First LED project - newbee

[morky.co.uk]

Hi all, after programming for 20+ years I have decided to turn my hand to some electronic projects. I am a completely new to this so sorry if I am asking some basic information here.

I have a quanity of 3mm white LEDs with a max power rating of 30mA and a power disposition of 80mW.

Is it possible to construct a circuit using only a 1.5V AA size battery and the LED's only? In parallel or series ?

How many LED's will I need to use before I need to use resistor(s)

Many thanks,

John

 

[Nigel Goodwin]

Hi all, after programming for 20+ years I have decided to turn my hand to some electronic projects. I am a completely new to this so sorry if I am asking some basic information here.

I have a quanity of 3mm white LEDs with a max power rating of 30mA and a power disposition of 80mW.

Is it possible to construct a circuit using only a 1.5V AA size battery and the LED's only? In parallel or series ?

1.5V isn't enough to overcome the forward voltage drop of an LED, the lowest ones are probably about 1.7V?. You also shouldn't put LED's in parallel, each LED (or series chain of LED's) should have it's own limiting resistor.

How many LED's will I need to use before I need to use resistor(s)

LED's ALWAYS need a current limiter, a resistor is the usual choice, you shouldn't use them without (the LED will soon die - and depending on the power available it could be almost instant!).

 

[JimB]

To expand on what Nigel just said, the voltage required to illuminate an LED depends on the colour of the LED and the technology process used to make it.
As a horrible generalisation, red LEDs require 1.7 to 2.0 volts
Orange and yellow LEDs require 2.0 to 2.2 volts
Green LEDs require about 2.2volts, but can require up to 3.5v depending on the technology.
Blue and white LEDs require 3.0 to 3.5 volts.

So, a single 1.5v AA cell will not do.
All LEDs require current limiting resistors.
To find the required resistor value:

Resistance = (SupplyVolts - LedVolts)/current

So for a LED requiring 2.2 volts, 20mA and running from a 5volt supply:

R = (5 - 2.2)/0.02 = 140 ohm

In practice use a 120 or 150 ohm resistor (preferred values, commonly available).

Finally you say that you have some white LEDs. Caution needed here, an LED in a white moulding probably will not emit white light. It is quite common to see red or green LEDs in a white moulded body.

JimB

 

[morky.co.uk]

Ok,

The spec for the LEDs I am using is :

Emitted Colour - White
Size (mm) - 3 mm
Lens Colour - Water Clear
Forward Voltage (V) - < = 4.5
Reverse Current (uA) - < = 50
Luminous Intensity Typ Iv (mcd) - 4000-5000
Life Rating - 100,000 Hours
Viewing Angle - 20 Degree
Absolute Maximum Ratings - ( Ta = 25°C )
Max Power Dissipation - PM = 80 mw
Max Continuous Forward Current - IFM = 30 mA
Max Peak Forward Current - IFP =75 mA
Reverse Voltage - 5 ~ 6 V

So, assuming I am now using a 9V PP3 battery, how many if the above can I place in series and what would the calculation be for working out which size resistor to use ? (sorry, getting a little confused as to which numbers to use from the Spec of the LEDs !!)

 

[morky.co.uk]

Ok,

The spec for the LEDs I am using is :

Emitted Colour - White
Size (mm) - 3 mm
Lens Colour - Water Clear
Forward Voltage (V) - < = 4.5
Reverse Current (uA) - < = 50
Luminous Intensity Typ Iv (mcd) - 4000-5000
Life Rating - 100,000 Hours
Viewing Angle - 20 Degree
Absolute Maximum Ratings - ( Ta = 25°C )
Max Power Dissipation - PM = 80 mw
Max Continuous Forward Current - IFM = 30 mA
Max Peak Forward Current - IFP =75 mA
Reverse Voltage - 5 ~ 6 V

So, assuming I am now using a 9V PP3 battery, how many if the above can I place in series and what would the calculation be for working out which size resistor to use ? (sorry, getting a little confused as to which numbers to use from the Spec of the LEDs !!)

 

[JimB]

OK, it is a real white LED.

The voltage required is 4.5 volts, the maximum current is 30mA but lets run it at 15mA.
So the resistance required is:

R = (9 -4.5)/0.015 = 300 ohm.

Problems:
A PP3 battery will not last very long at 15mA. The PP3 has a capacity of about 0.4 amp.hour, so at 15 mA, will last about 26 hours, maybe less because the battery voltage will drop to about 6 volts in that time.
It is better to use several AA cells in series.

JimB

 

[audioguru]

Hi Morky,
What device do you want to use your white LEDs in? A torch?
One white LED isn't bright enough for a torch. maybe just a keyring one up close. You will need to use a bunch of LEDs, each with its own current-limiting resistor.

As said by Jim, a little 9V battery doesn't have much power. Inside are six AAAA cells. Those cells are very slim and tiny. Tiny cells are very expensive when compared to common larger cells like AA size that have nearly 5 times the capacity.

There is a project that maybe is too advanced for you right now, but it explains the problems of little 9V batteries and compares them with larger cells. The project is an electronic circuit that boosts the voltage of two larger cells to 9V. If you go to the publisher's web-site, they want you to pay to see their project. However, if you go to www.google.com and enter into the search bar, Silicon Chip Never Buy, then the 1st link is to the entire article for free.
The publisher is Silicon Chip magazine in Australia, and the project is Never Buy a 9V Battery Again. Google's links are interesting, Never Buy a Dell Computer, Never Buy a Rusty Ford, etc. :lol:

 

[colin886]

thers a Joule Theif, might work?

 

[indecided]

However, if you go to www.google.com and enter into the search bar, Silicon Chip Never Buy, then the 1st link is to the entire article for free.

Audioguru, you're a savior - i once figured out how to get that off google, but recently I forgot... but now you reminded me! Thanks

However, I find that it's a icky process - sometimes it works, sometimes it doesn't... I just switch computers in the lab and it works again for a while. Bummer.

 

[cjk2]

Heres a circuit I designed/build that will power just about any LED off of one "AA". I have this circuit built and sitting on my desk right now. It has been running a red LED off a "dead" (.7v) AA battery for 48 hours now. It works very well and can even run several LED's in series(I tryed up to 4). Inductor is about 100 turns of 30awg wire on a 3/16" x 3/4" ferrite core. All transistors are generic NPN and all component values are non-critical.

 

[mstechca]

You should include a resistor in series with the LED.

 

[akg]

You should include a resistor in series with the LED.

not needed , the current handled by the led will be low .

 

[Someone Electro]

Shodnt the led be put anitparalel over the inductor?

 

[Nigel Goodwin]

Shodnt the led be put anitparalel over the inductor?

It makes no difference, electrically the LED is already in parallel with it (the positive and negative rails are the same at AC).

And, as already suggested, there's no resistor needed for the LED - which is obvious if you understand the simple circuit!.

 

[akg]

Shodnt the led be put anitparalel over the inductor?

a switchercad simulation has shown approx 1.6v across the led with 0.8 v batt . and without led approx 6v spikes at the collector-inductor jn:.

 

[akg]

You should include a resistor in series with the LED.

and with a series resistor the ckt will not function (as per the simulator)

 

[hyedenny]

Hi -
It also depends on how many LED's youre using... Add up all the forward voltage drops of the LED's in series to figure out the total forward drop for the current limiting resistor formula mentioned previously. Obviously, you'll be limited to only a few white LED's in series because of the relatively high voltage drop of the whiteys and the low voltage of the battery.