Line Driven LED + Waveforms

[MrAl]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jeff1500:
I got it working, but now I'm full of questions again.

(1)
Q1, pnp, NTE12, measured hFE=362
Q2, npn, NTE11, measured hFE=374

3. Here's the thing. It's hard to start it up. I've got R2 set up as four 1k resistors in series. When I first hook up power, the led(s) glow dimly, no voltage gain. If I jump the R2 series for an instant to short it to 2k, it lights up to full brightness and then runs well on R2 = 4k. It also starts if I pulse the positive battery connection kind of like striking a match. (It starts like an old car I used to have.) How do I fix this? I think all the connections are sound. Is it that big voltage rating on C1?

(2)
This is the circuit that uses two NPNs. The article says it also provides voltage regulation.

I think all I have to do is get a few more resistors and then I can build and test this one too. What resistor values would be appropriate for multiple leds? The pattern must be similar.<HR></BLOCKQUOTE>

(1)
You shouldnt have to worry about the
voltage rating of the cap.
It could be the PNP transistor is not
functioning the same as the 2N4403 PNP
would. Perhaps you can come down a little
on the 10k resistor (R1). Try 8k, then
6k, then 4k. What voltage are you
running it from? 1.5 or 3 volts?
I meant to tell you also that you dont need
a heavy duty transistor like the NTE12 for
that particular app.
Another problem could be an extremely
nonlinear coil characteristic. What are
you using for the coil now? The little
rf coils wont be as good as the hand wound
toroids, but they should work anyway.
One thing to think about also is that you
can provide a push button switch for the
'kick start' and have high and low setting
automatically-- the low setting would
provide a little night light output
while using very little battery power.
I built a small 2 LED flashlight like this
also, but i had to use a slide switch for
the low setting. It comes in handy when you
want to have a little light but dont want
to run down the battery at all.

(2)
The two NPN circuit:
I didnt get to look at this yet, but thanks
for reminding me. Since they do claim
that it voltage regulates, i'll take a
closer look at this circuit. I should have
some useable results by the end of the day
or tomorrow morning.

--Al

 

[MrAl]

Well, i took a look at that two NPN transistor
circuit, and i found that it appears to
regulate over some range of input voltage,
but then as soon as you put even a small
source impedance in series with the battery
(which every real battery has)
the regulation seems to go away, and it
doesnt regulate any longer.
My guess is that someone designed this
without checking it out with the battery
having some series resistance.
I'd like to build the actual circuit up,
but i dont know if i'll get to it or not.
Also, the regulation wont cover a wide
range of voltages, just a small range, if
it turns out to work at all using a real
life battery.
Also, i had to use 2N4401 transistors
as the 2N3904's didnt seem to have enough
gain and only drove the LED to about 5ma.
The 2N4401 took it up to 20ma, but only after
some resistor adjustments. Also, the wave
out of this circuit is very nasty looking,
driving the LED with high spikes instead of
a 50% duty cycle or a smooth dc. Efficiency
is the worst i have seen of all the circuits,
being some 60% or less. This equates to
using up 10 batteries when 6 would have
worked at 100%.
I cant say that i would want to use this
circuit.

--Al

 

[PeLu]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by MrAl:
Well, i took a look at that two NPN transistor
circuit, and i found that it appears to
regulate over some range of input voltage,<HR></BLOCKQUOTE>

As the description says, it behaves more like a constant power unit.

 

[MrAl]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by PeLu:
As the description says, it behaves more like a constant power unit.<HR></BLOCKQUOTE>

PeLu,

What is the significance of this?
Regulating the 'voltage across' or
'current through' an LED automatically
regulates the 'power' , right?

--Al

 

[jeff1500]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by MrAl:
Perhaps you can come down a little
on the 10k resistor (R1). Try 8k, then
6k, then 4k. What voltage are you
running it from? 1.5 or 3 volts? What are you using for the coil now? The little rf coils wont be as good as the hand wound toroids, but they should work anyway.
<HR></BLOCKQUOTE>

I'll vary R1 to see what happens. I get the same result using 3 volts or 2.4 volts. The coil is a hand wound ferrite torroid about 3/8" in diameter, 1/8" hole, 1/8" long, 20 turns of #26 wire.

The two npn circuit explanation says the size of the coil determins the oscillation frequency in that other design. Maybe I should try a different number of turns around the coil too.

