led's

[John Larkin]

On Fri, 2 Aug 2013 06:53:06 -0700 (PDT), George Herold <gherold@teachspin.com>
wrote:

On Thursday, August 1, 2013 3:51:40 PM UTC-4, John Larkin wrote:
On Thu, 1 Aug 2013 10:30:01 -0700 (PDT), George Herold

big snip other stuff


Speaking of modulating diode lasers, Cliff Stoll (who's quite a lovable 'character'.) Was visiting the other day. He does a bunch of educational outreach and uses a modulated diode laser, beam splitter, and cheap corner cube reflector to measure the speed of light. But what he needs is a cheap (fast) photodiode detector. I put him on to Phil's book... but I've been thinking it might be a nice project.



Phil is fond of ebay APDs, which are apparently surplus from some



expensive projects. How fast do you need? Pulse or sine wave?



Yeah, all good questions. I'll have to send an email to Cliff. I'm guessing it'll be easier to modulate with a sine wave. So at 100MHz I'd get a full 2*pi phase shift with a path lenght of 3 meters. That seems reasonable. But I guess a bit slower would work too ~50 MHz. (Sometimes I make something, measure it, and then define the spec.)



I'm not sure we can use ebay as a source, unless I can buy a few build it and then get several hundred more. And do I need APD's? I've been reverse biasing all sorts of diodes lately. The optoelectronics PD's I'm using list a maximum reverse bias of 30V, I've had 'em up to 60V and no problem. (I ran out of voltage.) I was wondering if I could make garden variety PD's avalanche.





A modulated laser, ballpark 1 milliwatt, will make gobs of signal into

an ordinary photodiode, no need to avalanche. If you're using sine

waves, and can use a tuned amp, even better.

Hey! That's interesting. Could I resonate the PD capacitance with some inductor? I could even tune it a bit with the PD reverse bias. (Or were you thinking of a tuned stage after the PD?)

You could do either, but resonating the PD itself, maybe into a gaasfet gate,
sounds good. You could emulate an RF front end, where a resonated phemt
amplifier can achieve noise temperatures in the 40K range. And you could tune it
with PD bias, nice idea. The sensitivity could be outrageous, and it would
reject room light pretty well.

(I had this 'crazy' idea in the past about using a T-coil* as part of a PD front end... only to find that Phil H. had already done it.)

That's better for wideband (time domain) stuff, I think.


--

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

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

 

[George Herold]

On Thursday, August 1, 2013 11:23:52 PM UTC-4, John Larkin wrote:

On Thu, 01 Aug 2013 19:35:47 -0500, John Fields <jfields@austininstruments.com

<snip>

You're confused.



Since Ohm's law is used to determine the relationship between voltage,

current, and resistance - AT A SINGLE POINT - it, alone, can say

nothing about the linearity of the relationship between voltage and

current.



That's goofy.



---

Not at all, it's just true.



If you choose to contest the veracity of my claim, then I expect you

to prove, mathematically, that I'm wrong.



Can you?

---



It's not a law, it's just two numbers that you enjoy

dividing because you think it means something.



---

Wrong.



It is a law, and it defines the instantaneous relationship between

voltage, current, and resistance in a circuit.



The way you use it, it's not a law, it's a definition.

Hey this is kinda interesting. (But let's not have a big John vs John confrontation.)

So last week I was running this workshop on noise. I knew I'd have some spare time while the attendees were doing stuff. So I took along a setup to measure the Johnson noise of a light bulb with a DC current going through it. (The measruements were a bit of a pain, I had to abandon the inductor I was using as a bias element and go with a simple resistor...anyway that's not important.)
So at some voltage across the light bulb I measured the current. And I took that ratio to be the resistance of the bulb. And then I assumed that the bulb would be making Johnson noise given by v^2 = 4kTR*BW. Where I'd see more noise because of increased temperature of the bulb.
(The idea was to try and measure the temperature.)
Do you think there is something wrong with this 'theory'?
Does the light bulb have resistance?
Does it have Johnson noise?
What's the 'correct' relation between them?

George H.

John Larkin Highland Technology Inc

www.highlandtechnology.com jlarkin at highlandtechnology dot com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom timing and laser controllers

Photonics and fiberoptic TTL data links

VME analog, thermocouple, LVDT, synchro, tachometer

Multichannel arbitrary waveform generators

 

[Phil Hobbs]

On 08/01/2013 11:23 PM, John Larkin wrote:

On Thu, 01 Aug 2013 19:35:47 -0500, John Fields <jfields@austininstruments.com
wrote:

On Thu, 01 Aug 2013 15:39:01 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:

On Thu, 01 Aug 2013 15:58:54 -0500, John Fields
jfields@austininstruments.com> wrote:

On Thu, 01 Aug 2013 12:47:49 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:

On Thu, 01 Aug 2013 14:13:26 -0500, John Fields
jfields@austininstruments.com> wrote:

On Thu, 01 Aug 2013 11:10:08 -0700, Daniel Pitts
newsgroup.nospam@virtualinfinity.net> wrote:

On 8/1/13 6:59 AM, John Fields wrote:
On Wed, 31 Jul 2013 16:23:44 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:

On Wed, 31 Jul 2013 08:58:12 -0500, John Fields
jfields@austininstruments.com> wrote:

On Tue, 30 Jul 2013 17:46:34 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:
http://www.avagotech.com/docs/AV02-1029EN

Those are not "3 volt leds." Their typ forward drop is 3.2, but they
are still not shown as being voltage operable; they are spec'd at 20
mA.

---
Well, with no voltage to drive current through the junction they
wouldn't work at all, so they are certainly "voltage operable".

