Transistor as a current limiter

[Jon Kirwan]

On Wed, 17 Jul 2013 19:19:00 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Wed, 17 Jul 2013 15:40:30 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:

On Wed, 17 Jul 2013 09:51:54 -0700, Jon Kirwan
jonk@infinitefactors.org> wrote:

I'd love to see creative improvements (WITHOUT OPAMPS OR
MOSFETS.)

Jon


No mosfets even?

---
Didn't he say: "WITHOUT OPAMPS OR MOSFETS."?

I did and was summarily ignored except that he later wrote
that I could replace the mosfets with BJTs if I wanted to.

I'm below 200mV of headroom required for surprisingly good
current control with BJTs only. I still need to fix it so
that the LEDs aren't in the middle of the circuit, but can
share a common anode or cathode. That part was just because
it was easier to think about at the time.

Jon

 

[John Larkin]

On Wed, 17 Jul 2013 19:42:17 -0700, Jon Kirwan <jonk@infinitefactors.org> wrote:

On Wed, 17 Jul 2013 19:34:50 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 17 Jul 2013 19:22:27 -0700, Jon Kirwan <jonk@infinitefactors.org> wrote:

On Wed, 17 Jul 2013 15:40:30 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:

https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Sources/LED_Regs.JPG

So I just decided to take a look (besides seeing the mosfet.)

Looking at schematic A: I see a diff-amp with 0.2V input (I
suppose you would tell me to use a R-divider if I were being
cheap about this.)

The +0.2 could be a shared rail. Current ewquirement is low.


One of its collectors (let's call that PNP
Q1 for now) collector is used to pull up on the base of
another NPN (call it Q2 for now) that ties the mosfet gate.
To get the voltage at the base of Q2 high enough that it is
slightly or usefully active, the collector of Q1 must reach
near to 0.65V or so. But the base is 0.2V. You see any
problem here?

No. It should work.

Explain why.

I can't explain control theory in a usenet post, and I can't explain transistor
theory either. Get a book or something.

You can take my circuits as starting points, or you can be hostile to
suggestions.

I guess you offered as much thought as you felt
I deserved.

With your attitude, you don't deserve much.

Saturated BJT in the diff-amp?

Nothing is saturated.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
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[Jasen Betts]

On 2013-07-17, Jon Kirwan <jonk@infinitefactors.org> wrote:

On Tue, 16 Jul 2013 17:45:33 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:

snip
This is sci.electronics.BASICS after all.
snip

Speaking of which, any thoughts on this issue:

By the way, right now I'm struggling with the idea of how to
actively enable individual half-current mirror BJTs in such a
way that I can selectively enable various combinations of
these extended mirror sections without impacting others which
remain "on." I'm already including emitter degeneration of 1
Ohm (or 100mV at 100mA), which improves significantly in
dealing with variations in Is and temperature variations with
each BJT. And it works fine on a protoboard, so I'm liking
that result. But if one of the BJTs has no load (infinite
resistance in effect), it saturates like crazy and messes
with the other BJTs. (And the degree of that problem gets
worse with emitter degeneration, too.) So I need a cheap and
easy way (I'm coming up with HORRIBLY EXPENSIVE and HARD ways
I don't want to use) to be able to selectively activate an
opposing switch (current source/sink on one side that is
under active control and a BJT switch on the other side which
selectively permits or blocks the programmed current.)

I'm just not seeing a simple way. It's all way too hairy for
my liking. (Can easily see how it might be approached with
MOSFETs, for example, but I get BJTs at better than 2 for a
penny and I get MOSFETs for a LOT MORE than that, even when
on sale.) I'm looking for a BJT and discrete (I can't do
designs with equal emitter areas on common subtrates, for
example, and I can't afford to throw dozens of BJTs at
everything either because I'm not doing an ASIC.)

Fun problem.

I would plan to design the power supply to provide about 1.5V
of headroom beyond what is needed by the LEDs at peak current
drive. That should provide a good 1.2V for the current mirror
side of things and 0.3V for the switch on the other end. (If
it can be done with less headroom, so much the better.)

The issue is this:

To have a controlled current sink (or source) capable of
supporting ... let's say at least 8 sinks/sources ... and
where each sink/source can be individually enabled or
disabled without affecting any of the others. Only BJTs as
active devices. Only discretes, no ICs, no opamps, etc.

My imagination fails me for a reasonable solution here. How
about yours?

all NPN emitter marked 'e'

I_set I_LED
V V
| |
+-----. |
\ | /
\| | |/
|--+----------|-----...
/| |\
e e
+ +
| |
/ \
\ 22R / 22R
/ \
| |
Q1 / /
+V |/ enable |/
--[1K]-| ---[1K]--|
|\ |\
e e
| |
--+-------------------+---...

scale the 22R as apropriate for your application 200mV drop is plenty
scale the 1K sufficient to ensure reasonabe saturation.

Q1 can probably be replaced by the apropriate resistor
(measure or calculate)


--
⚂⚃ 100% natural

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