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Hobby Lighting

M

Mike

Jan 1, 1970
0
Thanks for all the help from everyone.

I'm trying to calculate exactly what I need. I'm not sure how many LEDs but
let's say I had 10 and did the following.

batteries (3 x 9volt)
on/off switch
resistor
LEDs 10

Would that be correct?
As far as the resistor, I came up with the following (trying to learn).
If each LED is 20 mA and 2V, then 10 is 200 mA and 20V. If I have a 27V
power supply, then I need to remove 7V.
7V/.2A=35 Ohms
Then I'm lost. What resistor would I need?

As far as the wire, any special type? Should it be coated?

Thanks again.
Mike

PS. My son also built Shea Stadium (NY Mets, about 3' in diameter) and we
put false lights but are thinking to go back and put real ones. Would the
LEDs give off a nice glow to the field?
 
E

ehsjr

Jan 1, 1970
0
Mike said:
Thanks for all the help from everyone.

I'm trying to calculate exactly what I need. I'm not sure how many LEDs but
let's say I had 10 and did the following.

batteries (3 x 9volt)
on/off switch
resistor
LEDs 10

Would that be correct?

Yes - but you might want to use a 24V DC wall wart supply
instead. Something like DCTX-2450 at $4.50 from Allelectronics
http://www.allelectronics.com/
would supply power to your ten leds in series with a 470
ohm 1/2 watt resistor, and you wouldn't have to worry
about running the batteries down. And - with that supply,
you could run a number of 10 LED, 470 ohm resistor
strings in parallel, so you would not be limited to
10 LEDs.
As far as the resistor, I came up with the following (trying to learn).
If each LED is 20 mA and 2V, then 10 is 200 mA and 20V.

No - with the LEDs in series, the current stays the same, it
does not add. The voltage does add. So it would be 20 volts
at 20 mA.

If I have a 27V
power supply, then I need to remove 7V.
7V/.2A=35 Ohms

You have the right formula, but the current is 20 mA,
not 200 mA, so it's 7/.02 or 350 ohms.
Then I'm lost. What resistor would I need?

You can use a standard value 330 ohms, at 1/2 watt
resistor. Your LEDs would glow very brightly.
Personally, I would use a 470 ohm resistor. The LEDS
won't glow as brightly (but they will still be
bright) and the batteries will last longer.

Someone said LEDs require 20 mA. That is *not*
the best way to say it. It would be better to give
a range, or say something like "figure about 20 mA".
An LED will glow nicely at well under 20 mA - it's
just that 20 mA seems like a good design goal for
this project. I mention this because you may wonder
about the two different value of resistance above.

Understand that we are talking about LEDs that are not
white. They are red, amber, yellow, green - etc, but
not white. White LEDs require higher voltage (figure
3.6v per LED) and are more expensive. Also, some of
them have a blue-ish tint to the light.

As far as the wire, any special type? Should it be coated?

No special type needed. Just use some insulated wire that
is easy to work with.
Thanks again.
Mike

PS. My son also built Shea Stadium (NY Mets, about 3' in diameter) and we
put false lights but are thinking to go back and put real ones. Would the
LEDs give off a nice glow to the field?

No, unless you use white LEDs that don't have a blue-ish
tint to them. Even then, "nice glow" is subjective, so
its hard to say if you'd like it or not. Perhaps the blue
tint would be fine.

Ed
 
B

Ben Miller

Jan 1, 1970
0
Mike said:
Thanks for all the help from everyone.

I'm trying to calculate exactly what I need. I'm not sure how many LEDs
but let's say I had 10 and did the following.

batteries (3 x 9volt)
on/off switch
resistor
LEDs 10

Would that be correct?
As far as the resistor, I came up with the following (trying to learn).
If each LED is 20 mA and 2V, then 10 is 200 mA and 20V. If I have a 27V
power supply, then I need to remove 7V.
7V/.2A=35 Ohms
Then I'm lost. What resistor would I need?

Not quite. If you connect them in series, in one, out to the next, etc, then
the current remains at 20 mA, and the total LED voltage is 20 volts. The
resistor needs to drop 7 volts at 20 mA, so its value is 7/.02=350 ohms (390
is a standard value, so the current will be slightly lower). The power in
the resistor will be 7*.02 or .14 watt, so a 1/4 watt resistor will be fine.

Ben Miller
 
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