Also, notice that the data sheet shows a range of from 2.8 to 3.8V
across the junction with 20 mA through it, so 3.0V dropped across the
junction with 20mA through it is certainly within the range of
possibilities.

Consequently, for that LED, connecting 3.0V to it directly will force
20mA through it.
You've got cause and effect backwards here. Forcing 20ma through it will
cause ~3.2v drop across the LED. It is not necessarily the case that
3.2v will cause 20ma. Diodes have an exponential curve relating voltage
to current, a slight change in voltage can have a significant change in
current. Which is why you want to have some other device (eg, a
resistor) to help set the current.

Ergo, 3 volt LED.
Ergo, Nope.

---
It appears that you've entirely missed the point, which is that with
20mA forced through the LED, the voltage across it will vary from a
minimum of 2.8 to a maximum of 3.8V.

Such being the case, there will be some number of LEDs which, when
20mA is forced through them, will drop 3.0V.

Those LEDs, then, because of their own internal resistance:

Vf 3V
Rs = ---- = ------- = 150 ohms,
If 0.02A

will pass 20mA when they're excited by a non-current-limited 3 volt
supply.

It's meaningless and circular (and not predictive) to force "Ohm's
Law" onto a nonlinear device.

---
Complete and utter nonsense, since it's a very good way to determine
the linearity of a device.

For example, quite some time ago I measured - at various voltages -
the voltage across, and the current through several incandescent lamp
filaments and, by using Ohm's law was able to plot the resistance of
the filament as a function of the voltage across the filament.

What makes sense is to plot current vs voltage.

---
Of course, since that's all that's available, but it doesn't really
need to be plotted, - except for esthetics - it just needs to be
acquired, the E/I division done, and then the quotient plotted against
voltage.

Would you like to see the plots?
---

When you plotted resistance, was it E/I or dE/dI?

---
E/I
---

Was delta R/delta V a constant?

I think you know the answer to that one.

In much the same way, I used a diode instead of an incandescent lamp,
and was able to determine that a diode doesn't go ohmic until the
current through it causes the temperature of the diode to go high
enough to almost destroy it. I ran into measurement problems up
there, so the data's not all that reliable, but if there's any
interest I can do the experiment over again and tighten up the
environment.
---

It is a good way to blow up LEDs.

---
Ya think so???

OK, try this:

1. Get an LED and run its rated current through it.
2. Measure the voltage dropped across the LED.
3. Disconnect the supply and set it to the voltage you measured.
4. Leave the ammeter in place and connect the voltage supply across
the unconnected ends of the ammeter and the LED.
5. Post the currents measured in step 1 and step 4.
---

Ohm's Law isn't even a law. It's a statement that some devices more or
less sometimes have a linear relation between voltage and current.

---
You're confused.

Since Ohm's law is used to determine the relationship between voltage,
current, and resistance - AT A SINGLE POINT - it, alone, can say
nothing about the linearity of the relationship between voltage and
current.

That's goofy.

---
Not at all, it's just true.

If you choose to contest the veracity of my claim, then I expect you
to prove, mathematically, that I'm wrong.

Can you?
---

It's not a law, it's just two numbers that you enjoy
dividing because you think it means something.

---
Wrong.

It is a law, and it defines the instantaneous relationship between
voltage, current, and resistance in a circuit.

The way you use it, it's not a law, it's a definition.

There's a lot of interesting physics behind Ohm's law. Among other
things, it requires that the scattering length in the resistive medium
be very short compared with the distance required to acquire enough
momentum to cause impact ionization. Really ohmic things, like metals,
are pretty remarkable.

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

 

[Phil Hobbs]

On 08/02/2013 10:13 AM, George Herold wrote:

On Thursday, August 1, 2013 11:23:52 PM UTC-4, John Larkin wrote:
On Thu, 01 Aug 2013 19:35:47 -0500, John Fields <jfields@austininstruments.com

snip

You're confused.



Since Ohm's law is used to determine the relationship between voltage,

current, and resistance - AT A SINGLE POINT - it, alone, can say

nothing about the linearity of the relationship between voltage and

current.



That's goofy.



---

Not at all, it's just true.



If you choose to contest the veracity of my claim, then I expect you

to prove, mathematically, that I'm wrong.



Can you?

---



It's not a law, it's just two numbers that you enjoy

dividing because you think it means something.



---

Wrong.



It is a law, and it defines the instantaneous relationship between

voltage, current, and resistance in a circuit.



The way you use it, it's not a law, it's a definition.

Hey this is kinda interesting. (But let's not have a big John vs John confrontation.)

So last week I was running this workshop on noise. I knew I'd have some spare time while the attendees were doing stuff. So I took along a setup to measure the Johnson noise of a light bulb with a DC current going through it. (The measruements were a bit of a pain, I had to abandon the inductor I was using as a bias element and go with a simple resistor...anyway that's not important.)
So at some voltage across the light bulb I measured the current. And I took that ratio to be the resistance of the bulb. And then I assumed that the bulb would be making Johnson noise given by v^2 = 4kTR*BW. Where I'd see more noise because of increased temperature of the bulb.
(The idea was to try and measure the temperature.)
Do you think there is something wrong with this 'theory'?
Does the light bulb have resistance?
Does it have Johnson noise?
What's the 'correct' relation between them?

George H.

Johnson noise is pretty small in amplitude, so it's only sensitive to
the differential resistance. On time scales long compared with the
thermal time constant of the filament, you'd have to worry about the
nonlinearity, but at short time scales it would just look like a 2500K
resistor, I'd expect.

