Maker Pro
Maker Pro

Transistor as a current limiter

L

Lauri Alanko

Jan 1, 1970
0
I'm sorry if I gave that impression. I merely gave an example
calculation, with realistic sample values, to demonstrate my concern
with the circuit in question. I hoped to emphasize that this was
merely hypothetical by using words such as "suppose", "load would be",
"if we designed" etc.

Besides, in the same example the supply voltage was 12 V, and that
didn't prevent you from suggesting otherwise. Surely there was nothing
to prevent you from suggesting different leds, as well?

In any case, I don't see how anyone's time has been wasted. All the
replies I've received have been most pertinent and educational, and
the circuits I've seen seem useful regardless of whether one plugs in
four 2.2 V leds or three 3.6 V leds per series.


Lauri
 
J

Jon Kirwan

Jan 1, 1970
0
I'm sorry if I gave that impression. I merely gave an example
calculation, with realistic sample values, to demonstrate my concern
with the circuit in question. I hoped to emphasize that this was
merely hypothetical by using words such as "suppose", "load would be",
"if we designed" etc.

Besides, in the same example the supply voltage was 12 V, and that
didn't prevent you from suggesting otherwise. Surely there was nothing
to prevent you from suggesting different leds, as well?

In any case, I don't see how anyone's time has been wasted. All the
replies I've received have been most pertinent and educational, and
the circuits I've seen seem useful regardless of whether one plugs in
four 2.2 V leds or three 3.6 V leds per series.

I didn't feel misaligned. (Well, I'm a hobbyist anyway.)

In any case, this is the 'basics' group. Does a question have
be asked as though it were framed by an expert here? No. And
you framed the question better than many I've seen (and some
that I've asked.)

....

So... are you going to try something out? Or will this remain
hypothetical for a while?

Jon
 
P

petrus bitbyter

Jan 1, 1970
0
Jon Kirwan said:
On Sun, 7 Jul 2013 15:17:59 +0200, "petrus bitbyter"


I actually caught that mistake, right away. Note my
self-reply shortly after?

Missed it completely. That's to say my newsserver did. Now you mentioned it,
I found it in Google groups.

My IP, though "the best IP of the country," considers its newsserver as
experimental so it comes without any guaranty at all. Sometimes my own
postings do not show up even if wrote on that actual server.
 
J

Jon Kirwan

Jan 1, 1970
0
Missed it completely. That's to say my newsserver did. Now you mentioned it,
I found it in Google groups.

Thanks for the note about it. Yeah, I get it. Nice you caught
it, too. I might have missed it myself and that's a good
catch from you!

After my internet service (Verizon) decided to remove from
their contract, unilaterally and without my permission of
course, NNTP services... I pretty much was forced to "go find
something." I'm paying 10 euros (despite being in the US) per
year for a service offered in Europe and it appears to be
working well these last years (fast.) news.individual.net.

Seems to get everything I care about, perhaps except for
alt.binaries.schematics.electronic. I have to just live with
that for now.
My IP, though "the best IP of the country," considers its newsserver as
experimental so it comes without any guaranty at all. Sometimes my own
postings do not show up even if wrote on that actual server.

Well, at least you HAVE a newsserver with your internet
service.

I grew used to the idea that my internet service is supposed
to provide the usual suite, including domain name services,
mail, and NNTP. Traditional, I had thought. You take the good
with the bad to get the business. But it's really a monopoly,
so... they can take the good with the good and sell off the
bad.

Jon
 
J

Jon Kirwan

Jan 1, 1970
0
<snip>
Having read some of the suggestions, I notice very few of the minimalist
solutions I favour.

The minimalist 1 transistor current source: give the transistor a fixed base
bias (like the Vf of a LED) and ground the emitter via a resistance (the
value of which determines collector current). If current tries to increase;
the voltage on the emitter resistor creeps upward and cancels some of the
fixed base bias - which acts to regulate the current.

Its that simple.

I think that was already in the OP's original, very first
post... along with some comments from the OP in that same
post that showed thinking going on about it.

Jon
 
J

Jasen Betts

Jan 1, 1970
0
Thanks for the note about it. Yeah, I get it. Nice you caught
it, too. I might have missed it myself and that's a good
catch from you!

After my internet service (Verizon) decided to remove from
their contract, unilaterally and without my permission of
course, NNTP services... I pretty much was forced to "go find
something." I'm paying 10 euros (despite being in the US) per
year for a service offered in Europe and it appears to be
working well these last years (fast.) news.individual.net.