 

[PeLu]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by MrAl:
What is the significance of this?
Regulating the 'voltage across' or
'current through' an LED automatically
regulates the 'power' <HR></BLOCKQUOTE>

In a give range (of load) it should 'feed' the same amount of power into it. When you connect a LED with 1.8V it will put some 20mA through it, when you use a 3.6V one, there will be only 10mA flowing. That's the theory. It neither regulates voltage nor current.
Usual flyback converters worked in a way that some amount of energy is put into a coil at the charging part every cycle. On the discharge half-cycle the coil has to get rid off this energy and its left over to the coil how to do that. If load resistance is higher, output voltage will be higher and vice versa. Although regulation is not very good.
(I tried to use non EE terms above .-)
(of course not for you, Mral)

So you may use different leds and getting all the power.
And if you connect more than 1 LED, you will not get more power (unlike in a constant current or constant voltage application).

 

[MrAl]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jeff1500:
I'll vary R1 to see what happens. I get the same result using 3 volts or 2.4 volts. The coil is a hand wound ferrite torroid about 3/8" in diameter, 1/8" hole, 1/8" long, 20 turns of #26 wire.

The two npn circuit explanation says the size of the coil determins the oscillation frequency in that other design. Maybe I should try a different number of turns around the coil too.<HR></BLOCKQUOTE>

Hi again Jeff,

I have gotten this circuit to oscillate
with 5 to 25 turns on the small toroid.
The thing is, the PNP transistor was a
2N4403 in all my test circuits, and this
is a much lighter current device. I cant
say for sure using the NTE12 will act the
same, but changing R1 might increase current
to the NTE11 NPN. If you measure the
collector voltage of the PNP, it should be
close to the battery voltage on peaks.
By shorting part of the series makeup of
R2, you are providing more current to the
NPN.
If all else fails, try to get ahold of a
lower current PNP transistor, like the
2N4403 if you can.

Good luck with it,
--Al

 

[PeLu]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by MrAl:
What would be the purpose of providing
a circuit that regulates 'power' to an
LED ?
<HR></BLOCKQUOTE>

I think its only a side effect.

 

[jeff1500]

It's a great circuit. I'll get it to go. It might take me a little while though. Things suddenly got real busy around here; work, travel, stuff like that.

I'll bet a small change to R1 with maybe a subsequent small change to R2 will make it start right up. I'll post results.

It's running in the other room right now. Nice and bright with just the right amount of current in each of the two leds I've got hooked up at the moment. I just tap it with a safety pin to start it up.

My goal has been to make a huricane light to use if the power goes out in a storm.

 

[MrAl]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by PeLu:

When you connect a LED with 1.8V it will put some 20mA through it, when you use a 3.6V one, there will be only 10mA flowing. That's the theory. It neither regulates voltage nor current.

So you may use different leds and getting all the power.

<HR></BLOCKQUOTE>

Oh ok thanks for clearing that up 🙂
Interestingly enought though, is that it
does regulate the current though the LED
with a small range of battery voltages
(unlike those converters that dont make
any attempt at all to in the design)
but if you put a small resistance in series
with the battery the regulation gets worse
and worse as the resistance goes up. Since
every battery has some resistance, the
useability of this circuit for regulating
current through the LED somes into question.

The useability for driving a range of
different types or colors of LED's is
great, but then who, once the color or type
is determined, would then go ahead and
change the led type or color later on?
I guess the purpose then would be to
provide a single circuit that could
drive many types of led's without changing
any parts. That's not a bad idea, but
using the right circuit for the particular
LED seems best to me, especially when it
comes to driving white led's.

I also dont see how much good this circuit
will do for us if it cant regulate current.
Because it appears to regulate with no series
resistance, i assumed when they said it
regulates 'power' to the led they really
meant 'current' , as sometimes people say
'power' when they really mean 'current' or
'voltage' or even 'brightness'.

I got a circuit working that regulates
current and uses common parts, but it
requires a total of 4 transistors. I was
hoping to keep it to 2, but its not that
easy. I did come up with something that
uses 2 transistors that regulates to some
degree, but not as good as i had hoped.
Perhaps that will be good enough, ill have
to test it more.

I'll post results that come up good, mainly
for driving white LED's (or other 3.5v ones).

Good luck with all your LED circuits 🙂

--Al

 

[MrAl]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by PeLu:
When you connect a LED with 1.8V it will put some 20mA through it, when you use a 3.6V one, there will be only 10mA flowing. That's the theory. It neither regulates voltage nor current.
So you may use different leds and getting all the power.
<HR></BLOCKQUOTE>

Oh i just realized something else when
reading my first reply (above) over.

