:04 GMT, "Sam Nickaby" <(E-Mail Removed)> wrote:

>James has a 60 LED Christmas Lights made by Phillips. Each LED takes
>4-Volts . They run on 120-Volt AC.
>
>Question: If James brother steals 10 LEDs from the 60 LEDs, James now
>has 50 LEDs. What is the value of the resistor in Watts and resistance
>that James should put in place of the missing LEDs in order to prevent the
>LED from burning out?
>
>Thanks
>

45 megohms.

John

ehsjr
Guest
Posts: n/a

 02-27-2006, 05:05 AM
John Larkin wrote:
> On Mon, 27 Feb 2006 01:04:04 GMT, "Sam Nickaby" <(E-Mail Removed)> wrote:
>
>
>>James has a 60 LED Christmas Lights made by Phillips. Each LED takes
>>4-Volts . They run on 120-Volt AC.
>>
>>Question: If James brother steals 10 LEDs from the 60 LEDs, James now
>>has 50 LEDs. What is the value of the resistor in Watts and resistance
>>that James should put in place of the missing LEDs in order to prevent the
>>LED from burning out?
>>
>>Thanks
>>

>
>
> 45 megohms.
>
> John
>

Ahhhhh - but what value do you use if his *sister*
steals the bulbs?

Ed

John Fields
Guest
Posts: n/a

 02-27-2006, 07:09 AM
On Mon, 27 Feb 2006 01:04:04 GMT, "Sam Nickaby" <(E-Mail Removed)>
wrote:

>James has a 60 LED Christmas Lights made by Phillips. Each LED takes
>4-Volts . They run on 120-Volt AC.
>
>Question: If James brother steals 10 LEDs from the 60 LEDs, James now
>has 50 LEDs. What is the value of the resistor in Watts and resistance
>that James should put in place of the missing LEDs in order to prevent the
>LED from burning out?

---

--
John Fields
Professional Circuit Designer

PeteG
Guest
Posts: n/a

 02-27-2006, 10:38 AM

"Sam Nickaby" <(E-Mail Removed)> wrote in message
news:8esMf.40783\$(E-Mail Removed). com...
> James has a 60 LED Christmas Lights made by Phillips. Each LED takes
> 4-Volts . They run on 120-Volt AC.
>
> Question: If James brother steals 10 LEDs from the 60 LEDs, James now
> has 50 LEDs. What is the value of the resistor in Watts and resistance
> that James should put in place of the missing LEDs in order to prevent the
> LED from burning out?
>
> Thanks
>
>

60 leds x 4 volts is 240volts so they never worked.

If they're connected as two sets of 30 leds wired back to back so they
conduct on alternate halves of the AC cycle which you don't say but it's
kind of implied since 30 leds x 4 volts = 120 then you need to know did his
brother take 5 leds from each leg? If he didn't you need two resistors.

And whatever resistor(s) you put in it still won't work because there are 10
open connections were the leds have been removed :-)

Pete

redbelly
Guest
Posts: n/a

 02-27-2006, 12:21 PM

PeteG wrote:

>
> 60 leds x 4 volts is 240volts so they never worked.
>
> If they're connected as two sets of 30 leds wired back to back so they
> conduct on alternate halves of the AC cycle which you don't say but it's
> kind of implied since 30 leds x 4 volts = 120 then you need to know did his
> brother take 5 leds from each leg? If he didn't you need two resistors.
>
> And whatever resistor(s) you put in it still won't work because there are 10
> open connections were the leds have been removed :-)
>
> Pete

That wouldn't work either. A 120 Vrms signal has an amplitude of 170
V, which will fry the LED's.

Mark

PeteG
Guest
Posts: n/a

 02-27-2006, 01:31 PM

"redbelly" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
>
> PeteG wrote:
>
> >
> > 60 leds x 4 volts is 240volts so they never worked.
> >
> > If they're connected as two sets of 30 leds wired back to back so they
> > conduct on alternate halves of the AC cycle which you don't say but it's
> > kind of implied since 30 leds x 4 volts = 120 then you need to know did

his
> > brother take 5 leds from each leg? If he didn't you need two resistors.
> >
> > And whatever resistor(s) you put in it still won't work because there

are 10
> > open connections were the leds have been removed :-)
> >
> > Pete

>
> That wouldn't work either. A 120 Vrms signal has an amplitude of 170
> V, which will fry the LED's.
>
> Mark
>

Yeah I'd thought of that too although it doesn't say whether it's 120V peak,
or 120V rms so we assume rms because that's what we'd expect in the real
world. The whole scenario as he has presented it here is flawed.

I don't like contrived 'real world' type questions for this reason.

They'll probably cover AC peak and rms voltages in next weeks lesson.

Pete

James Douglas
Guest
Posts: n/a

 02-28-2006, 11:17 AM
Sam Nickaby wrote:
> James has a 60 LED Christmas Lights made by Phillips. Each LED takes
> 4-Volts . They run on 120-Volt AC.
>
> Question: If James brother steals 10 LEDs from the 60 LEDs, James now
> has 50 LEDs. What is the value of the resistor in Watts and resistance
> that James should put in place of the missing LEDs in order to prevent the
> LED from burning out?
>
> Thanks
>
>

None, just go to WalMart and get another one for \$1.99. I know this was
a trick question.

redbelly
Guest
Posts: n/a

 02-28-2006, 01:54 PM

PeteG wrote:

> Yeah I'd thought of that too although it doesn't say whether it's 120V peak,
> or 120V rms so we assume rms because that's what we'd expect in the real
> world. The whole scenario as he has presented it here is flawed.
>
> I don't like contrived 'real world' type questions for this reason.
>
> They'll probably cover AC peak and rms voltages in next weeks lesson.
>
> Pete

The thing is, even a question for a basic electronics class homework
wouldn't be flawed like this. If it was a troll post, I guess I've
been hooked. Oh well.

Mark

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