Seems to get everything I care about, perhaps except for
alt.binaries.schematics.electronic. I have to just live with
that for now.

perhaps next year send $50 to astraweb and receive a terrabyte
of usenet with no use-before date.
 
J

Jon Kirwan

Jan 1, 1970
0
perhaps next year send $50 to astraweb and receive a terrabyte
of usenet with no use-before date.

Hmm. On first blush, that sounds like a lot. But when I think
about what a terrabyte will likely mean to me... hmm. Do they
carry binaries like alt.binaries.schematics.electronic? (I
guess I can go look and find out... but if you already know,
I wouldn't mind your opinion about it, too.)

Thanks,
Jon
 
J

Jon Kirwan

Jan 1, 1970
0
<snip>
perhaps next year send $50 to astraweb and receive a terrabyte
of usenet with no use-before date.

Just looked at the site and it's not clear what they carry
and what they don't carry. I'll have to email them about
details, I suppose.

Also, I used to get unlimited NNTP, a few email accounts,
access to their dial-up "56k" modems, and a shell account for
$10/month in a bundle. (Did NOT include my ground line at the
home, of course.) The ground line cost me $45 per month,
then. (And I had to buy my own modem, of course.) That was
more than 15 years ago.

Now I still pay for home fiber on three optical wavelengths
if I can believe what they told me. Phone service, broadband,
and internet access. Then I pay for NNTP, separately. And pay
for shell accounts, a name on name servers, and email as a
bundle, separately. And I pay more, of course.

My NNTP access has to be around a megabyte or so per day.
Unless the protocol is totally stupid and intentionally
inflates packets with random garbage, it can't exceed 10 meg
per day. I don't do news server binaries, except perhaps the
electronics one. 1000 gig would last me a while, I imagine. I
might die before I get my money's worth. ;)

Jon
 
J

Jon Kirwan

Jan 1, 1970
0
Reviewing the OP's posts to date, I notice 32 chains are intended - the
single transistor current source doesn't have to be repeated 32 times - just
use a 32 element current mirror.

That's been discussed.

Jon
 
J

Jon Kirwan

Jan 1, 1970
0
ABSE is pretty much a dead group with occasional political rants from JT -
even JF is having trouble finding people to rag on!

Oh... well.... Then I will most certainly be dead before I
could use up a 1Tb purchase.

Jon
 
J

Jon Kirwan

Jan 1, 1970
0
Over the years I've downloaded at least 1Tb from
alt.binaries.e-book.technical

Very likely I'll be dead before I can read that many books & magazines.

Hmm. Haven't looked at that one. And yes, it looks like one
way or another I'll be dead (and still have more capacity to
download.)

By the way, I did get the list of currently suppported groups
at astraweb located at:

http://www.news.astraweb.com/active.html

For anyone else wondering like I was.

No information at all on how long they retain things, though.

Jon
 
L

Lauri Alanko

Jan 1, 1970
0
---
Indeed, and my entries were intended to illustrate how you could use a
simple, inexpensive single-transistor common emitter circuit to
achieve your stated goals.

But, with no feedback until now, it was impossible to determine
whether or not _my_ time was wasted in trying to help you.

Sorry for not responding to your posts earlier. I was a bit
overwhelmed by the number of replies to what seemed to be a relatively
straightforward query. After the current mirror circuit, which seemed
ideal, the point of the additional suggestions seemed lost to me,
especially since there was no explication of how they would be
superior.

You are suggesting using a resistor as a current limiter, and a
transistor as an on/off switch for PWM. This doesn't seem ideal to me.
Firstly, it requires multiple components per chain (though there is an
obvious variant where there is a single power transistor switching the
supply to all the chains). But more importantly, a resistor is a
fragile current limiter: the current will depend on fluctuations in the
supply voltage and the I-V curves of the leds (which, I hear, are
prone to individual variations). And if a single led shorts, the
current to the others will increase, decreasing their lifetime as
well. I doubt these issues will be very relevant for my toy project,
but I want to learn to do things properly.

The reason I asked about using a _transistor_ as a current limiter is
that transistors are mostly immune to such fluctuations at the
collector, and only their (relatively low) base voltage needs to be
well-regulated. This seems superior to using a resistor, and there is
the added advantage that the same transistor can also be controlled to
provide e.g. PWM.

However, I recently learned that PWM is not actually very efficient
with leds: it seems that leds provide more light per watt when run at
constant low currents than when pulsed at a high current. PWM, though,
provides better linearity and stable color. But even if I want to use
adjustable current instead of PWM, the current mirror still works:
just attach a voltage-controlled current source to the mirror. So I
think I'm going to stick with that one.

