Bipolar current limit

On Wed, 22 Feb 2017 07:35:05 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

On Tuesday, February 21, 2017 at 8:41:12 PM UTC-5, John Larkin wrote:
On Mon, 20 Feb 2017 13:35:26 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

On Monday, February 20, 2017 at 3:11:11 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 11:14:20 -0800, George Herold wrote:

OK this is perhaps overkill.. see footnote.
So I've got an indicator LED (bi-color) the input is the error term from
a thermal loop, and I turn that into an LED current.

I was thinking it might be nice to turn up the gain on the current,
but then I worried about burning out the LED with too much current from
the opamp.* So I drew this up... and then added the diodes to stop the
transistors "running backwards". And the cap to stop the oscillations
when the transistors turn on.

Assuming that this is for a visual indicator, just use a rail-rail output
op-amp with a series resistor. In normal operation the op-amp is within
its voltage limits and current is as commanded. When the input is big,
the op-amp "asks" for too much current, hits the rail, the resistor
limits the current, and Bob's yer Uncle.
I've done that, but the one issue is the turn on voltage of the LED.
I've got a few volt dead space near zero volts.
(I want to say again that perhaps this is a silly idea, and I'll bag
the whole thing.)
So by feeding back of the current I eliminate that dead space.
Then I dreamed of more gain.. so when the loop is closed but a little
unstable maybe you could see the oscillations in the error term in the
intensity of the LED... or if near zero by having it change color.
But then some student leaves it in high gain with maximum error and the
current from the opamp heats up the led. (potential failure...)

So I wanted to limit the current to the LED.
(I never can seem to put enough background into my questions...)


There's a lot of reasons not to want to let an op-amp hit the rail, but
as long as you have one that does so in a reasonably well-behaved manner
you should be OK.
Hmm well I run opamps into the rails all the time.
For slow stuff, I've never had problems.

In this case I'm talking about running opamps at their current limit.
I've never done that (on purpose) before.

George H.

I

--
Tim Wescott
Control systems, embedded software and circuit design
I'm looking for work! See my website if you're interested
http://www.wescottdesign.com


Would this do what you want?

https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Limiters/Lin_LED.JPG

Right that's what I started with.. I then made a wrong turn.
Jasen straightened me out.
(I've now got a 50 ohm sense resistor and ~700 ohms in series with the LED..
Which limits maximum current to ~15-20 mA or so.)
Click to expand...

OK, that lets you scale full LED brightness to some smaller voltage
input, independent of the current limit.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics
 
On Wed, 22 Feb 2017 10:58:01 -0800, George Herold wrote:

Thank God for SEB and SED.

George H.
(being agnostic, I think I'm still allowed to thank God for stuff. :^)
Click to expand...

I have reconciled my combination of agnosticism and faith by firmly
believing a whole lot of stuff that God is not, and a bit of stuff that
God is, if in fact God exists.

Living your life in a manner that's consistent with being good if God
exists and if God does not exist is a significant aid to staying on the
straight and narrow. Being utterly crappy at lying and conspiracy is
better, though.

For example: if God exists then giving thanks is the right thing to do;
if God doesn't exist, then giving thanks doesn't matter one way or
another.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!
 
On Wednesday, February 22, 2017 at 2:56:49 PM UTC-5, Tim Wescott wrote:
On Wed, 22 Feb 2017 10:58:01 -0800, George Herold wrote:

Thank God for SEB and SED.

George H.
(being agnostic, I think I'm still allowed to thank God for stuff. :^)

I have reconciled my combination of agnosticism and faith by firmly
believing a whole lot of stuff that God is not, and a bit of stuff that
God is, if in fact God exists.

Living your life in a manner that's consistent with being good if God
exists and if God does not exist is a significant aid to staying on the
straight and narrow. Being utterly crappy at lying and conspiracy is
better, though.
Click to expand...
Right, for me at least, God is that thing which makes us all behave
good. (excuse my grammar) I don't need to probe much deeper.
(Well when I was younger I had much more angst..)
Be nice, and others respond in kind, tell the truth as
well as you can.. etc.

George H.
with deepest respect to all my more devote colleagues.

For example: if God exists then giving thanks is the right thing to do;
if God doesn't exist, then giving thanks doesn't matter one way or
another.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!
Click to expand...
 
On 02/22/2017 01:58 PM, George Herold wrote:
On Wednesday, February 22, 2017 at 11:23:59 AM UTC-5, John Larkin wrote:
On Wed, 22 Feb 2017 07:35:05 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

On Tuesday, February 21, 2017 at 8:41:12 PM UTC-5, John Larkin wrote:
On Mon, 20 Feb 2017 13:35:26 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

On Monday, February 20, 2017 at 3:11:11 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 11:14:20 -0800, George Herold wrote:

OK this is perhaps overkill.. see footnote.
So I've got an indicator LED (bi-color) the input is the error term from
a thermal loop, and I turn that into an LED current.