The filament is obviously not in thermodynamic equilibrium, so the usual
math underlying the Johnson noise formula doesn't quite work, but with
DC excitation I'd expect it to be very close. It would certainly work
if the filament were at equilibrium in an isothermal 2500K oven, and
metal conduction is pretty well exactly ohmic, so I wouldn't expect the
comparatively small net drift velocity to make much difference at all.

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

 

[George Herold]

On Friday, August 2, 2013 12:04:13 PM UTC-4, John Larkin wrote:

On Fri, 2 Aug 2013 07:13:30 -0700 (PDT), George Herold <gherold@teachspin..com

wrote:



On Thursday, August 1, 2013 11:23:52 PM UTC-4, John Larkin wrote:

On Thu, 01 Aug 2013 19:35:47 -0500, John Fields <jfields@austininstruments.com



snip



You're confused.







Since Ohm's law is used to determine the relationship between voltage,



current, and resistance - AT A SINGLE POINT - it, alone, can say



nothing about the linearity of the relationship between voltage and



current.







That's goofy.







---



Not at all, it's just true.







If you choose to contest the veracity of my claim, then I expect you



to prove, mathematically, that I'm wrong.







Can you?



---







It's not a law, it's just two numbers that you enjoy



dividing because you think it means something.







---



Wrong.







It is a law, and it defines the instantaneous relationship between



voltage, current, and resistance in a circuit.







The way you use it, it's not a law, it's a definition.



Hey this is kinda interesting. (But let's not have a big John vs John confrontation.)



So last week I was running this workshop on noise. I knew I'd have some spare time while the attendees were doing stuff. So I took along a setup to measure the Johnson noise of a light bulb with a DC current going through it. (The measruements were a bit of a pain, I had to abandon the inductor I was using as a bias element and go with a simple resistor...anyway that's not important.)

So at some voltage across the light bulb I measured the current. And I took that ratio to be the resistance of the bulb. And then I assumed that the bulb would be making Johnson noise given by v^2 = 4kTR*BW. Where I'd see more noise because of increased temperature of the bulb.

(The idea was to try and measure the temperature.)

Do you think there is something wrong with this 'theory'?

Does the light bulb have resistance?

Does it have Johnson noise?

What's the 'correct' relation between them?



George H.



Sure, any resistor has Johnson noise.



If you were to drive the filament through an inductor or a noiseless current

source, and couple a small AC signal into it with a capacitor, at higher

frequencies it would look like a resistor of some ohms, and it would have the

corresponding Johnson noise. If the filament is incandescent, the Johnson noise

will be high.

Yup, that's what I tried (inductor from voltage source) (Driving a light bulb with a current source is asking for trouble. IMHO)
Problem was I didn't have a big enough toroid and the coil inductor picked up magnetic noise big time. (I already knew this, but sometimes I need my face rubbed into something a few times till I remember.)

So I got some data with a resistor as bias, but at the highest temperatures the bias resistance was much smaller than the DC bulb resistance and it shunted most of the noise to ground.

That equivalent Johnson resistance would not be the DC E/I and will (probably)

be close to the slope, dE/dI at the operating point. Thermal mass makes dE/dI

frequency dependant. The exact math is beyond my pay grade.

Hmm.. OK I'll have to think some more... why dE/dI?

RE the thermal mass: The time constant for a bulb is something like a second maybe 10mS at the fastest. So only at low frequency is that going to be an issue.

I bet a filament has mountains of low frequency noise. Might be interesting to

measure.

Well I had the thing perched on bubble wrap to try and keep the building shake out of it. Better might be to suspend it. I would sometimes see a bunch of low frequency 'crude'. Which I assumed was adjacent filament loops bumping into each other. But there were long periods of relative quite. (I also could filter out the LF stuff.)

One issue is that the filament loops were also an inductor and they would pickup local magnetic field interference. (Mostly from the room lights, which I couldn't turn off because those in the workshop would have objected.) The pickup would increase at higher bulb currents.. which still has me a bit confused. So before I try this again I need some big torodial inductors and some mu metal to shield the bulb.

But this resistance question is great! What does resistance really mean? (Should I be thinking in terms of damping or energy?)

George H.

--



John Larkin Highland Technology Inc

www.highlandtechnology.com jlarkin at highlandtechnology dot com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom timing and laser controllers

Photonics and fiberoptic TTL data links

VME analog, thermocouple, LVDT, synchro, tachometer

Multichannel arbitrary waveform generators

 

[Phil Hobbs]

On 08/02/2013 12:10 PM, John Larkin wrote:

On Fri, 2 Aug 2013 06:53:06 -0700 (PDT), George Herold <gherold@teachspin.com
wrote:

On Thursday, August 1, 2013 3:51:40 PM UTC-4, John Larkin wrote:
On Thu, 1 Aug 2013 10:30:01 -0700 (PDT), George Herold

big snip other stuff


Speaking of modulating diode lasers, Cliff Stoll (who's quite a lovable 'character'.) Was visiting the other day. He does a bunch of educational outreach and uses a modulated diode laser, beam splitter, and cheap corner cube reflector to measure the speed of light. But what he needs is a cheap (fast) photodiode detector. I put him on to Phil's book... but I've been thinking it might be a nice project.



Phil is fond of ebay APDs, which are apparently surplus from some



expensive projects. How fast do you need? Pulse or sine wave?



Yeah, all good questions. I'll have to send an email to Cliff. I'm guessing it'll be easier to modulate with a sine wave. So at 100MHz I'd get a full 2*pi phase shift with a path lenght of 3 meters. That seems reasonable. But I guess a bit slower would work too ~50 MHz. (Sometimes I make something, measure it, and then define the spec.)