What would be the purpose of providing
a circuit that regulates 'power' to an
LED ?
In other words, why would someone want
to regulate a 1.8v LED at 20ma and a
3.6v LED at only 10ma?
If the circuit could regulate current,
ANY color LED used would conduct 20ma.
That seems pretty universally applicable
to me. You could use any LED you want
and be sure it would get 20ma no matter
what color it was.

Any ideas?
--Al

 

[MrAl]

Oh yes ok Jeff, that sounds cool.
So your overdriving your led's to 30ma each?

I made a small light with 2 led's
and put three settings on it:
low(very low), medium, and high.

The low setting is for long runs
or night light. The medium and
high setting were kind of to try out
two different settings:
20ma, and 40ma overdrive.
What i did notice at 40ma is that the light
output doesnt appear to be twice as bright
as the light output at 20ma. According
to the curve of intensity vs current
it is suppose to be almost twice as bright.
I'll have to do a more careful test, but
it really doesnt look twice as bright.
Perhaps 30ma is the best choice.

--Al

 

[PeLu]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by MrAl:
What i did notice at 40ma is that the light
output doesnt appear to be twice as bright
as the light output at 20ma.<HR></BLOCKQUOTE>

Taking 20mA as 100%, it has at 40mA about 78% of the efficiency or about 165% of the 20mA brightness.

 

[jeff1500]

I got it to work. Hurray.

It's over by the phone. So while I was talking I poked at it a little. I put another 10k resistor in parallel with the existing R1 to get 5k. Wouldn't start. Then I put it in series for R1 = 20k. No good.

Then I took out C2, the output smoothing capacitor. That let it start right up as soon as I made the battery connection.

I'll bet C2=47 micro F is way too big. I took some readings:

LED current (each), give or take a little:
R1=10k,R2=4k,
with C2=47 uF and a jump start, 23 mA
without C2, with diode, 17 mA
without C2, without diode, 20 mA

Looks like there's a little improvement with C2. Better current flow, better wave shape?

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR> C2=2uf per LED seems ok, the current doesnt have to be perfectly smooth. This means 2uf for 1 LED, 4uf for 2 LEDs, 8uf for 4 LEDs.<HR></BLOCKQUOTE>

Do you still like these values for C2?

Funny that the 47 uF capacitor works okay with the one transistor, double coil circuit?

 

[MrAl]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jeff1500:
I got it to work. Hurray.

I'll bet C2=47 micro F is way too big.

Do you still like these values for C2?

<HR></BLOCKQUOTE>

Glad to hear you got it working ok.

I found even 1uf per LED to work ok, but
0.5uf is getting a little skimpy because of
the waveshape. The smoother the waveshape,
the better, but it doesnt have to be
perfectly dc either 🙂

Of course all the tests i did were at 20ma
per LED also, but i dont think 30ma will
be that different.

I'll look at very large values for C2 also,
like 47uf. I havent done that yet.
Should be interesting to see what happens.
I was assuming you were setting the cap to
about 2uf per LED.

Take care for now,
--Al

 

[MrAl]

Oh i forgot to mention something:
when you disconnect the output cap,
you cant rely on the measurement of
current thought the LED using a digital
meter. The only time you can use this
kind of measurement is when the cap
and diode are both installed. The reason
is that the current pulses are high spikes
and the meter cant average them out
correctly. This means you cant compare
readings without the cap installed and
with the cap installed.

I took a look at the circuit with the larger
cap, and i can see it takes longer to
start up with larger output caps, but it
does in fact start eventually. Perhaps
in the real world circuit it wont start
because it needs a higher pulse of current
through the inductor to start it oscillating.
The lower the input voltage (battery) the
harder it is to start also.
I have also found that with very low inputs,
like 1.2vdc, the resistor R2 had to come
down as low as 500 ohms to get enough
current flowing through the LED with some
NPN transistors.

If you still have startup problems, you can
also try inserting a resistor in series with
the output cap, say about 1 ohm to start.
If that works, fine, but if not try 2 ohms.
You shouldnt have to go above 5 ohms.
The lower the better on this resistor.

Good luck with it and let me know how you make out with it...

BTW, im starting a new topic with this circuit which will really be a continuation
of this thread.

--Al