I was thinking it might be nice to turn up the gain on the current,
but then I worried about burning out the LED with too much current from
the opamp.* So I drew this up... and then added the diodes to stop the
transistors "running backwards". And the cap to stop the oscillations
when the transistors turn on.

Assuming that this is for a visual indicator, just use a rail-rail output
op-amp with a series resistor. In normal operation the op-amp is within
its voltage limits and current is as commanded. When the input is big,
the op-amp "asks" for too much current, hits the rail, the resistor
limits the current, and Bob's yer Uncle.
I've done that, but the one issue is the turn on voltage of the LED.
I've got a few volt dead space near zero volts.
(I want to say again that perhaps this is a silly idea, and I'll bag
the whole thing.)
So by feeding back of the current I eliminate that dead space.
Then I dreamed of more gain.. so when the loop is closed but a little
unstable maybe you could see the oscillations in the error term in the
intensity of the LED... or if near zero by having it change color.
But then some student leaves it in high gain with maximum error and the
current from the opamp heats up the led. (potential failure...)

So I wanted to limit the current to the LED.
(I never can seem to put enough background into my questions...)


There's a lot of reasons not to want to let an op-amp hit the rail, but
as long as you have one that does so in a reasonably well-behaved manner
you should be OK.
Hmm well I run opamps into the rails all the time.
For slow stuff, I've never had problems.

In this case I'm talking about running opamps at their current limit.
I've never done that (on purpose) before.

George H.

I

--
Tim Wescott
Control systems, embedded software and circuit design
I'm looking for work! See my website if you're interested
http://www.wescottdesign.com


Would this do what you want?

https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Limiters/Lin_LED.JPG

Right that's what I started with.. I then made a wrong turn.
Jasen straightened me out.
(I've now got a 50 ohm sense resistor and ~700 ohms in series with the LED..
Which limits maximum current to ~15-20 mA or so.)

OK, that lets you scale full LED brightness to some smaller voltage
input, independent of the current limit.

Right I'm putting a pot on the input to adjust the "gain".
(photons per volt.)

I'm mostly working by myself.. no real electronics people to bounce ideas
off of. It's weird how you can get stuck in some design path.. know it's
not good... (see my original schematic.) But not see the way clear,
until someone gives you a head slap.

Thank God for SEB and SED.

George H.
(being agnostic, I think I'm still allowed to thank God for stuff. :^)
Click to expand...

Yeah, but you're not allowed to complain about him. ;)

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
 
On Wed, 22 Feb 2017 17:47:50 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

On Wednesday, February 22, 2017 at 2:56:49 PM UTC-5, Tim Wescott wrote:
On Wed, 22 Feb 2017 10:58:01 -0800, George Herold wrote:

Thank God for SEB and SED.

George H.
(being agnostic, I think I'm still allowed to thank God for stuff. :^)

I have reconciled my combination of agnosticism and faith by firmly
believing a whole lot of stuff that God is not, and a bit of stuff that
God is, if in fact God exists.

Living your life in a manner that's consistent with being good if God
exists and if God does not exist is a significant aid to staying on the
straight and narrow. Being utterly crappy at lying and conspiracy is
better, though.
Right, for me at least, God is that thing which makes us all behave
good. (excuse my grammar) I don't need to probe much deeper.
(Well when I was younger I had much more angst..)
Be nice, and others respond in kind, tell the truth as
well as you can.. etc.

George H.
with deepest respect to all my more devote colleagues.
Click to expand...

Your LED circuits wouldn't be very useful if He hadn't invented light.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics
 
On Saturday, February 25, 2017 at 11:35:03 PM UTC-5, John Larkin wrote:
On Wed, 22 Feb 2017 17:47:50 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

On Wednesday, February 22, 2017 at 2:56:49 PM UTC-5, Tim Wescott wrote:
On Wed, 22 Feb 2017 10:58:01 -0800, George Herold wrote:

Thank God for SEB and SED.

George H.
(being agnostic, I think I'm still allowed to thank God for stuff. :^)

I have reconciled my combination of agnosticism and faith by firmly
believing a whole lot of stuff that God is not, and a bit of stuff that
God is, if in fact God exists.

Living your life in a manner that's consistent with being good if God
exists and if God does not exist is a significant aid to staying on the
straight and narrow. Being utterly crappy at lying and conspiracy is
better, though.
Right, for me at least, God is that thing which makes us all behave
good. (excuse my grammar) I don't need to probe much deeper.
(Well when I was younger I had much more angst..)
Be nice, and others respond in kind, tell the truth as
well as you can.. etc.

George H.
with deepest respect to all my more devote colleagues.


Your LED circuits wouldn't be very useful if He hadn't invented light.
Click to expand...
Grin.. "The Last Question" by Asimov is a fav. "Let there be light...."
But you should know that God is women. See Dogma,
https://en.wikipedia.org/wiki/Dogma_(film)

George H.

