Hello,
I am trying to replace all the incandescents in my Bandai Voyager with LED's. I have started by putting 4 LED's in series and using 6 AA batteries to power the LED's. My question for you experienced modellers is can I use a 9 Volt battery instead. I am ssuming it will have less battery life or would a 9 volt put out more amps than 6 AA's? I like the 9 volt battery because it takes up so little space.

Any comments would helpfull,

Oggy

The 9 volt "transistor radio" battery will work fine if you don't mind the greatly reduced life expectancy. Don't forget the resistor to control the current through the leds.

Thanks for the quick response. Yes I am using resistors on each LED. I thought 9 volts would only power 3 LED's (1x3.5 volts and 1x3.7 volts with 68 ohm resistors each and 1x2.4 volts with 120 ohms.) But I added another LED at 2.4 volts without too much decrease in light output. I am pleased but perplexed. It shouldn't light up according to my calculations.

I am not home right now and I am guessing about the resistor strength, although I am very close.

Oggy

In series voltages are additive: 3.5+3.7+2.4=9.6 volts. A 9V cell would be inadequate for a series circuit with these LEDs even without resistors. I believe you are wiring in parallel in which case the resistors you're using would be too small. You can wire a lot of LEDs in parallel from a 9V battery before suffering a "brown out".

A 9V energizer alkaline (http://data.energizer.com/PDFs/522.pdf) can produce about 600 milli Amp hours (mAh) before dying, or about 0.5 amps for one hour. That's a lot of light. An LED draws about 20 mA so one LED could run about 30 hours on a 9V battery.

There are advantages to series over parallel wiring in terms of current draw, but typically it's better to run in parallel: that is each LED makes a separate circuit to the battery.

Can you take a picture that shows the layout, or make a drawing and post it (by hand and scanned if necessary).

I appreciate the input and I will provide a drawing ASAP. If you like I will describe what i have done for now. This is to light the nacelles and impulse of the Bandai Voyager.

Power Source: 6x AA batteriews = 9 volts

4 LED's in series in order of placement.Definitly not in Parallel.

Red LED : V= 2.4 20mA(?) resistor 120 ohms
Blue LED : V=3.7 20mA(?) resistor 68 ohms
White LED :V=3.5 20mA 3mm 7000 MDC resistor 72 ohms (49.9 + 22)
red LED: V=2.4 20mA(?) resistor 120 ohms (100+20)

(?) this is a guess.

BTW the 49.9 resistor is yellow,white, white, gold. Whats up with that second white?

I used an online LED calculator.

Alan

You are using a precision resistor, which uses 4 stripes rather than the usual 3. So you get 4 (yellow), 9 (white), 9 (white) *0.1 (gold) = 49.9 ohms.

Hello again. Here is the electronics schematic for my Bandai Voyager. I thou ght I would only be able to light 3 LED's but I found I could light 4. Thats great except I haven't figured out how its working. By my calculations the voltage should have dropped to near zero in a series circiut after 3 LED's. So any ideas about what is happening?

Thanks for looking,

That's screwed up dude. Here's one example of how it can be wired (there's several different ways). The value of the resistors represents about the maximum brightness that can be run without hurting the LED life span. If the LEDs are too bright, the associated resistor values can be increased to lower the current an dim them down.

http://members.dslextreme.com/users/md/led_2_example.jpg

edit: in response to oggy, but trust Renegade (he's in the biz, I just do software).

well.... that's series for sure.

But both forward voltages and resistance are cumulative.

I've got LED voltage = 12V
resistance = 379.9 ohms

If you've got a healthy young 9V battery it might be providing enough power to light them, but not terribly bright. As the battery starts to wear out, voltage drops, and some LEDs will stop lighting at all when their minimum voltage threshold is passed.

In any case 9V isn't enough to drive the 4 LEDs for long (in series) even without any resistors.

Here's a calculator (http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator)that shows some variations of circuits. Check it by inputting just a single LED, say white at 3.5V and 20mA with a 9V source: it wants you to supply a 330ohm resistor.

Try the series diagram with just 3 LEDs (all with the same voltage/amperage) say 3.5V and 20mA with a 9V source. It quickly tells you to reduce the number of LEDs or increase supply voltage.

There is a parallel option for multiple LEDs using just one resistor, but I don't recommend it: this option tends to result in one or more LEDs failing to light if they're even slightly different.

Best way to set it up is to have a separate circuit for each LED back to the battery (or to a "bus" wire pair that runs around the LED locations). Each LED has it's own resistor and each LED is provided with 9V. This way your resistors should all be around 330ohms.

Thanks guys for your advice. I guess I have a lot to learn about LED's. I was inspired to do a serial circuit when I saw a professionally made circuit on another site for the Monogram Voyager. It was neatly done . By the way I see Renegade had the two red LED's with just one resistor, I thought all LED's had to have their own resistor? Are there exceptions?
But again thanks.

Oggy

...I see Renegade had the two red LED's with just one resistor, I thought all LED's had to have their own resistor? Are there exceptions?
You'll notice that the two red LED's are in series. In this case, only one resistor is needed to limit the current to both. And I've seen Xmas LED strings that use no resistors at all. But you have to know what you're doing, as LED's I-V characteristics are nowhere near linear.

Thanks guys for your advice. I guess I have a lot to learn about LED's. I was inspired to do a serial circuit when I saw a professionally made circuit on another site for the Monogram Voyager. It was neatly done . By the way I see Renegade had the two red LED's with just one resistor, I thought all LED's had to have their own resistor? Are there exceptions?
But again thanks.

Oggy

They don't all have to have their own resistor, but the sum of their forward voltages has to be less than the power supply when in series. Plugging this in to the handy series (http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator)calc (2 red LEDs @ 2.4V 20mA current) you get one resistor in series with them of 220ohms (Unless your red LEDs are "superbright" they're probably less than 2V each. Running them with a fresh 9V battery and only 220ohms is going to shorten their lives, maybe dramatically. That's why Renegade suggests 360ohms. He designs bullet proof circuits.)

This site (http://www.kpsec.freeuk.com/components/led.htm) has a better explanation of what's going on.

You'll note that series, when possible, has the advantage of saving power. (It also has the disadvantage of failing to light everything in the series when only one LED fails. Not good for real rockets, but OK in models).