But thanks for your suggestions, regardless.


Lauri
 
J

Jon Kirwan

Jan 1, 1970
0
<snip>
However, I recently learned that PWM is not actually very efficient
with leds: it seems that leds provide more light per watt when run at
constant low currents than when pulsed at a high current.

Your statement here needs clarification. It's not as nuanced
as it probably should be.

PWM is simply a method of using a duty cycle (from 0% to
100%) to adjust the apparent intensity. It is not necessarily
the case that it is less efficient with LEDs. It may be. But
not necessarily.

Let's say you are using PWM to adjust the brightness of a
single LED. Normally, at 100% duty cycle the LED is at
"normal and desired" brightness. Now you use PWM to reduce
this intensity. In this case, quite to the opposite of your
conclusion, PWM is actually a MORE efficient method than
others. In this case, your nominal power required is 100%
when operating at full brightness and will correspond to the
duty cycle % when operating at other brightness levels. So
operating at 10% duty cycle will require 10% of the power.
Other methods would dissipate 90% elsewhere so that 10% would
be dissipated in the LED and would be HORRIBLE, by
comparison. So you would WANT to use PWM, here, to save
power.

But let's say you are using PWM because you are multiplexing
a complex LED display. So, here, let's say you are
multiplexing by a factor of 5 because you have 5 columns (or
rows -- pick your terminology) to operate. In order to
achieve a "nominal 100% brightness" in a column, you must
drive it at 5 times the nominal current. So if the current is
20mA, nominally, you need to run them at 100mA, but at 20%
duty cycle. The other columns will also be operated at 20%
for their nominal brightness level. To adjust their
brightness from 0% to 100%, you would PWM them from 0% to
20%. In this case, because the LED voltage will be higher at
5X the nominal current, the power dissipated will be more
than just 5X nominal. But the brightness is determined by the
current, not the power. So in this case one could argue that
PWM wastes some power that, had the LEDs had individual
drivers and weren't muxed instead, would be less for the same
effect. But you pay this price because of the convenience and
reduced cost.

Keep in mind that as far as human perceptions go, so long as
the repetition rate is high enough that the brain cannot
follow it, the brightness perceived will be based upon the
average (integral) of the incident light flux. It will not
depend upon the pulse intensity. (There are other factors,
such as the spatial size and spatial frequency, position on
or off axis of the eye, and the surrounding intensities
nearby that also affect perception... but let's keep this
focused.) If you reduce the repetition rate, then at some
point the brain starts to perceive the peak pulse and will
show a change in brightness perception, along with it. But
it's also confounded by the fact that the pulse is being
noticed as well and that usually isn't desired. So the best
rule of thumb to stick with is that your average current
value represents the perceived flux.

The only other rule to keep in mind is that human perception
is logarithmic, so halving the average current does NOT
reduce the apparent brightness in half. To maintain a
constant rate of decline in brightness, you must multiply the
duty cycle by a constant factor for each time unit. So, for
example:

100%
50%
25%
12.5%
6.25%

Would yield constant changes in apparent brightness. If you
achieved those values with fixed time intervals between them,
a human would perceive a "smooth" diminuation in brightness
that appeared linear.

Regardless, PWM is by itself not necessarily inefficient. And
even when you must pay a small price in efficiency because
you are multiplexing and have to use higher pulse currents
because of that, it's still better than the alternative of
paying for individual drives for every LED and not terribly
inefficient, anyway. Other alternatives would be either
excessively expensive for very little gain, or simply worse.
PWM, though, provides better linearity

Of what? Again, note that human perception is logarithmic. So
while PWM can easily be controlled linearly, it's not going
to be perceived that way if that is how you use it. In fact,
PWM would be more usable as a brightness control if the
hardware could PWM accurately in a logarithmic way -- it's
actually a pain to use PWM correctly for brightness control
if you care about human perceptions of relative brightness.
It's advantage is that it is just easy to apply, is all.
and stable color.

I guess I should mention something else just to complicate
things (and agree with you.)

If you operate a monochromatic (single) LED at differing
currents (and, consequently, different voltages to achieve
it), then you will get a different wavelength distribution
out of it, too. If you use a spectrophotometer you will see
the shifts. This also means the "color perception" of a
single color LED shifts a little.

So one could argue that keeping the LED current fixed while
PWM'ing to achieve brightness shifts is actually achieving
"stable color" for these reasons (the peak current remains
fixed, just the duty cycle changes.)

