Unknown LEDS

[Tipsy]

I want to illuminate my microscopes viewing area using some white LED's I have. The goal is to create as much light as possible in the limited space available. Research and searches show I need to know the LED specs before I can make accurate calculations for voltage and resistance but they are not available to me. All I've been told is they are 3v.

Are there some tests I can do to determine the pertinent data I need?

Would a 9v battery be the best choice for power vs space?

How would you proceed?

 

[kinarfi]

2 AA and you may not need a resistor, or you'll toast it on the first try.

 

[Vizier87]

The ultra-bright LEDs light up dazzlingly with a supply of 5V regulated output out of a 9V battery, series with a 1K ohm resistor.

 

[Hero999]

What package size are the?

If they're 5mm and white they'll have a typical forward voltage of 3.5V and a maximum operating current of between 20mA and 30mA.

I'd recommend a Joule thief, which can be made using a single transistor and a transformer or an inductor plus a couple of transistors.
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[joespaz7]

I want to illuminate my microscopes viewing area using some white LED's I have. The goal is to create as much light as possible in the limited space available. Research and searches show I need to know the LED specs before I can make accurate calculations for voltage and resistance but they are not available to me. All I've been told is they are 3v.

Are there some tests I can do to determine the pertinent data I need?

Would a 9v battery be the best choice for power vs space?

How would you proceed?

you need about 5 volts, and around 15mA-20mA going though so ohms law..E/I ends up being around 330 Ω. so theres your circuit 2 AA batteries your L.E.D's and a resistor around 330Ω . by the way, the average voltage that a LED drops is 1.8V from my experance. Good luck!

 

[Vizier87]

That Joule Robber ( ) gets my vote!!!!!

 

[jbeng]

by the way, the average voltage that a LED drops is 1.8V from my experance...

A red LED's drop may be about 1.8v, but a white LED's voltage drop will be closer to 3 volts...

 

[Tipsy]

Yes, they are 5mm (or 6mm including the rim) and advertised as super bright (but don't they all!) white LED's of which I'd like to use as many as possible for best illumination and power efficiency. What are your hints and tips on that score?

Below is the 'scope in question and as you can see, has ample space for a string of LED's all around the black cover which measures 4.5cm by 2cm.

 

[Vizier87]

I suppose if you build that joule bandit circuit, the only thing left to optimize is to put reflectors around the LED... focus the light onto your specimen.

 

[audioguru]

I suppose if you build that joule bandit circuit, the only thing left to optimize is to put reflectors around the LED... focus the light onto your specimen.

Reflectors are used with incandescent light bulbs because they shine all over the place. Most LEDs shine a narrow beam in only one direction so a reflector won't do anything.

 

[Vizier87]

Oh.. you're right. But I saw these really small ultra-bright LEDs used in nokia cellphones, cuboid shaped and it seemed that the light goes all over the place..

 

[Tipsy]

As already mentioned, I'd like to use more than one LED so do the above recommendations still hold or will they require revising?

 

[Hero999]

How many LEDs do you want to use?

The Joule Theif circuits aren't very good for more than a couple of LEDs run from a single AA cell.

Resistors is the easy way but it's less efficient.

What power supply are you using? Battery, mains?

If it's a battery, what sort? Voltage? Ah rating?

If you're using batteries you might want to use a SMPS.

 

[Tipsy]

Being impatient and carefree I tried an experiment using six LED's and a 9V PP3 battery. I made two blocks holding 3 LED's each. The LED's are wired in series and the blocks wired in parallel. ie each block receives 9V which is fed in series to 3 LED's.

It works at the moment but I don't know how long it'll last. What are the dangers? Too much draw draining the battery rapidly or too much power burning out the LED's?

 

[Hero999]

What resistors are you using?

The current might be too high which will cause the LEDs to overheat.

It's also possible that the current is very low and the LEDs won't last long and will stop working when the voltage drops much below 9V.

 

[Vizier87]

Uncle Hero: I'd like to ask since this joule looters caught my undivided attention now rather than the LEDs, can we actually connect several joule thieves together to get a larger output current/voltage to feed hungrier devices?

 

[Tipsy]

I have no resistors at all as I don't believe I have all the data required to make the calculation.

My (novice) thinking is this;

If the battery drains too fast, add a second in parallel.
If the LED's burn out (replace them) and add resistance.

 

[audioguru]

A new 9V alkaline battery has enough current to instantly burn out three 2V red LEDs in series.
If the three LEDs are 3.2V white or blue ones then they will be very dim when the 9V battery is new and will not light when the battery is run down a little.

 

[Tipsy]

The reality is the battery is not new and the white ~3V LED's provide very good illumination at first glance. Going by what you're saying Uncle Scrooge, I won't burn out the LED's but might find the battery's won't last very long? What about two PP3's in parallel?

 

[audioguru]

When a battery voltage runs down a little then the current (and brightness) of series LEDs without a current-limiting resistor will drop very quickly.

Two 9V batteries in parallel will last a little longer than double the life of a single battery.