I'm not sure we can use ebay as a source, unless I can buy a few build it and then get several hundred more. And do I need APD's? I've been reverse biasing all sorts of diodes lately. The optoelectronics PD's I'm using list a maximum reverse bias of 30V, I've had 'em up to 60V and no problem. (I ran out of voltage.) I was wondering if I could make garden variety PD's avalanche.





A modulated laser, ballpark 1 milliwatt, will make gobs of signal into

an ordinary photodiode, no need to avalanche. If you're using sine

waves, and can use a tuned amp, even better.

Hey! That's interesting. Could I resonate the PD capacitance with some inductor? I could even tune it a bit with the PD reverse bias. (Or were you thinking of a tuned stage after the PD?)

You could do either, but resonating the PD itself, maybe into a gaasfet gate,
sounds good. You could emulate an RF front end, where a resonated phemt
amplifier can achieve noise temperatures in the 40K range. And you could tune it
with PD bias, nice idea. The sensitivity could be outrageous, and it would
reject room light pretty well.

(I had this 'crazy' idea in the past about using a T-coil* as part of a PD front end... only to find that Phil H. had already done it.)

That's better for wideband (time domain) stuff, I think.

There's a theorem of Bode's (*) to the effect that if you have a
parallel RC, the integral over all frequencies of the log magnitude of
the insertion gain can't exceed pi/RC, regardless of what matching
network you pick. T-coils can give you a 2.8x rise time improvement for
a given load resistance, which is better than you can usually get with
other simple matching networks.

Using the PD as a varactor is a good trick, but one thing you have to
watch for is the way the Q varies with bias. It also changes the
responsivity dramatically at longer wavelengths, where the light makes
it all the way to the bottom of the depletion zone.

Cheers

Phil Hobbs

(*) Pronounced bohd-uh, as in Dutch and German, not bo-dee, as in Hindi
and soggy canned spaghetti. (Reference: his obituary pointed this out
particularly.)

--
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

 

[John Larkin]

On Fri, 2 Aug 2013 09:52:14 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Friday, August 2, 2013 12:04:13 PM UTC-4, John Larkin wrote:
On Fri, 2 Aug 2013 07:13:30 -0700 (PDT), George Herold <gherold@teachspin.com

wrote:



On Thursday, August 1, 2013 11:23:52 PM UTC-4, John Larkin wrote:

On Thu, 01 Aug 2013 19:35:47 -0500, John Fields <jfields@austininstruments.com



snip



You're confused.







Since Ohm's law is used to determine the relationship between voltage,



current, and resistance - AT A SINGLE POINT - it, alone, can say



nothing about the linearity of the relationship between voltage and



current.







That's goofy.







---



Not at all, it's just true.







If you choose to contest the veracity of my claim, then I expect you



to prove, mathematically, that I'm wrong.







Can you?



---







It's not a law, it's just two numbers that you enjoy



dividing because you think it means something.







---



Wrong.







It is a law, and it defines the instantaneous relationship between



voltage, current, and resistance in a circuit.







The way you use it, it's not a law, it's a definition.



Hey this is kinda interesting. (But let's not have a big John vs John confrontation.)



So last week I was running this workshop on noise. I knew I'd have some spare time while the attendees were doing stuff. So I took along a setup to measure the Johnson noise of a light bulb with a DC current going through it. (The measruements were a bit of a pain, I had to abandon the inductor I was using as a bias element and go with a simple resistor...anyway that's not important.)

So at some voltage across the light bulb I measured the current. And I took that ratio to be the resistance of the bulb. And then I assumed that the bulb would be making Johnson noise given by v^2 = 4kTR*BW. Where I'd see more noise because of increased temperature of the bulb.

(The idea was to try and measure the temperature.)

Do you think there is something wrong with this 'theory'?

Does the light bulb have resistance?

Does it have Johnson noise?

What's the 'correct' relation between them?



George H.



Sure, any resistor has Johnson noise.



If you were to drive the filament through an inductor or a noiseless current

source, and couple a small AC signal into it with a capacitor, at higher

frequencies it would look like a resistor of some ohms, and it would have the

corresponding Johnson noise. If the filament is incandescent, the Johnson noise

will be high.

Yup, that's what I tried (inductor from voltage source) (Driving a light bulb with a current source is asking for trouble. IMHO)
Problem was I didn't have a big enough toroid and the coil inductor picked up magnetic noise big time. (I already knew this, but sometimes I need my face rubbed into something a few times till I remember.)

So I got some data with a resistor as bias, but at the highest temperatures the bias resistance was much smaller than the DC bulb resistance and it shunted most of the noise to ground.



That equivalent Johnson resistance would not be the DC E/I and will (probably)

be close to the slope, dE/dI at the operating point. Thermal mass makes dE/dI

frequency dependant. The exact math is beyond my pay grade.

Hmm.. OK I'll have to think some more... why dE/dI?

RE the thermal mass: The time constant for a bulb is something like a second maybe 10mS at the fastest. So only at low frequency is that going to be an issue.

I did once make a photo-optical link that transmitted usable audio,
from a radio. With a flashlight bulb.

I bet a filament has mountains of low frequency noise. Might be interesting to

measure.

Well I had the thing perched on bubble wrap to try and keep the building shake out of it. Better might be to suspend it. I would sometimes see a bunch of low frequency 'crude'. Which I assumed was adjacent filament loops bumping into each other. But there were long periods of relative quite. (I also could filter out the LF stuff.)

