Bipolar current limit

G

George Herold

Guest
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.
 
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.
Click to expand...
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.
Click to expand...
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
Click to expand...
 
On Monday, February 20, 2017 at 5:07:37 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 13:35:26 -0800, George Herold 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...)

I'm not saying don't do the current feedback -- I'm just saying that if
you do that AND put a resistor in series with the LED, then when the op-
amp can it'll regulate current, and when it can't, the resistor will
cleanly limit current.

You could even make the current-sense resistor into your current-limit
resistor, and get plenty of signal thereby.
Click to expand...
Hmm yeah that's my 1 k ohm series resistor.. well 1k to 0 k.
when the opamp is railed, 15V -> 15ma... then when changes
get smaller so ~1V or smaller signals 1V ~15 mA.

George H.
--

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

I'm looking for work -- see my website!
Click to expand...
 
On Monday, February 20, 2017 at 5:42:13 PM UTC-5, Jim Thompson wrote:
On Mon, 20 Feb 2017 11:14:20 -0800 (PST), George Herold
gherold@teachspin.com> 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.

Looks like an OpAmp "snuffer" to me... there is no bound on the
current you are pulling current directly from the output.
Click to expand...

Right. I'm not sure where the voltage drop is.. (opamp or transistor)
but the current is limited by whatever the opamp delivers.

All my opamps (in stock) are ~30-40 mA max output current.
They are all rated to have the outputs shorted continuously.
(It's not something, I've looked at carefully.)
I thought that current might kill the leds.. even the
spec of 25 mA max probably limits the lifetime.
So I'm snuffing the opamps above ~14 mA (0.7 V is about
where it turns on hard.)

George H.
...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Thinking outside the box... producing elegant solutions.
Click to expand...
 
On Monday, February 20, 2017 at 7:15:03 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 13:35:26 -0800, George Herold 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.

This:

Let R = maxcurrent / (Vcc - Vfwd)

It assumes that Vcc = -Vee, and that the two diodes' Vfwd is reasonably
close, but it'll get the job done. Can be easily rearranged for a single
supply, if you have enough overhead voltage. If you need more gain put a
resistor in the leg between R and the V- input, with another resistor to
ground.


Vcc
o
|\|
.------|-\
| | >----o----.
o--------)------|+/ | |
| |/| | |
| o - V -
| Vee ^ -
| | |
| o----'
| |
'--------------o
|
.-.
| | R
| |
'-'
|
|
===
GND
(created by AACircuit v1.28.7 beta 02/28/13 www.tech-chat.de)

--

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

I'm looking for work -- see my website!
Click to expand...

Right.. did my dropbox link not work?

George H.
 
On Monday, February 20, 2017 at 7:48:53 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 16:29:53 -0800, George Herold wrote:

On Monday, February 20, 2017 at 7:15:03 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 13:35:26 -0800, George Herold 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.

This:

Let R = maxcurrent / (Vcc - Vfwd)

It assumes that Vcc = -Vee, and that the two diodes' Vfwd is reasonably
close, but it'll get the job done. Can be easily rearranged for a
single supply, if you have enough overhead voltage. If you need more
gain put a resistor in the leg between R and the V- input, with another
resistor to ground.


Vcc
o
|\|
.------|-\
| | >----o----.
o--------)------|+/ | |
| |/| | |
| o - V -
| Vee ^ -
| | |
| o----'
| | '--------------o
|
.-. | | R | | '-'
|
|
=== GND
(created by AACircuit v1.28.7 beta 02/28/13 www.tech-chat.de)

--

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

I'm looking for work -- see my website!

Right.. did my dropbox link not work?

George H.

Yes it did -- but you have a bunch of extra stuff in that schematic
that's not necessary if you size R correctly.
Click to expand...
I want to be able to change R..photons/ volt...
(though leds are not all that great at low current)
from ~1k ohm to ~50 ohms...

Speaking of which I'm using a green /red bipolar led,
not all that great at matching the eyes response...
(the green is much brighter than the red, eye-wise)
would red/orange... green/blue* be better?

George H.
--

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

I'm looking for work -- see my website!
Click to expand...
 
