4 Transistor 4 White LED Current Regulating Circuit

MrAl

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Here's a new circuit to add to the collection:)

It regulates to about 2% in simulations
with battery voltages from 2 to 3 volts
which would make it ideal for two cell
NiCd or Alkaline applications.
It regulates to about 10% with battery
voltages from 1.5 to 4 volts.
It uses 4 transistors.
Please read all the notes if you intend to
use it.

Here it is:
http://members.aol.com/xaxo/page1.html

Good luck, and if you use it please let us
know how you make out with it ok? Thanks!

Al
 

MrAl

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jeff1500:
Very nice. I'll certainly post results if I build it.

How come four diodes all in a row for D2 to D5?

What do you think about this? It's a zener diode voltage regulator. Just by luck of the search engine, it looks like it's from Satcure.
http://www.netcentral.co.uk/satcure/design/tranex1.htm
<HR></BLOCKQUOTE>


Hi again Jeff,
The four diodes in a row are used as a
voltage reference. This is a very
important part of the design because
when the output voltage rises too much,
the diodes conduct more heavily, turning
the transistor Q3 on harder, which serves
to regulate the output current in the long
run. As i noted, the diodes forward
voltage drop changes with temperature,
approx 2mv per degree C. Since Q3's BE
diode also changes that much, the total
change is about 4 diode drop changes
minus 1 diode drop change, or about
3 diode drop changes total, which comes out
to about 2mv*3 = .006 volt per degree C.
This means the average voltage across the
output capacitor (not the led's) changes by
about .006 volts per degree C change in
temperature.
Lets now look at a +20 degree change in temperature.
For that change, the output voltage changes
by .006*20= .120 volts. Since each 10 ohm
resistor drops about 0.22 volts while running
each LED at about 22ma and 3.5 volts, the output
voltage is about 3.5+.22=3.72 volts.
Now changing that voltage by -0.120 volts results in
an output voltage of 3.60 volts. Applying that
output voltage to the series combo of 10 ohm and
one white LED results in the current dropping
to about 18ma, which is about -10% regulation
with a temperature change of 20 degrees C.

Hmmm, this isnt quite as bad as i thought.
Worst case, with the combination of very very
low input voltage of 1.5 volts and a 45 degree
ambient temperature, the regulation would drop
to -20% (or about 16ma), but only at the extreme
of 1.5 volts input and 20 degrees C increase in
temperature.
The thing is, replacing the 4 series diode with
a 2.2 volt zener diode (anode towards Q3's base),
and it looks like the temperature stability of
the circuit improves by a factor of 10. This
could be the answer, instead of playing with
a thermistor. This would make the output
current drop by only about 1ma with a 20 degree
C change in temperature.
I'd have to check this out though,
because those very low voltage zener diode vi
curves arent very sharp. It would need actual
real life testing for temperature stability.

I checked out the zener regulator circuit, and
that may or may not be a circuit you would
want to use for LED drivers running off of
batteries. It will act as a constant current
regulator, but at a cost of low efficiency.
For example, if the input voltage is 6 volts
and the LED voltage is 3.5 volts at 22ma, then
the power in is 6*.022=.142 watts, and the
power out is 3.5*.022=.077, and the eff is
Pout/Pin=0.58, only 58%. This means for
every 100 batteries you buy for the light,
about 42 are wasted, and only 58 go to
powering the light. It is a solution
just the same :)
If your running a few LED's off the car battery,
with the engine turned off the eff drops to about
29%, but who cares in that app. If your running
off of a stand alone battery you may not like this,
because for every 100 batteries you buy, 71 are
wasted and only 29 actually go toward powering the
light. Instead, using a switching regulator
(down converter) you can get at least 85% eff.

I dont know if your following the thread on
the ZXSC300 chip, but in that thread we are
trying to implement this chip for sort of
general purpose LED driving. We hope to
get it to work for a wide range of outputs
and input voltages. You might find this of
interest also. Apparently the chips are getting
easier to get now too, but there are other
chips out there too which deserve consideration.
One of the things i think i realized was that
for driving heavy output currents using a
single cell may not be a good idea anyway, as
the current draw goes WAY up for multiple
LED's (like 10 or so). So perhaps two
separate general solutions are in order anyway:
one for less then maybe 5 LED's, and another
one for more then 5 LED's. Or maybe a few solutions,
depending on how many LED's and what kind of
battery you want to use (except for more then
5 LED's you would have to use at least two cells
in series).