The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

One issue is that the filament loops were also an inductor and they would pickup local magnetic field interference. (Mostly from the room lights, which I couldn't turn off because those in the workshop would have objected.) The pickup would increase at higher bulb currents.. which still has me a bit confused. So before I try this again I need some big torodial inductors and some mu metal to shield the bulb.

But this resistance question is great! What does resistance really mean? (Should I be thinking in terms of damping or energy?)

AC coupled into the filament, it looks just like a resistor. I'm not
sure what the small-signal value will be.


--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation

 

[Jasen Betts]

On 2013-08-02, John Larkin <jlarkin@highlandtechnology.com> wrote:

The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

Flashlight bulbs used to be "filled" with a vacuum, I think they're
using krypton in some now.

One issue is that the filament loops were also an inductor and they
would pickup local magnetic field interference. (Mostly from the room
lights, which I couldn't turn off because those in the workshop would
have objected.) The pickup would increase at higher bulb currents..
which still has me a bit confused. So before I try this again I need
some big torodial inductors and some mu metal to shield the bulb.

probably magnetic fields were moving the filament, changing is emission
pattern and therefore measured output.


--
⚂⚃ 100% natural

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

 

[John Fields]

On Fri, 02 Aug 2013 12:49:08 -0700, John Larkin
<jlarkin@highlandtechnology.com> wrote:

The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

---
The filament of an incandescent lamp is made of pure tungsten, which
has a positive temperature coefficient of resistance and is
extensively heat-treated in order to make it ductile enough to survive
the drawing and coiling process required to form it into the shape
required for a particular filament.

Thermistors are an entirely different breed of cat in that they're
constructed by pressing and sintering metal oxides, generally have a
negative temperature coefficient, and can only be enticed to emit
humanly visible photons once, and then only for a very short time.
---

One issue is that the filament loops were also an inductor and they would pickup local magnetic field interference. (Mostly from the room lights, which I couldn't turn off because those in the workshop would have objected.)

---
Marie and Pierre Curie sifted through a ton of pitchblende, in the
winter and in unheated quarters, and discovered Radium.

You couldn't make your measurements after everyone else left your
comfy lab and you had it all to yourself?
---

So before I try this again I need some big torodial inductors and some mu metal to >>shield the bulb.

---
Why?
---

But this resistance question is great! What does resistance really mean? (Should I be thinking in terms of damping or energy?)

---
It's arguably best to think of resistance as collisions between
electrons which liberate heat.
----

AC coupled into the filament, it looks just like a resistor. I'm not
sure what the small-signal value will be.

---
With AC coupled into the filament, its wound nature will cause it to
look inductive, so it can't look like a pure resistance regardless of
how lightly it's driven.

--
JF

 

[Phil Hobbs]

On 7/31/2013 7:28 PM, John Larkin wrote:

On Wed, 31 Jul 2013 08:00:56 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Tuesday, July 30, 2013 12:00:38 PM UTC-4, John Larkin wrote:
On Tue, 30 Jul 2013 07:15:57 -0700 (PDT), George Herold <gherold@teachspin.com

wrote:
On Monday, July 29, 2013 5:25:15 PM UTC-4, John Larkin wrote:

On Tue, 30 Jul 2013 01:28:23 +0800, "Mr. Man-wai Chang"
snip previus tuff
I was surprsded by the sub-nA light output. At roughly 100 mV/decade



current, I'd have guessed that the voltage drop would be so low that

it wouldn't have enough energy to make photons.

Ahh, but it's those electrons on the far end of the exponential 'Boltzmann tail' that have enough extra thermal energy to make it into the depletion region. (I'll have to try I-V curves for LED's at low temperatures someday, dunking into LN2 should be pretty easy.)



Well, now you're talking about physics and science and stuff like that.
Grin.. well lots of electronics looks like a branch of science to me.


Seems to me that you need X volts to make a photon of X electron-volts energy.


Can thermal energy bootstrap an electron to make a photon that's more EV than

the voltage across the junction?
There was some news 'splash' from (maybe) mit of someone getting more light out of an led than power in.. at some ridiculously low current.

(The good thing about electrons is there are a whole lot of them, so there are a few with many kT's of kinetic energy.)

There's also this 'common' physics demo where they look at the led wavlength and forward voltage drop for a bunch of different color leds and plot things up to get a measure of Planck's constant. (A bit of a bogus experiment when you look at the details.)



Modern LEDs are remarkable gadgets. It would be fun to explore the corner cases,

"someday" as you say.



We're currently experimenting with cheap (like, $12) semiconductor lasers to

find some that accidentally make clean picosecond light pulses when whacked by

one of our laser drivers. The manufacturers certainly don't know if they might.

Most laser data sheets are pitiful.

Just spent over a grand on Thorlabs hardware to hold a laser in place and get

some of its light into a fiber.
Ouch.. the only thing more expensive than optics is high vacuum.

Speaking of modulating diode lasers, Cliff Stoll (who's quite a lovable 'character'.) Was visiting the other day. He does a bunch of educational outreach and uses a modulated diode laser, beam splitter, and cheap corner cube reflector to measure the speed of light. But what he needs is a cheap (fast) photodiode detector. I put him on to Phil's book... but I've been thinking it might be a nice project.

Phil is fond of ebay APDs, which are apparently surplus from some
expensive projects. How fast do you need? Pulse or sine wave?


There was that few hundred pieces of the 1.3 um APDs surplused from

Terabeam that went for about 1 cent on the dollar three or four years
ago, but nothing interesting since.

We should still figure out something useful to do with those.

