questions about led s and batteries

[betty.k]

i was wondering what the correct wiring up of led s is. i ve noticed there is usually a resistor in the curcuit, but i usually just wire them straight up, making sure i stick to about 3V.
next question is re batteries. my little tank chews through the batt s, so i was wondering how the following types compare to the 4xAA alkaline batt s i currently use:

grandcell rechargable

nicad rc packs

anything i might have missed.
i m interested in charge/run times, mah and longevity. thanks:8ball:

[PandaBear]

Quote:

grandcell rechargable

They work just like normal alkalines, but you can top them
up in the (special) charger. But they don t like deep cycling;
if you discharge them deeply they will never recharge back to
100%, best for shallow cycles only.

You can probably get several cycles out of them like that, but
given their limitations AND the fact that all mine have *leaked*,
I don’t buy Grandcells anymore.

Quote:

nicad rc packs

These are just packs of standard AA or AAA cells.
When using Nicad/NiMH, one should use 5 cells x 1.2V
to replace 4 x 1.5V drycells.

[trash]

You can recharge normal alkalines in the grandcell charger too. There is nothing special about them. Just put them in the charger as soon as you finish using them. I ve got a many as 20 charges out of them before their cactus.

You can also recharge normal carbon batteries,
(not in the grandcell charger). Again, you need to charge them as soon as you finish using them.
You can buy a carbon battery charger, they cost about $60. Try TVSN or one of the crazy clints shops. Silicon chip had a charger kit about 10 years ago, it had a lot of info on recharging carbon batteries. BTW, you can’t recharge a 9V carbon for some reason.

I’m not too fond of NiCd’s, NiMH’s are a nicer choice these days. I prefer lithiums, but you have to be more careful with them.

LED’s, the resistor is designed to limit the current. Usually about 10 to 20mA. Lucky for you, your little 3V button batteries have a hard time giving you that much current. Else the smoke would get out of them.

[betty.k]

ok, but what if i want to run them off a 3.6V nimh. i tried running headlights (green) and tail lights (red) on my dragon rc tank from button cells, but the headlights were a lot dimmer than the rear. so i ran them off the tank batt and the headlights went orange and they all got hot enough to melt plastic. what s going on here?:8ball:

[jamiekulhanek]

If they are superbright LED s 3.6 volts is PERFECT for them. They are different types of leds so they need different voltages.

If they are standard LEDS 3.6v is way too much.

[trash]

You should be able to get specs for the LED s your using from where you bought them.
DSE catalog used to have them in the back.

[betty.k]

didn t buy them anywhere, just cannibalising old mobile phones. mobile antennae work really well on some rc gadgets by the way!:8ball:

[ph2t]

betty, some hints n tips.

Leds have different ON voltages, they vary with colour. Here’s some ballpark figures (all approx values of course):

Red = 2V
Yellow = 2.2V
Green = 2.2V
Blue = 3.5V – 4.0V
White = 3.6V – 4.0V

What you will find is that the ON voltage is generally linked to the intensity (brightness, measured in mini-candelas, or mcd) for the LEDS. Most blue and white leds are classed as high intensity (> 1000 mcd) hence the operating votlage for these LEDS is higher.

The average driving current for most of these LEDs is around 20mA – 30mA. You achieve this current by putting a resistor in series with the LED.

Here’s a usefull equation to help calculate all the bits involved:

R = (E-Vf) * 1000/I

R = Value of the resistor (in Ohms)
E = Source voltage of the circuit
Vf = ON voltage of the LED
I = Current across the LED (in mA).

So for a 3V circuit using a Red LED, what is the value of the resistor?

Well you know from the datasheet that the Red LED in question needs 20mA to run and has an ON voltage of 2V.

Therefore:

E = 3V, Vf = 2V, I = 20mA

R = (3 – 2) * 1000/20
R = 1 * 50
R = 50 Ohms

The E24 value for R would be then 47 Ohms. (resistors only come in set values and ranges, this does change with the tolerances though, here I’m quoting standard 5% carbon values).

Hope this doesn’t confuse too much an helps you out.

Just go and buy a Dick Smith Catalogue, it has lots of usefull techo crap in the back like this info.

ph2t.

[twizm]

this is somewhat helpful. im going to be putting an underglow in my car, preferably red to match the body. its gonna be a tight fit in there with a dual cell mod though =
im such a say-er and not a do-er

[betty.k]

not confusing at all, and very helpful. saves a lot of trial and error. thanks plenty for the info ph2t:smiley16::8ball:

[trash]

Doh !
Millicandella … 1/1000th of a candle power brightness. Don t confuse candle power with real power.
A 10mW red led is not the same brightness as a 10mW green led. Your eyes are more responsive to green than red. The candle power is colour biased already, so a 1000mcd red LED is the same brightness as a 1000mcd green. Jaycar’s latest 20000mcd red LED is a bodge. It is only about 8000mcd.

Good stuff ph2t, be buggered if I could even guess LED voltages.
The voltage is a characteristic of the doping, like Gallium Arsnide or Indium Phosphide. So it is possible that three kinds of RED leds all have different voltages.

[Author]

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