But if you are comparing, say, 20mA vs 100mA, then there is
also a color shift in doing so. So I take your point that PWM
achieves "stable color."
But even if I want to use
adjustable current instead of PWM, the current mirror still works:
just attach a voltage-controlled current source to the mirror. So I
think I'm going to stick with that one.

It's what I'd probably do (or use an IC, if I could find one
that is likely to exist for a while, is readily available,
could handle the dissipation, and is cheap enough.)

Jon
 
J

Jasen Betts

Jan 1, 1970
0
On 11 Jul 2013 07:30:08 GMT, Jasen Betts <[email protected]>
wrote:

Hmm. On first blush, that sounds like a lot. But when I think
about what a terrabyte will likely mean to me... hmm. Do they
carry binaries like alt.binaries.schematics.electronic? (I
guess I can go look and find out... but if you already know,
I wouldn't mind your opinion about it, too.)

Yeah, as far as I can tell, all the binaries groups are available.
that one certainly is.
 
J

Jasen Betts

Jan 1, 1970
0
On Thu, 11 Jul 2013 21:05:10 +0100, "Ian Field"


Hmm. Haven't looked at that one. And yes, it looks like one
way or another I'll be dead (and still have more capacity to
download.)

By the way, I did get the list of currently suppported groups
at astraweb located at:

http://www.news.astraweb.com/active.html

For anyone else wondering like I was.

No information at all on how long they retain things, though.

http://www.astraweb.com/tools/stats.html

currently almost 5 years
 
J

Jasen Betts

Jan 1, 1970
0
I only download headers via NNTP and then, selectively,
download bodies when I care to read them.

yeah, but does your software use XHDR or HEAD? The former pulls a single
header from several messages whereas the latter pulls all headers from
a single message.

If your killfiles only use four headers, 4 XHDR requests will be more
much more efficient than 100 HEAD requests,


Also headers are often larger than message bodies.
Probably lots less than that, then, since most of what I get
from SED remains headers-only.

Hmm... I can count that.

jasen@gonzo:/var/spool/news/sci/electronics/design$ find -type f -ctime -183 -exec 'sed' '/^$/,$ d' {} ';' | wc
457382 1757014 29908634

Hbout 30 megabytes of headers in the last 183 days.

total message size:
find -type f -ctime -183 -exec cat {} ';' | wc
1567860 8477618 70408901

Hmm, 70 megs, looks like I over-estimated total volume

So 40 megs message body, So the header:body ratio is
approximately 3:4

How much is quoted content?

find -type f -ctime -183 -exec cat {} ';' | sed '/^ *>/ p;d' | wc
643846 4338102 24717942

Almost 25 megs, well over half. (ignoring the quotes by those who use
non-standard quoting styles, mainly Phil A. and one of the political
activists whose handle I don't recall)

so, headers are more almost twice the volume of the new content.
 
J

Jon Kirwan

Jan 1, 1970
0
yeah, but does your software use XHDR or HEAD? The former pulls a single
header from several messages whereas the latter pulls all headers from
a single message.

Interesting. So I went to this site:

http://www.forteinc.com/agent/faq.php#D8212751186A878A852571C800049E02

where it discusses my reader (Agent.) Apparently, by default,
it does not use XHDR... but it can.
If your killfiles only use four headers, 4 XHDR requests will be more
much more efficient than 100 HEAD requests,

I'm still learning about this. And thanks for making me more
aware.
Also headers are often larger than message bodies.

I can easily believe that. Lots of message bodies are very
short.
Hmm... I can count that.

jasen@gonzo:/var/spool/news/sci/electronics/design$ find -type f -ctime -183 -exec 'sed' '/^$/,$ d' {} ';' | wc
457382 1757014 29908634

Hbout 30 megabytes of headers in the last 183 days.

Okay. Believable, too. I only examine 3 newsgroups on a daily
basis. sci.electronics.basics, sci.electronics.design, and
comp.arch.embedded. That's it.
total message size:
find -type f -ctime -183 -exec cat {} ';' | wc
1567860 8477618 70408901

Hmm, 70 megs, looks like I over-estimated total volume

So 40 megs message body, So the header:body ratio is
approximately 3:4

How much is quoted content?

find -type f -ctime -183 -exec cat {} ';' | sed '/^ *>/ p;d' | wc
643846 4338102 24717942

Almost 25 megs, well over half. (ignoring the quotes by those who use
non-standard quoting styles, mainly Phil A. and one of the political
activists whose handle I don't recall)

so, headers are more almost twice the volume of the new content.