I think i might breadboard that 4 transistor
circuit just to see how well it performs
relative to the simulations :)
Interestingly, because it uses a real error
integration scheme the individual gains
of the transistors arent as important as they
are in the pure brinkmann circuit. The only
adjustment should be that one resistor!
BTW, all the parts in that circuit are all low
cost, but did it look like it had too many parts
to be practical?

Well thanks for your input on these circuits
and for the other ideas too.
--Al
 

MrAl

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Just a note:

I've updated the schematic of the
4 transistor 4 LED driver circuit to
use only one series diode.
Since the temp co's of the diode and
Q3's BE diode are about equal, the
circuit should perform adequately over
a wide variation in temperature.
I'll have to build it up and take it
outside on a cold night and try it out:)

Take care,
--Al

Added later:
Scratch that:) the diodes are in series
and so there will be some temperature
error still, just not quite as large
as before. Perhaps it will be low enough
though.

--Al
 

MrAl

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I finally got around to building the circuit
up :)
It works pretty close to what the
simulations predicted. From what you can
see below, the circuit works pretty darn
good for a total cost of under $2.00 about.
It turned out to be a very good circuit.
I have a feeling there is a way to get
it to work down lower yet for one
battery operation with maybe 4 LED's or less.

Here are the final measurements:

4 transistor LED current regulating circuit

Notes:
Single white LED with 10 ohm in series.
R2 changed to 1k for single LED app., which means
250 ohms for 4 leds might be ok too, depending on
the gain of Q2.
Changed R7 to a standard 15k resistor and adjusted
R8 to about 18k, which gave 20ma out at 2.0 volts input.
The 1uf cap (C4) was a cheap electrolytic type.
The output cap (C2) was 10uf, which would probably
work with 4 led's in parallel too.
Two inductor values were tried, and both values worked
about the same; they were 200uH and 1000uH.

Actual circuit measurements 11/2001:

Vin Iout

1.2v 10ma
1.5v 14ma
2.0v 20ma
2.5v 22ma
3.0v 22ma
3.5v 24ma
3.8v 26ma
4.2v 26ma
4.4v 30ma
4.5v 40ma (osc shuts down, direct feed though.)

As shown above, the current is very well
regulated from 2.0v to 3.0v, and not bad
outside that range either. This means it
would look like an almost perfect regulator
for 2 cell apps.
Also shown above, the current with 4.5 volts
input shuts down the oscillator, which isnt
that bad in itself, but the current goes up
to 40ma, so i wouldnt recommend using it
above 4.5 volts, and in most cases less then
4.1 volts unless you dont mind driving
the led's at 40ma at this high input.
I drive some of my flashlights at 40ma
because i want a lot of brightness and
i dont intend to keep them on for too long
at a time.

Good luck with your LED circuits,
--Al
 

jeff1500

Enlightened
Joined
Jul 9, 2001
Messages
417
Very good.

How long do you think two "AA" batteries would last with two or three leds?

Does the value of R7 or R9 depend on transistor gain or how many leds are used?

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR> I dont know if your following the thread on the ZXSC300 chip, but in that thread we are trying to implement this chip for sort of general purpose LED driving. We hope to get it to work for a wide range of outputs and input voltages. <HR></BLOCKQUOTE>

How big is the chip? How are you going to make connections to it?
 

MrAl

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jeff1500:
Very good.

How long do you think two "AA" batteries would last with two or three leds?

Does the value of R7 or R9 depend on transistor gain or how many leds are used?

How big is the chip? How are you going to make connections to it?
<HR></BLOCKQUOTE>


Hi there again Jeff,

The circuit will be just as efficient as
the Brinkmann circuit, so however long
the battery lasts with that should be
about the same with this circuit.
R7 and R8 shouldnt depend on circuit gain
at all. Once one of these values
is adjusted, the transistor gain in the
oscillator isnt important anymore because
the circuit measures the error between
set point and what it really is and uses
that error as feedback to adjust the output
current. Since these gains are in the
forward regulation path, a change in gain
in the oscillator isnt noticed after the
integration period of about 3ms or so.
Oh one other note, to make sure the
circuit oscillates, you might want to
short the junction of R1 and R5 to ground
with an input of about 2.0 volts dc.
Thats a good check to make sure the
oscillator part of the circuit is wired
correctly before the circuit attemps to
regulate.
Also, i would recommend operating the circuit
with lower output current to make sure
all the connections are correct before
hooking up several LED's. R2 can be changed
to 1k temporarily for this test. If it works
with one LED, then reduct R2 to about 250 ohms
and try it with the 4 LED's.
I really liked the way it regulated or
i would drop this circuit in a second:)
It only took about 20 mins to plug in
all the parts into the socket board for
testing.