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 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[John Larkin]

On Fri, 2 Aug 2013 19:21:16 -0700 (PDT), George Herold <gherold@teachspin.com>
wrote:

On Friday, August 2, 2013 2:38:54 PM UTC-4, Phil Hobbs wrote:
On 08/02/2013 10:13 AM, George Herold wrote:

On Thursday, August 1, 2013 11:23:52 PM UTC-4, John Larkin wrote:

On Thu, 01 Aug 2013 19:35:47 -0500, John Fields <jfields@austininstruments.com



snip



You're confused.







Since Ohm's law is used to determine the relationship between voltage,



current, and resistance - AT A SINGLE POINT - it, alone, can say



nothing about the linearity of the relationship between voltage and



current.







That's goofy.







---



Not at all, it's just true.







If you choose to contest the veracity of my claim, then I expect you



to prove, mathematically, that I'm wrong.







Can you?



---







It's not a law, it's just two numbers that you enjoy



dividing because you think it means something.







---



Wrong.







It is a law, and it defines the instantaneous relationship between



voltage, current, and resistance in a circuit.







The way you use it, it's not a law, it's a definition.



Hey this is kinda interesting. (But let's not have a big John vs John confrontation.)



So last week I was running this workshop on noise. I knew I'd have some spare time while the attendees were doing stuff. So I took along a setup to measure the Johnson noise of a light bulb with a DC current going through it. (The measruements were a bit of a pain, I had to abandon the inductor I was using as a bias element and go with a simple resistor...anyway that's not important.)

So at some voltage across the light bulb I measured the current. And I took that ratio to be the resistance of the bulb. And then I assumed that the bulb would be making Johnson noise given by v^2 = 4kTR*BW. Where I'd see more noise because of increased temperature of the bulb.

(The idea was to try and measure the temperature.)

Do you think there is something wrong with this 'theory'?

Does the light bulb have resistance?

Does it have Johnson noise?

What's the 'correct' relation between them?



George H.

Johnson noise is pretty small in amplitude, so it's only sensitive to
the differential resistance. On time scales long compared with the
thermal time constant of the filament, you'd have to worry about the
nonlinearity, but at short time scales it would just look like a 2500K
resistor, I'd expect.

OK, I was thinking about this. (I couldn't wait for the weekend.)
So there's some energy dissipated in the bulb given by the DC V*I.
But that doesn't come into the Johnson noise.
Then I was thinking of the derivation of Johnson noise that starts with a parallel RC and the equipartition theorem. (the capacitor holds 1/2 kT of energy = 1/2 CV^2) and then you integrate over the RC bandwidth to get the Johnson noise formula, So 'R' in this case is, as you say, the dynamic R. And that's pretty cool!

So the data for my first crack at this was pretty crappy. (not much signal, since it was all lost in the small bias resistor.. big error bars.) But I got a number for the temperature of about 1500K for a bulb that was a little past orange and getting to yellow. And that number seemed small. But I was using a DC resistance value. The dynamical resistance looks to be ~50% bigger..

So what is the AC, small-signal resistance of a hot filament?

I'm guessing that's it's about equal to the gross resistance, E/I, at that
operating point. The DC slope resistance, dE/dI, at that point will be higher,
maybe 2x or thereabouts.


--

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

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

 

[George Herold]

On Friday, August 2, 2013 2:38:54 PM UTC-4, Phil Hobbs wrote:

On 08/02/2013 10:13 AM, George Herold wrote:

On Thursday, August 1, 2013 11:23:52 PM UTC-4, John Larkin wrote:

On Thu, 01 Aug 2013 19:35:47 -0500, John Fields <jfields@austininstruments.com



snip



You're confused.







Since Ohm's law is used to determine the relationship between voltage,



current, and resistance - AT A SINGLE POINT - it, alone, can say



nothing about the linearity of the relationship between voltage and



current.







That's goofy.







---



Not at all, it's just true.







If you choose to contest the veracity of my claim, then I expect you



to prove, mathematically, that I'm wrong.







Can you?



---







It's not a law, it's just two numbers that you enjoy



dividing because you think it means something.







---



Wrong.







It is a law, and it defines the instantaneous relationship between



voltage, current, and resistance in a circuit.







The way you use it, it's not a law, it's a definition.



Hey this is kinda interesting. (But let's not have a big John vs John confrontation.)



So last week I was running this workshop on noise. I knew I'd have some spare time while the attendees were doing stuff. So I took along a setup to measure the Johnson noise of a light bulb with a DC current going through it. (The measruements were a bit of a pain, I had to abandon the inductor I was using as a bias element and go with a simple resistor...anyway that's not important.)

So at some voltage across the light bulb I measured the current. And I took that ratio to be the resistance of the bulb. And then I assumed that the bulb would be making Johnson noise given by v^2 = 4kTR*BW. Where I'd see more noise because of increased temperature of the bulb.

(The idea was to try and measure the temperature.)

Do you think there is something wrong with this 'theory'?

Does the light bulb have resistance?

Does it have Johnson noise?

What's the 'correct' relation between them?



George H.

Johnson noise is pretty small in amplitude, so it's only sensitive to
the differential resistance. On time scales long compared with the
thermal time constant of the filament, you'd have to worry about the
nonlinearity, but at short time scales it would just look like a 2500K
resistor, I'd expect.

OK, I was thinking about this. (I couldn't wait for the weekend.)
So there's some energy dissipated in the bulb given by the DC V*I.
But that doesn't come into the Johnson noise.
Then I was thinking of the derivation of Johnson noise that starts with a parallel RC and the equipartition theorem. (the capacitor holds 1/2 kT of energy = 1/2 CV^2) and then you integrate over the RC bandwidth to get the Johnson noise formula, So 'R' in this case is, as you say, the dynamic R.. And that's pretty cool!

