Maker Pro
Maker Pro

Real Quick LED Driver Question

T

Tom

Jan 1, 1970
0
I am trying to drive an LED (2.1Vf, 20mA) from a digital signal
(3.3V). From what I have been able to understand, the logic level
output I am using does not provide enough current to drive an LED
brightly. I have another power souse (7.4V) on board to drive the
LEDs, but I do not know what transistor to use. There are so many
different ones. What would be my best option? Can someone point me
out to a model number?


Thanks,
Tom
 
D

DJ Delorie

Jan 1, 1970
0
Tom said:
What would be my best option? Can someone point me out to a model
number?

Pretty much anything will do, if it's just for blinkenlights (if it's
a fiberoptic laser LED, that's a different issue). You need either
an NPN bipolar or N-channel MOSFET.

On my furnace board, I used ON Semi's DTC144EET1G, mostly because I
wanted a SOT-523 (i.e. really small) package. This is an NPN bipolar
with a built-in base resistor, since you need some sort of resistor
when driving bipolar transistors. Then you have to do the math - how
much Ice current do you want, divide by worst-case Hfe (gain) to get
Ibe, determine voltage difference between worst case Voh of your
driver and highest Vbe, select base resistor.

For N-MOSFETs it's a little easier, as no base resistor is needed and
the turn-on voltage drop across the MOSFET is very little; you can
almost pick the resistor as if the resistor/led were connected right
to the voltage rails. You just have to pick any N-MOSFET with a Vgs
turn-on voltage sufficiently below your minimum Voh, and a Ids current
capability high enough for your LED.

To recap:

1. Select a physical size, like TO-92 or SOT-23.

2. Choose between bipolar or MOSFET.

3. Current capability: For bipolar, select for Ice. For MOSFET,
select for Ids. Choose at least 2x your expected LED current.

4. Pick the cheapest one that's still listed ;-)
 
J

John Fields

Jan 1, 1970
0
I am trying to drive an LED (2.1Vf, 20mA) from a digital signal
(3.3V). From what I have been able to understand, the logic level
output I am using does not provide enough current to drive an LED
brightly. I have another power souse (7.4V) on board to drive the
LEDs, but I do not know what transistor to use. There are so many
different ones. What would be my best option? Can someone point me
out to a model number?
 
P

Paul E. Schoen

Jan 1, 1970
0
My personal favorite NPN is MPSA06 (TO92), and for higher power PNP I like
MJE170 (TO225). Mostly because I have several hundred of the one and a
couple thousand of the other, among my surplus stock. I design many of my
projects around these and other components I have in bulk, unless I have to
compromise too much on specs.

For driving 8 LEDs (or even relays) it's hard to beat the 18 pin ULN2803
octal darlington driver.

Paul
 
M

Meat Plow

Jan 1, 1970
0
I am trying to drive an LED (2.1Vf, 20mA) from a digital signal
(3.3V). From what I have been able to understand, the logic level
output I am using does not provide enough current to drive an LED
brightly. I have another power souse (7.4V) on board to drive the
LEDs, but I do not know what transistor to use. There are so many
different ones. What would be my best option? Can someone point me
out to a model number?


Thanks,

Wasn't quick enough.
--
#1 Offishul Ruiner of Usenet, March 2007
#1 Usenet Asshole, March 2007
#1 Bartlo Pset, March 13-24 2007
#10 Most hated Usenetizen of all time
Pierre Salinger Memorial Hook, Line & Sinker, June 2004
COOSN-266-06-25794
 
F

feebo

Jan 1, 1970
0
I am trying to drive an LED (2.1Vf, 20mA) from a digital signal
(3.3V). From what I have been able to understand, the logic level
output I am using does not provide enough current to drive an LED
brightly. I have another power souse (7.4V) on board to drive the
LEDs, but I do not know what transistor to use. There are so many
different ones. What would be my best option? Can someone point me
out to a model number?


Thanks,
Tom

any GP NPN signal transistor will do really - 2N4124, BC107, BC184,
ZTX300 anything really - provided you drive it in common emitter mode
(i.e the LED & resistor to the collector, emitter to ground and drive
the base with your 3.3v signal via a resistor). Base resistor needs to
be about 1K5 and your LED resistor will be about 270R. Should light up
nicely.
 
Top