The 300 chip is a rather small chip so it's
going to present some problems with soldering.
Perhaps a very small heak sinking
pliers clamped on the lead right next to the
plastic and quick soldering right on the tip
of the lead. Any ideas here?
Any ideas how much a special soldering
station would cost to solder these
small outline chips? Or where to get them?


--Al
 

MrAl

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Update:

I was able to find a simpler solution
then the 4 transistor circuit.
It uses only two transistors and one
very common 8 pin ic chip and one
voltage reference diode.
This is a down and dirty full regulator
circuit now :)

The nice thing is, the simulations show
almost no perceptable change in the
output current with inputs from
1.4 volts to 3.6 volts. Im guessing its
about 0.1 percent on regulation
(that's one tenth of one percent!)

Also, there wont be any temperature
problems either, as the right reference
diode will already be temperature
compensated. he he :)

I should have it up and running in real life
in a few days hopefully, then ill post the
results and details.

Add another circuit to the list :)
--Al
 

jeff1500

Enlightened
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Messages
417
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR> I was able to find a simpler solution then the 4 transistor circuit. It uses only two transistors and one very common 8 pin ic chip and one voltage reference diode. <HR></BLOCKQUOTE>

Low parts count. That's a nice improvement.
 

KenBar

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Messages
766
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
It uses only two transistors and one
very common 8 pin ic chip and one
voltage reference diode.


<HR></BLOCKQUOTE>

I have a problem finding what is common parts for some. Would it be possible to post a source for the chip,etc and a new thread when you get to it please? This has got me wanting to build something again!
 
D

**DONOTDELETE**

Guest
A good source of parts is www.digikey.com.

Go online and request a free 832-page catalog. When ordering, mail them a check (instead of using a credit card online) and get free shipping. Order $25 or more and waive the $5 handling charge.
 

MrAl

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Hi again,

Jeff: yeah im trying to lower the parts count :) its not that easy
because it has to have a certain minimum amount of functioning.

Kenbar: Ever see a LM358 or LM339 in a surplus catalog? About 50 cents
or something :) I think the LM339 will work out better then the LM358.
Another chip might work too, like the LM393 8pin comparator, but i havent
tryed that chip yet, only the 358 and 339 and the 339 works better because
it's output can swing from +V to ground.

Duggg: You mean digi key doesnt charge for shipping if you mail a check?
That sounds pretty nice, how about online credit card? What if i only
request one part for 1.60, after they tack on $5 handling they wont charge
shipping on top of that?

Good luck with your LED circuits,
--Al
 

KenBar

Enlightened
Joined
Jan 24, 2001
Messages
766
Thanks Duggg. Mr Al, am I on the right track with this. It does not look discontinued.
Here is a novel use for a 339.
board.jpg


Power use for 339

358

358
 
D

**DONOTDELETE**

Guest
MrAl,

If you use a credit card (online, by phone, or by mail), they will charge you shipping (at cost), plus a $5 handling charge if the subtotal (excluding shipping) is under $25.

If you don't want to order $25 worth of stuff, mail them a check for the subtotal plus the $5 handling charge, and they will not add shipping on top of that.

The key is, to get free shipping, you must send them a check or money order.
 

MrAl

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Yeah thats the part Kenbar ! he he.
Did you build that thing up?

Duggg:
thanks for the info, i find that quite
unusual that they seem to prefer checks
over credit cards? Not paying shipping
is nice :) I hate shipping, but i also
hate under 25 dollar extra charges, but
i guess its better then both charges.

Thanks again,
--Al
 

Steelwolf

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MrAl: Not sure if the LM339 or 358 comparators will work all that well if you intend them for single cell operation. Data sheet says that the minimum input voltage required is 3V.

So it might need a jump start, or it might be suitable only as a regulator in a buck configuration. Also, the LM339 chip is quite large, but I think only 1 of the 4 gates needs to be used, so not space efficient.
 