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.
Click to expand...

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.

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.

--
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
 
On Mon, 20 Feb 2017 13:35:26 -0800, George Herold 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...)
Click to expand...

I'm not saying don't do the current feedback -- I'm just saying that if
you do that AND put a resistor in series with the LED, then when the op-
amp can it'll regulate current, and when it can't, the resistor will
cleanly limit current.

You could even make the current-sense resistor into your current-limit
resistor, and get plenty of signal thereby.

--

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

I'm looking for work -- see my website!
 
On Mon, 20 Feb 2017 11:14:20 -0800 (PST), George Herold
<gherold@teachspin.com> 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...

Looks like an OpAmp "snuffer" to me... there is no bound on the
current you are pulling current directly from the output.

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Thinking outside the box... producing elegant solutions.
 
On Mon, 20 Feb 2017 13:35:26 -0800, George Herold 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.
Click to expand...

This:

Let R = maxcurrent / (Vcc - Vfwd)

It assumes that Vcc = -Vee, and that the two diodes' Vfwd is reasonably
close, but it'll get the job done. Can be easily rearranged for a single
supply, if you have enough overhead voltage. If you need more gain put a
resistor in the leg between R and the V- input, with another resistor to
ground.


Vcc
o
|\|
.------|-\
| | >----o----.
o--------)------|+/ | |
| |/| | |
| o - V ->
| Vee ^ -
| | |
| o----'
| |
'--------------o
|
.-.
| | R
| |
'-'
|
|
===
GND
(created by AACircuit v1.28.7 beta 02/28/13 www.tech-chat.de)

--

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

I'm looking for work -- see my website!
 
On Mon, 20 Feb 2017 16:29:53 -0800, George Herold wrote:

On Monday, February 20, 2017 at 7:15:03 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 13:35:26 -0800, George Herold 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.

This:

Let R = maxcurrent / (Vcc - Vfwd)

It assumes that Vcc = -Vee, and that the two diodes' Vfwd is reasonably
close, but it'll get the job done. Can be easily rearranged for a
single supply, if you have enough overhead voltage. If you need more
gain put a resistor in the leg between R and the V- input, with another
resistor to ground.


Vcc
o
|\|
.------|-\
| | >----o----.
o--------)------|+/ | |
| |/| | |
| o - V -
| Vee ^ -
| | |
| o----'
| | '--------------o
|
.-. | | R | | '-'
|
|
=== GND
(created by AACircuit v1.28.7 beta 02/28/13 www.tech-chat.de)

--

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

I'm looking for work -- see my website!

Right.. did my dropbox link not work?

George H.
Click to expand...

Yes it did -- but you have a bunch of extra stuff in that schematic
that's not necessary if you size R correctly.

--

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

I'm looking for work -- see my website!
 
On 2017-02-20, George Herold <gherold@teachspin.com> 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?
Click to expand...

I think you have the brighness control in the wrong place.
You can use a cheaper pot and/or get more reliable operation
when you're not putting 10mA though the wiper

Once you move that, current limit consists of building a gain stage
and letting the op-amp reach for the rails. it doesn't need to be a
rail-to-rail op-amp it just needs to be one that remains sensible
whist making an effort.

eg:

--.
B<--+|\_L_
| -|/ |
| |--R1-'
| R2
| |
~ ~

B brightness pot (eg:20K)
L led
~ ground

--
This email has not been checked by half-arsed antivirus software
 
On Monday, February 20, 2017 at 9:31:01 PM UTC-5, Jasen Betts wrote:
On 2017-02-20, George Herold <gherold@teachspin.com> 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?

I think you have the brighness control in the wrong place.
You can use a cheaper pot and/or get more reliable operation
when you're not putting 10mA though the wiper

Once you move that, current limit consists of building a gain stage
and letting the op-amp reach for the rails. it doesn't need to be a
rail-to-rail op-amp it just needs to be one that remains sensible
whist making an effort.

eg:

--.
B<--+|\_L_
| -|/ |
| |--R1-'
| R2
| |
~ ~

B brightness pot (eg:20K)
L led
~ ground
Click to expand...
Ahh... OK I think that works! (Seems fine on paper.)
Thanks! You know I often get stuck down some rabbit hole of
a design, and instead of digging deeper I need to back out and
crawl down some other hole.