So the data for my first crack at this was pretty crappy. (not much signal, since it was all lost in the small bias resistor.. big error bars.) But I got a number for the temperature of about 1500K for a bulb that was a little past orange and getting to yellow. And that number seemed small. But I was using a DC resistance value. The dynamical resistance looks to be ~50% bigger..

The filament is obviously not in thermodynamic equilibrium, so the usual

math underlying the Johnson noise formula doesn't quite work, but with

DC excitation I'd expect it to be very close. It would certainly work

if the filament were at equilibrium in an isothermal 2500K oven, and

metal conduction is pretty well exactly ohmic, so I wouldn't expect the

comparatively small net drift velocity to make much difference at all.

Ah OK the drift velocity looks pretty small compared to all the other corrections.

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

 

[Ian Field]

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:ktha04$ku1$1@gonzo.reversiblemaps.ath.cx...

On 2013-08-02, John Larkin <jlarkin@highlandtechnology.com> wrote:

The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

Flashlight bulbs used to be "filled" with a vacuum, I think they're
using krypton in some now.

Last time I checked - the only way you can buy a flashlight bulb is buy a
flashlight with one in it.

There are still one or two discount stores that carry bulb type
flashlights - but increasingly they're dual type with a ring of LEDs round
the reflector.

 

[John Larkin]

On Sat, 3 Aug 2013 16:26:06 +0100, "Ian Field" <gangprobing.alien@ntlworld.com>
wrote:

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:ktha04$ku1$1@gonzo.reversiblemaps.ath.cx...
On 2013-08-02, John Larkin <jlarkin@highlandtechnology.com> wrote:

The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

Flashlight bulbs used to be "filled" with a vacuum, I think they're
using krypton in some now.


Last time I checked - the only way you can buy a flashlight bulb is buy a
flashlight with one in it.

You can buy spare Maglite bulbs.


--

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

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

 

[John Fields]

On Sat, 3 Aug 2013 16:26:06 +0100, "Ian Field"
<gangprobing.alien@ntlworld.com> wrote:

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:ktha04$ku1$1@gonzo.reversiblemaps.ath.cx...
On 2013-08-02, John Larkin <jlarkin@highlandtechnology.com> wrote:

The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

Flashlight bulbs used to be "filled" with a vacuum, I think they're
using krypton in some now.


Last time I checked - the only way you can buy a flashlight bulb is buy a
flashlight with one in it.

---

http://www.bulbtown.com/Standard_s/594.htm

http://www.bulbs.com/B--Type/results.aspx?adpos=1s1&AffID=6&creative=26228201718&device=c&matchtype=b&network=g

http://www.radioshack.com/family/index.jsp?categoryId=2032296

--
JF

 

[Ian Field]

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:b6bqv8demo1lahn0sbfcvdo6scpoqcdla9@4ax.com...

On Sat, 3 Aug 2013 16:26:06 +0100, "Ian Field"
gangprobing.alien@ntlworld.com
wrote:



"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:ktha04$ku1$1@gonzo.reversiblemaps.ath.cx...
On 2013-08-02, John Larkin <jlarkin@highlandtechnology.com> wrote:

The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

Flashlight bulbs used to be "filled" with a vacuum, I think they're
using krypton in some now.


Last time I checked - the only way you can buy a flashlight bulb is buy a
flashlight with one in it.

You can buy spare Maglite bulbs.

I bought a Maglite - and quickly formed the conclusion that they're
overrated.

A 3W LED flashlight is brighter, uses less batteries and the 'bulb' will
probably outlast me.

 

[John Larkin]

On Sat, 3 Aug 2013 18:44:58 +0100, "Ian Field" <gangprobing.alien@ntlworld.com>
wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:b6bqv8demo1lahn0sbfcvdo6scpoqcdla9@4ax.com...
On Sat, 3 Aug 2013 16:26:06 +0100, "Ian Field"
gangprobing.alien@ntlworld.com
wrote:



"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:ktha04$ku1$1@gonzo.reversiblemaps.ath.cx...
On 2013-08-02, John Larkin <jlarkin@highlandtechnology.com> wrote:

The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

Flashlight bulbs used to be "filled" with a vacuum, I think they're
using krypton in some now.


Last time I checked - the only way you can buy a flashlight bulb is buy a
flashlight with one in it.

You can buy spare Maglite bulbs.

I bought a Maglite - and quickly formed the conclusion that they're
overrated.

A 3W LED flashlight is brighter, uses less batteries and the 'bulb' will
probably outlast me.

The Maglites are physically very rugged, sealed, waterproof. Lots of LED lights
are junk and *don't* last long. The Maglite bulbs last because such lights are
not used for weeks at a time. They usually have a spare inside, too.

I got stuck in the dark on a cliff-side hiking trail on the north coast of
Kauai. The sun goes down fast, splat-sizzle, at that latitude, and then the
killer frogs attack. The trail was steep, dangerous, and covered by slippery
wormy guava fruits. My little Maglite saved my life, and my GFs as an added
bonus.


--

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

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

 

[Ian Field]

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:j2kqv8h5o1ee7soju61guatk37gmmhrfv3@4ax.com...

On Sat, 3 Aug 2013 18:44:58 +0100, "Ian Field"
gangprobing.alien@ntlworld.com
wrote:



"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:b6bqv8demo1lahn0sbfcvdo6scpoqcdla9@4ax.com...
On Sat, 3 Aug 2013 16:26:06 +0100, "Ian Field"
gangprobing.alien@ntlworld.com
wrote:



"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:ktha04$ku1$1@gonzo.reversiblemaps.ath.cx...
On 2013-08-02, John Larkin <jlarkin@highlandtechnology.com> wrote:

The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

Flashlight bulbs used to be "filled" with a vacuum, I think they're
using krypton in some now.