--

John Larkin Highland Technology, Inc

lunatic fringe electronics
Click to expand...
 
In article <5267fd91-b530-466c-b2f4-e5455a250943@googlegroups.com>,
gherold@teachspin.com says...
On Saturday, February 25, 2017 at 11:35:03 PM UTC-5, John Larkin wrote:
On Wed, 22 Feb 2017 17:47:50 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

On Wednesday, February 22, 2017 at 2:56:49 PM UTC-5, Tim Wescott wrote:
On Wed, 22 Feb 2017 10:58:01 -0800, George Herold wrote:

Thank God for SEB and SED.

George H.
(being agnostic, I think I'm still allowed to thank God for stuff. :^)

I have reconciled my combination of agnosticism and faith by firmly
believing a whole lot of stuff that God is not, and a bit of stuff that
God is, if in fact God exists.

Living your life in a manner that's consistent with being good if God
exists and if God does not exist is a significant aid to staying on the
straight and narrow. Being utterly crappy at lying and conspiracy is
better, though.
Right, for me at least, God is that thing which makes us all behave
good. (excuse my grammar) I don't need to probe much deeper.
(Well when I was younger I had much more angst..)
Be nice, and others respond in kind, tell the truth as
well as you can.. etc.

George H.
with deepest respect to all my more devote colleagues.


Your LED circuits wouldn't be very useful if He hadn't invented light.
Grin.. "The Last Question" by Asimov is a fav. "Let there be light...."
But you should know that God is women. See Dogma,
https://en.wikipedia.org/wiki/Dogma_(film)

George H.
Does that make him a drag queeen?
Click to expand...

Jamie
 
On Monday, February 20, 2017 at 2:14:27 PM UTC-5, George Herold wrote:
OK this is perhaps overkill.. see footnote.
So I've got an indicator LED (bi-color) the input is the error
term from a thermal loop, and I turn that into an LED current.

I was thinking it might be nice to turn up the gain on the current,
but then I worried about burning out the LED with too much current from the
opamp.* So I drew this up... and then added the diodes to stop the
transistors "running backwards". And the cap to stop the oscillations
when the transistors turn on.

https://www.dropbox.com/s/t14fdf6hng399ou/Current-limit.JPG?dl=0

Don't laugh too loud.
Is there some easier way to do this?

George H.

*Right one easy answer would be to find an opamp with ~20 mA maximum current.
(uA741 ??? :^)
Do you have any suggestions? All our "in stock opamps are higher currents.
Click to expand...

Yeah it is called a fucking resistor..
 
On Tuesday, February 28, 2017 at 1:02:35 AM UTC-5, jurb...@gmail.com wrote:
On Monday, February 20, 2017 at 2:14:27 PM UTC-5, George Herold wrote:
OK this is perhaps overkill.. see footnote.
So I've got an indicator LED (bi-color) the input is the error
term from a thermal loop, and I turn that into an LED current.

I was thinking it might be nice to turn up the gain on the current,
but then I worried about burning out the LED with too much current from the
opamp.* So I drew this up... and then added the diodes to stop the
transistors "running backwards". And the cap to stop the oscillations
when the transistors turn on.

https://www.dropbox.com/s/t14fdf6hng399ou/Current-limit.JPG?dl=0

Don't laugh too loud.
Is there some easier way to do this?

George H.

*Right one easy answer would be to find an opamp with ~20 mA maximum current.
(uA741 ??? :^)
Do you have any suggestions? All our "in stock opamps are higher currents.

Yeah it is called a fucking resistor..
Click to expand...

Hah, I could also use a couple of depletion fets and resistor, but I only
learned that today.

George H.
(I don't see any need to be rude.)
 
On Sat, 25 Feb 2017 20:34:59 -0800, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

On Wed, 22 Feb 2017 17:47:50 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

On Wednesday, February 22, 2017 at 2:56:49 PM UTC-5, Tim Wescott wrote:
On Wed, 22 Feb 2017 10:58:01 -0800, George Herold wrote:

Thank God for SEB and SED.

George H.
(being agnostic, I think I'm still allowed to thank God for stuff. :^)

I have reconciled my combination of agnosticism and faith by firmly
believing a whole lot of stuff that God is not, and a bit of stuff that
God is, if in fact God exists.

Living your life in a manner that's consistent with being good if God
exists and if God does not exist is a significant aid to staying on the
straight and narrow. Being utterly crappy at lying and conspiracy is
better, though.
Right, for me at least, God is that thing which makes us all behave
good. (excuse my grammar) I don't need to probe much deeper.
(Well when I was younger I had much more angst..)
Be nice, and others respond in kind, tell the truth as
well as you can.. etc.

George H.
with deepest respect to all my more devote colleagues.


Your LED circuits wouldn't be very useful if He hadn't invented light.
Click to expand...

---
Or time.

John Fields
 
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