Thanks for the results. It makes it pretty clear that I would
have quite a long ride with 1000 gig of metered newsgroups.
Even if it were a gig a year for the 3 groups, and if I
didn't allow myself to die until it expired, I'd be the new
Methuselah.

Even adding a binary group, if I don't download things I
don't want, should work okay.

Best way to find out is to buy the tiniest package they offer
and see how long it lasts. That will be the benchmark, then.
Assuming I live long enough to see even that expire....

Jon
 
J

Jon Kirwan

Jan 1, 1970
0

Nice link. Using a diff-amp in a controlled feedback loop.
Thanks.

I do NOT like the capacitor there. When inactive (OFF), C1
charges up to Vcc across it, if C1 is small enough (if C1 is
big, then it never reaches Vcc across it.) When active (ON),
the common node between C1 and R4 jumps up towards 1.2V below
Vcc and in doing so causes the collector of Q2 to be driven
below ground, Vcc-1.2V-Vc1=-1.2V since Vc1=Vcc. As C1
discharges it actually borrows current from Q3's collector
(rather than from R3) causing a lower current in R3, yielding
a lower voltage potential for Q2's base... leading to
diversion of R2's current towards Q1 and thus MORE drive
current into Q4 through Q7. The upshot is that upon turn-on,
the peak current in the LEDs is actually HIGHER than designed
and it then gently settles down to the desired level as C1
discharges to a new (lower) potential across its leads. This
is spikey behavior instead of a "soft start." Make C1 bigger
and it gets worse, too. The sharp leading edge will reach
even higher initial currents in the LEDs that way.

So it's value should be kept small. I'm not yet convinced
about the danger of oscillations or the source of them in
this circuit. There are very low impedances on every node of
Q2. It just doesn't seem necessary to me. I don't see how the
small Cbc value and the value of Rb' can conspire to make a
problem here, even with the collector making a small jump at
turn on. And it actually seems to have a downside where I'd
rather simply remove it.

I might also invert the whole structure and delete R1 and D1,
if my LED supply was a volt or more above the micro rail
voltage. The pin output won't be loaded much and will
probably be a very close reflection of the micro rail
voltage. Of course, I'd adjust values, accordingly.

For example, if my micro were running on 3.6V and my LED
supply were 6V (hypothetically) and I wanted 100mA and 5
chains of them (not 4), then I might invert the polarities
and use R3=36 ohms, R2=470 ohms, and R4=270 ohms, kill C1,
and just drive the base of Q1 directly.

I'd start the design by estimating the total base currents
required for Q3 through Q7 (600mA/beta=200, or about 3mA.)
Then I'd double that for the diff-amp pair to 6mA. Given 3.6V
drive and an estimated 0.7V drop on Q1, I get 2.9V at Q1's
emitter. So 2.9V/6mA is 483 ohms. 470 is a standard value and
gives a slight increase in estimated current, so I'd use that
value instead. I also want 3.6V across R3 at 100mA, so 36
ohms. Changing that to 33 or 39 ohms would change the LED
currents by 10% or more. Luckily, 5% resistors do come in 36
ohms. So I'd stick with that. Then, since R4 should be
(assuming 3mA in each branch, hopefully) at about one 100mA
sized Vbe (about 110mV added to 700mV as a guess), so R4
should be about 0.81/3mA, or 270 ohms. Nice. Standard value.
So that's where I'd get my values.

I'm still stuck trying to understand C1's benefit.

Jon
 
J

Jasen Betts

Jan 1, 1970
0
Interesting. So I went to this site:

http://www.forteinc.com/agent/faq.php#D8212751186A878A852571C800049E02

where it discusses my reader (Agent.) Apparently, by default,
it does not use XHDR... but it can.


I'm still learning about this. And thanks for making me more
aware.


I can easily believe that. Lots of message bodies are very
short.


Okay. Believable, too. I only examine 3 newsgroups on a daily
basis. sci.electronics.basics, sci.electronics.design, and
comp.arch.embedded. That's it.


Thanks for the results. It makes it pretty clear that I would
have quite a long ride with 1000 gig of metered newsgroups.
Even if it were a gig a year for the 3 groups, and if I
didn't allow myself to die until it expired, I'd be the new
Methuselah.

Even adding a binary group, if I don't download things I
don't want, should work okay.

ABSE content in the last 6 months
$ find -type f -ctime -183 -exec cat {} ';' | wc -c
11504635

11.5 megs since 14-January.
Best way to find out is to buy the tiniest package they offer
and see how long it lasts. That will be the benchmark, then.
Assuming I live long enough to see even that expire....

$10 for 25G, probably enough.
 
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