George H.
--
This email has not been checked by half-arsed antivirus software
Click to expand...
 
On Tuesday, February 21, 2017 at 9:17:52 AM UTC-5, Phil Hobbs wrote:
On 02/20/2017 08:08 PM, George Herold wrote:
On Monday, February 20, 2017 at 7:48:53 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 16:29:53 -0800, George Herold wrote:

On Monday, February 20, 2017 at 7:15:03 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 13:35:26 -0800, George Herold 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.

This:

Let R = maxcurrent / (Vcc - Vfwd)

It assumes that Vcc = -Vee, and that the two diodes' Vfwd is reasonably
close, but it'll get the job done. Can be easily rearranged for a
single supply, if you have enough overhead voltage. If you need more
gain put a resistor in the leg between R and the V- input, with another
resistor to ground.


Vcc
o
|\|
.------|-\
| | >----o----.
o--------)------|+/ | |
| |/| | |
| o - V -
| Vee ^ -
| | |
| o----'
| | '--------------o
|
.-. | | R | | '-'
|
|
=== GND
(created by AACircuit v1.28.7 beta 02/28/13 www.tech-chat.de)

--

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

I'm looking for work -- see my website!

Right.. did my dropbox link not work?

George H.

Yes it did -- but you have a bunch of extra stuff in that schematic
that's not necessary if you size R correctly.

I want to be able to change R..photons/ volt...
(though leds are not all that great at low current)
from ~1k ohm to ~50 ohms...

Speaking of which I'm using a green /red bipolar led,
not all that great at matching the eyes response...
(the green is much brighter than the red, eye-wise)
would red/orange... green/blue* be better?

If you use a Howland current source (1 op amp plus one sense resistor
plus one quad R pack) you can just put the pot as a "volume control" in
front of it.
Click to expand...

Thanks Phil, I've never used a Howland current source.
Is there some advantage over the more traditional circuit?
(Like Jasen's above.)
The Howland keeps the load grounded... but I don't see how that helps me here...

George H.
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
Click to expand...
 
On 02/20/2017 08:08 PM, George Herold wrote:
On Monday, February 20, 2017 at 7:48:53 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 16:29:53 -0800, George Herold wrote:

On Monday, February 20, 2017 at 7:15:03 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 13:35:26 -0800, George Herold 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.

This:

Let R = maxcurrent / (Vcc - Vfwd)

It assumes that Vcc = -Vee, and that the two diodes' Vfwd is reasonably
close, but it'll get the job done. Can be easily rearranged for a
single supply, if you have enough overhead voltage. If you need more
gain put a resistor in the leg between R and the V- input, with another
resistor to ground.


Vcc
o
|\|
.------|-\
| | >----o----.
o--------)------|+/ | |
| |/| | |
| o - V -
| Vee ^ -
| | |
| o----'
| | '--------------o
|
.-. | | R | | '-'
|
|
=== GND
(created by AACircuit v1.28.7 beta 02/28/13 www.tech-chat.de)

--

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

I'm looking for work -- see my website!

Right.. did my dropbox link not work?

George H.

Yes it did -- but you have a bunch of extra stuff in that schematic
that's not necessary if you size R correctly.

I want to be able to change R..photons/ volt...
(though leds are not all that great at low current)
from ~1k ohm to ~50 ohms...

Speaking of which I'm using a green /red bipolar led,
not all that great at matching the eyes response...
(the green is much brighter than the red, eye-wise)
would red/orange... green/blue* be better?
Click to expand...

If you use a Howland current source (1 op amp plus one sense resistor
plus one quad R pack) you can just put the pot as a "volume control" in
front of it.

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 02/21/2017 09:54 AM, George Herold wrote:
On Tuesday, February 21, 2017 at 9:17:52 AM UTC-5, Phil Hobbs wrote:
On 02/20/2017 08:08 PM, George Herold wrote:
On Monday, February 20, 2017 at 7:48:53 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 16:29:53 -0800, George Herold wrote:

On Monday, February 20, 2017 at 7:15:03 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 13:35:26 -0800, George Herold 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.