MrAl

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Steelwolf:
MrAl: Not sure if the LM339 or 358 comparators will work all that well if you intend them for single cell operation. Data sheet says that the minimum input voltage required is 3V.

So it might need a jump start, or it might be suitable only as a regulator in a buck configuration. Also, the LM339 chip is quite large, but I think only 1 of the 4 gates needs to be used, so not space efficient.
<HR></BLOCKQUOTE>

Yeah, there's a trick to it :)
Hint: the output has to drive a 3.5v led
right? :)
That's how it works now down to less
then 2 volts as is.

Also, im looking at using the
LM393, which only has two comparators,
so its an 8 pin dip package. It's
pretty small i guess. This wont be
the smallest circuit on earth though,
but it sure is looking a lot smaller
now.
We are still working on getting some
of the Zetex 300 chips also, which could
work out pretty nice.
Also, i got another design now that im
working on that only uses one transistor
and maybe the LM393 comparator chip.
I got it working from 2 volts
to 3 volts, but have no success with
1 to 1.5 volts with perfect regulation.
I was hoping to use it with one cell
or two cells. Looks like it will only
work really good with two cells, and
somewhat decent but not perfect with
one cell. Then i'm trying to simplify
the circuit as much as possible to reduce
overall parts count. One drawback to
this newer one transistor/opamp design
is that it requires winding a transformer
similar to the earlier one transistor
circuit, but the windings cant be combined
into a center tapped config, they have to
remain totally isolated windings. It's not
that much different, but its harder to wind
this kind of coil then a simple inductor.
I'll post results as soon as i think i
have something stable working. Right now
it still sorta conceptual, although the
2 to 3 volt version simulation shows that
it does work with extremely good regulation.
I still tend to like the two transistor
one opamp circuit better though, its more
stable to begin with. Besides, i have real
life testing on the 4 transistor circuit
that tells me the basic ideas are pretty
sound.
I'll see what happens over the next few days
with the one transistor/opamp circuit.
Mercator (also on here) and i are trying to
get the parts to build up that zetex chip
design and see how that works. We do have
high hopes for that circuit. If that doesnt
work, we may go to the Zetex 100 chip we have
discussed, although that chip is harder to
deal with because its a regular 8 pin SM
chip.

Good luck with all your LED circuits,
--Al
 

MrAl

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Hello again,

I just got my samples from National today
so soon i'll be able to test the two
transistor/op amp circuit. It looks
very promising and very low cost, with
no critically selected or hard to get parts.

So far the parts list looks like this:
2 common, low cost transistors
1 common op amp (dip or SM)
2 caps
5 resistors
1 reference diode or zener diode

For the die hard current regulation
fanatics, i'll be creating an absolute
current measurement scheme also, which will
actually measure the current through the
LED (or LS) and correct it as necessary,
to near absolute perfection.
In fact, the typical error over the
complete temperature range of -40 degrees C
to +85 degrees C will only be about 0.2
percent (two tenths of one percent).

Now just in case your a super current
regulation fanatic,(LS or multiple LED apps
only), you can get the regulation down about
10 times better than that with the selection
of a better reference diode ($5.00 US).
Because the second config will actually
regulate the current, it will adapt to
any voltage LED, and even if the voltage
changes over temperature the current will
STILL remain constant.
The only drawback is this second config will
require about 4 more resistors.

I'll be posting info as soon as i get it
up and running. Right now it is
theoretically a very sound design, using
techniques that are found in very
high current regulator circuits.

Now if you will excuse me, i have some
VERY tiny soldering to do :) he he.
Actually, i have to update my soldering
equipment first :-(
The other day i picked up a pack of
soft steel 10 penny nails, so i have to
get out the tap and die set and grinder and
create some tiny soldering tips. I've used
8 penny nails in the past, i think they tap
to size 6-32 without grinding down the
diameter first. The 10 penny ones look
better for 8-32 size iron tips. I have
also used brass rod stock, commonly available
at hobby shops, which conducts pretty well
although the tip has to be shorter than
when using copper stock.

Good luck with your LED circuits,
--Al
 

MrAl

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Messages
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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jeff1500:
Sounds promising.<HR></BLOCKQUOTE>

Oh hi there Jeff, i was wondering if you
were still with us :)
Have you been trying out any new (or old)
LED circuits lately?

--Al
 
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