Two Simple Regulating LED Driver Circuits

[MrAl]

Hello,

Here are a couple of LED driver circuits for the smaller
LED's but could probably be modified for larger ones too.
They both use transistors commonly available all over
the world. The two methods of regulation require an
extra transistor and one or two resistors, but these
are very cheap transistors anyway. The inductor used
is a standard two lead device that doesnt require any
extra windings to complicate the construction.

The first circuit regulates by measuring the current
getting to the output, and so with low output voltages
(3.5v) the base emitter drop of the current measuring
transistor eats up about 15 percent efficiency. With
higher voltage outputs (10v or more) it's not nearly
as bad though.

The second circuit regulates by measuring the output
voltage and comparing it with a white LED running at
lower current. This technique doesnt lower the
efficiency like the direct current measurement method
although it wont regulate as well, although not that bad
either :-)

Here they are...

Note that you can always run them without any regulation
as many people do.

If you try any of these please post results if you can.


Take care,
Al

 

[eebowler]

Thank you MrAl. /ubbthreads/images/graemlins/happy14.gif Now, my problem lies in getting a damn coil! /ubbthreads/images/graemlins/jpshakehead.gif

I yanked out some inductors from a discman the other day. Do yo think they will work? I have no idea what their values are.

 

[vicbin]

Hi again sensei, /ubbthreads/images/graemlins/bowdown.gif

Again, thank you so much and these two circuits are now in my Discrete LED Booster circuit collections album. /ubbthreads/images/graemlins/grin.gif

Now questions :

- Although not as mighty as FMMT617, here I can get these low VCE sat transistor (2SC2500, VCESat=0.2V @ 2Amp) PDF Datasheet for 2 cents a piece ! /ubbthreads/images/graemlins/smile.gif Any modification needed if I replace Q2 with it ?

- Since the Q3 for both design is sensing the current right ? What if I can replace it with generic Germanium transistor (lower VBE), will there any "significant" improvement on the effficiency ?


To other poor CPFer like me,

The keyword here is "finding the correct inductor" and if you can't afford a LC meter, this circuit might help you to your conquest to find a correct inductor.

Simple RF Inductance Meter Circuit

PS: These days, cheap China made LCR meter cost only about 10 bucks.

Thanks & regards,
Vic

 

[ViReN]

Hello Mr Al,

Amazing... thats all i can say /ubbthreads/images/graemlins/smile.gif

Thank you for putting up such a great circuit. I will be working on this on Monday as i am going out of station for a while.

I will surely post the results, PCB & the Optimized layout of this circuit.

The Main trouble for me will be finding out an Inductor... as it is one of the Key Components. Since i will be out, i will be searching there, if i could get any better components (especially) the inductor.

What kind of efficiency should i expect from the circuit? It will be helpful for me to compare this with the actual results.

Also, since i will be using lower Currents & Higher Voltages at output, is it ok if i use 1N4148 Diodes ?

The First Circuit is Current Regulating, where as the second circuit is Voltage Regulating ?? (assuming that we are sensing Current in first and Voltage in later?)

Thanks & Regards,
ViReN

 

[MrAl]

Hello again,

Vic:
Oh thanks for the link to the meter...looks interesting.
I think you can use that transistor without too much
trouble if you try different values of resistor for
either R1 or R2. I dont know if the Vsat will make
all that much difference or not at low currents but
at higher currents (maybe 4 or more LED's) or
higher output voltages it may make a difference.
I didnt think it was 'that' good, but yes it's probably
better than the 2N4401 with higher collector current.

What makes a big difference is the inductor.
If you can order from Digikey, find an inductor with
low series resistance (0.5 ohms or less). They are
a little expensive for one part (maybe $2) but it will
work very well.

I've tested this circuit (without regulation) down to
1.0 volts and was able to get 30ma output with a good
inductor of 68uH and 0.25 ohms series resistance
with one LED. I made a mistake on the transistor type in
the simulation environment, so the resistances R1 and R2
can both be doubled if using those exact transistors.
Maybe i'll correct the schematic, but usually you want to
try different values for those anyway.
At 30ma the LED is nice and bright too. Maybe i'll try
40ma too.

If you have a really hard time getting inductors, try
winding 20 or more turns of #22 wire around a 0.25 inch
form. This will make an air core inductor.
Wind the turns close together and even more if you
wish. I dont know
how bad the efficiency will be, but you can still easily
get 10ma out and if you modify resistor values you'll
probably get more too.

Using a germanium transistor is a good idea too,
if you lower the sense resistor on the base.
That should bring the eff back up quite a bit with
3.5v output.


Viren:
Oh im glad you like the circuits... i like these two also and have
tried one with out the regulation and got better than expected
results (of course using good 68uH, 0.25 ohms inductor).
I've also tried air core inductor and it did work although less
output for same resistor values (R1=8k and R2=100 ohms).

Those little SMD inductors seem to work pretty well. I got
30ma output for only 1v input! Very nice if you ask me :-)
Digikey has inductors but you could also try the air core
design if you want to. I know digikey has high shipping
to your part of the world, im sorry about that. Each inductor
is rather high too at around $2 for one part while everything
else is pennies. Doesnt seem fair :-)
What would it cost me to mail you one inductor over there
to try out?

The efficiency you should expect is from 60 to 70 for the
'voltage' regulating model, and only 50 percent for the
current regulating model. Using a germanium transistor
for the current sense (as Vic here has suggested)
would raise this back to maybe 60 percent.

I would think 1N4148 diodes should be ok if you have high
output voltage because that particular drop doesnt matter as
much as when you have only 3.5 volt output. In fact, the loss
roughly halves for each double in output voltage. If you have
three times the output voltage then you have about 1/3 the loss
that you would at 3.5 volts out.

Yes the first circuit actually measures the output current while
the second circuit uses another method which is like voltage
regulation, although it uses a white LED for sensing too so it
should be better than pure voltage regulation over temperature.


Take care,
Al