Last time I checked - the only way you can buy a flashlight bulb is buy
a
flashlight with one in it.

You can buy spare Maglite bulbs.

I bought a Maglite - and quickly formed the conclusion that they're
overrated.

A 3W LED flashlight is brighter, uses less batteries and the 'bulb' will
probably outlast me.

The Maglites are physically very rugged, sealed, waterproof. Lots of LED
lights
are junk and *don't* last long. The Maglite bulbs last because such lights
are
not used for weeks at a time. They usually have a spare inside, too.

I got stuck in the dark on a cliff-side hiking trail on the north coast of
Kauai. The sun goes down fast, splat-sizzle, at that latitude, and then
the
killer frogs attack.

Have you been licking toads again!!!

(If you're playing sniper in the jungle - you can stick a frog on the end of
your rifle to quench the muzzle flash).

As I do a fair bit of cycling, and often after dark - I keep a spare 5-LED
cycle light in my pocket, it does for almost anything I need a flashlight
for. At 1GBP I'm not too worried about losing one ever once in a while.

Some of the single, power LED ones are nearly as pricey as a maglite, so I
use them mainly where they can't go far if I put them down without thinking.

 

[John Larkin]

On Sat, 3 Aug 2013 20:49:36 +0100, "Ian Field" <gangprobing.alien@ntlworld.com>
wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:j2kqv8h5o1ee7soju61guatk37gmmhrfv3@4ax.com...
On Sat, 3 Aug 2013 18:44:58 +0100, "Ian Field"
gangprobing.alien@ntlworld.com
wrote:



"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:b6bqv8demo1lahn0sbfcvdo6scpoqcdla9@4ax.com...
On Sat, 3 Aug 2013 16:26:06 +0100, "Ian Field"
gangprobing.alien@ntlworld.com
wrote:



"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:ktha04$ku1$1@gonzo.reversiblemaps.ath.cx...
On 2013-08-02, John Larkin <jlarkin@highlandtechnology.com> wrote:

The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

Flashlight bulbs used to be "filled" with a vacuum, I think they're
using krypton in some now.


Last time I checked - the only way you can buy a flashlight bulb is buy
a
flashlight with one in it.

You can buy spare Maglite bulbs.

I bought a Maglite - and quickly formed the conclusion that they're
overrated.

A 3W LED flashlight is brighter, uses less batteries and the 'bulb' will
probably outlast me.

The Maglites are physically very rugged, sealed, waterproof. Lots of LED
lights
are junk and *don't* last long. The Maglite bulbs last because such lights
are
not used for weeks at a time. They usually have a spare inside, too.

I got stuck in the dark on a cliff-side hiking trail on the north coast of
Kauai. The sun goes down fast, splat-sizzle, at that latitude, and then
the
killer frogs attack.

Have you been licking toads again!!!

The GF was quite pretty, in fact.


--

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

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

 

[John Fields]

On Sat, 3 Aug 2013 20:49:36 +0100, "Ian Field"
<gangprobing.alien@ntlworld.com> wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:j2kqv8h5o1ee7soju61guatk37gmmhrfv3@4ax.com...
On Sat, 3 Aug 2013 18:44:58 +0100, "Ian Field"
gangprobing.alien@ntlworld.com
wrote:



"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:b6bqv8demo1lahn0sbfcvdo6scpoqcdla9@4ax.com...
On Sat, 3 Aug 2013 16:26:06 +0100, "Ian Field"
gangprobing.alien@ntlworld.com
wrote:



"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:ktha04$ku1$1@gonzo.reversiblemaps.ath.cx...
On 2013-08-02, John Larkin <jlarkin@highlandtechnology.com> wrote:

The filament is a heated thermistor that makes its own turbulence in
the gas. Gotta be noisy.

Flashlight bulbs used to be "filled" with a vacuum, I think they're
using krypton in some now.


Last time I checked - the only way you can buy a flashlight bulb is buy
a
flashlight with one in it.

You can buy spare Maglite bulbs.

I bought a Maglite - and quickly formed the conclusion that they're
overrated.

A 3W LED flashlight is brighter, uses less batteries and the 'bulb' will
probably outlast me.

The Maglites are physically very rugged, sealed, waterproof. Lots of LED
lights
are junk and *don't* last long. The Maglite bulbs last because such lights
are
not used for weeks at a time. They usually have a spare inside, too.

I got stuck in the dark on a cliff-side hiking trail on the north coast of
Kauai. The sun goes down fast, splat-sizzle, at that latitude, and then
the
killer frogs attack.

Have you been licking toads again!!!

(If you're playing sniper in the jungle - you can stick a frog on the end of
your rifle to quench the muzzle flash).

---
Quenching the muzzle flash is of no use at all when using a supersonic
projectile traversing a long distance, which is what snipers do, since
the flash will have died out long before the target is rendered meat
and the direction of the shot undiscernable.

Sticking a frog on the business end of the barrel to quench the muzzle
flash would seriously affect the trajectory of the bullet, idiot.
---

As I do a fair bit of cycling, and often after dark - I keep a spare 5-LED
cycle light in my pocket, it does for almost anything I need a flashlight
for. At 1GBP I'm not too worried about losing one ever once in a while.

Some of the single, power LED ones are nearly as pricey as a maglite, so I
use them mainly where they can't go far if I put them down without thinking.

---
Then you probably lose a lot of them.


--
JF