This:

Let R = maxcurrent / (Vcc - Vfwd)

It assumes that Vcc = -Vee, and that the two diodes' Vfwd is reasonably
close, but it'll get the job done. Can be easily rearranged for a
single supply, if you have enough overhead voltage. If you need more
gain put a resistor in the leg between R and the V- input, with another
resistor to ground.


Vcc
o
|\|
.------|-\
| | >----o----.
o--------)------|+/ | |
| |/| | |
| o - V -
| Vee ^ -
| | |
| o----'
| | '--------------o
|
.-. | | R | | '-'
|
|
=== GND
(created by AACircuit v1.28.7 beta 02/28/13 www.tech-chat.de)

--

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

I'm looking for work -- see my website!

Right.. did my dropbox link not work?

George H.

Yes it did -- but you have a bunch of extra stuff in that schematic
that's not necessary if you size R correctly.

I want to be able to change R..photons/ volt...
(though leds are not all that great at low current)
from ~1k ohm to ~50 ohms...

Speaking of which I'm using a green /red bipolar led,
not all that great at matching the eyes response...
(the green is much brighter than the red, eye-wise)
would red/orange... green/blue* be better?

If you use a Howland current source (1 op amp plus one sense resistor
plus one quad R pack) you can just put the pot as a "volume control" in
front of it.

Thanks Phil, I've never used a Howland current source.
Is there some advantage over the more traditional circuit?
(Like Jasen's above.)
The Howland keeps the load grounded... but I don't see how that helps me here...
Click to expand...

Either one works.

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 Mon, 20 Feb 2017 17:08:02 -0800, George Herold wrote:

On Monday, February 20, 2017 at 7:48:53 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 16:29:53 -0800, George Herold wrote:

On Monday, February 20, 2017 at 7:15:03 PM UTC-5, Tim Wescott wrote:
On Mon, 20 Feb 2017 13:35:26 -0800, George Herold 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.

This:

Let R = maxcurrent / (Vcc - Vfwd)

It assumes that Vcc = -Vee, and that the two diodes' Vfwd is
reasonably close, but it'll get the job done. Can be easily
rearranged for a single supply, if you have enough overhead voltage.
If you need more gain put a resistor in the leg between R and the
V- input, with another resistor to ground.


Vcc
o
|\|
.------|-\
| | >----o----.
o--------)------|+/ | |
| |/| | |
| o - V -
| Vee ^ -
| | |
| o----'
| | '--------------o
|
.-. | | R | | '-'
|
|
=== GND
(created by AACircuit v1.28.7 beta 02/28/13 www.tech-chat.de)

--

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

I'm looking for work -- see my website!

Right.. did my dropbox link not work?

George H.

Yes it did -- but you have a bunch of extra stuff in that schematic
that's not necessary if you size R correctly.

I want to be able to change R..photons/ volt...
(though leds are not all that great at low current)
from ~1k ohm to ~50 ohms...
Click to expand...

What I drew is a voltage follower on the voltage on R. If you want more
current/volt, then you can either reduce R or you can make the thing into
a voltage amplifier on the voltage on R by putting a resistor from V- to
ground, and a resistor from the junction of R and the photodiode to V-.

Speaking of which I'm using a green /red bipolar led,
not all that great at matching the eyes response...
(the green is much brighter than the red, eye-wise)
would red/orange... green/blue* be better?
Click to expand...

Oh, a green/red LED matches my eye response just fine -- I can't tell the
difference between them.

I'm not sure what would be better.

--

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

I'm looking for work -- see my website!
 
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
Click to expand...

Would this do what you want?

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


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
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
Click to expand...

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.)

Oh Jasen, if you are ever in Buffalo, I owe you a beer, or your beverage of choice.

George H.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
Click to expand...
 
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.
Click to expand...

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. :^)


--

John Larkin Highland Technology, Inc

lunatic fringe electronics
Click to expand...
 
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