Voltage Question

[webley445]

Mfg. - TOSHIBA
Mfg. # TLYH180P(U2)
Operates on 2.1 - 2.5
VDC @ 20 mA 8000 mcd typical
luminosity T1-3/4 (5mm),
colorless, transparent Super
ultra bright, lights yellow beam
Brightest 5mm LED
*****************************************
would it be safe to direct the described led off of 2AA or AAA batts? what if I have 2-4 in series?

 

[INRETECH]

Since the LED runs at 2.1-2.5v, one battery would not work - you would need two, and then a limiting resistor to protect the LED

Good old Ohms Law again saves the day

I=E/R

Or

R=E/I

Since 2 batteries would be 3v, we need to "get rid of" 0.5v:

R=0.5/.020

Or about 25 ohms

When designing with LEDs, simply put the number of LEDs 1-N into a string to best match your power supply, whatever voltage is left - must be dissipated by a resistor

If you wanted to run 2.5v LEDs from a 12v power source, it would be 4 LEDs - which would use up 10v, you would need to get "rid of" 2v

R=2/.020

Approx 100 ohms

 

[EMPOWERTORCH]

This is the Maplin Part No PF07Y or something simi;lar... I havent got a catalogue in front of me but I've used all the LED's in the series in direct-drive torch applications. If you heatsink the cathode leg well, (I use the torch's switch ring), you should have no problems with direct drive. In practice I use Zinc chloride cells as these are cheap to buy and still give many hours operation of an LED. They have a comparitively high internal resistance themselves which limits the current flow anyway. NiCads and NiMH rechargeables will also power your torch using direct drive (1.2 * 2 = 2.4V).
Incidentally, this LED forms the "Lemons" part of "Oranges & Lemons". This torch has operated reliably as an Edc for several weeks now!
The light colour of he Toshiba yellow LED is almost identical to the colour of low pressure sodium bulb light (street lights). The beam of a single LED is very filment-torch like, this model having a very nsrrow 8 degree beam angle.
The green in the series isn't as bright as the rest of he LED's, Maplin quote 1600mcd. But, a torch using the green LED is remarkably effective owing to the visiblity of the light! Other colours include red and orange.

 

[INRETECH]

Alkaline batteries have a pretty high ESR so you might be able to not use the resistor; always use a current meter when designing circuits

The parts are designed for 20ma, running them higher will affect their MTBF

The small leads on the 5mm package are very poor for heatsinking

 

[php_44]

Originally posted by INRETECH:
Alkaline batteries have a pretty high ESR....

<font size="2" face="Verdana, Arial">I think you meant to say "Alkaline batteries have a pretty LOW internal resistance" (effective series resistance). In fact, fresh alkaline cells have an internal resistance of about 0.1 ohm. Effectively, directly connecting an LED with no series resistor would mean you are relying on the LED's dynamic resistance to prevent it from self destructing. I'd use a series resistor - even a small one.

Some LEDs have a V/I curve that shows a tendancy for the LED to self limit current at voltages higher than the LED's voltage at 20mA. Given two brands of RED LED that drop 2.4V at 20mA, one may only consume 35mA at 3V and another 60mA. I might direct connect the first to a pair of alkaline cells, but not the second.

 

[INRETECH]

Compared to NiCads, Alkaline batteries have a higher ESR

But, whenever the current is higher than the companies reccomended value; one should always use a resistor to limit it down

 

[php_44]

Originally posted by INRETECH:
Compared to NiCads, Alkaline batteries have a higher ESR

But, whenever the current is higher than the companies reccomended value; one should always use a resistor to limit it down

<font size="2" face="Verdana, Arial">Yup, NiCads & NiMH cells have a lower internal resistance than alkaline cells. The data sheets I have specify NiCD and NiMH AA size cells as having an internal resistance of 0.04 ohms. The same size alkaline cell is about 0.1 ohms fresh, and a "heavy duty" cell is about 0.4 ohms fresh. Still too low for direct connection unless you're drawing a lot of current.