Finally got around to modifying my Fluke 845ab with LED's
It should work fine. The phototransistor does have some sensitivity in the blue region. It uses a fast comparator circuit (with a little hysteresis). One input is the average/filtered phototransistor output, the other is the raw signal. It can detect the PWM signal from a small flashlight (a couple of lumens) in a room lit by 10,000+ lumens of overhead LED lighting. It can also detect the light output fluctuations caused by DC-DC converter switching.
What would your PWM sensor circuit do if all of the AC was in the blue, and the
white light appeared as a DC bias on the phototransistor?
Ok. My point being that absent some way of telling what color light
your phototransistor is responding to, the modulation you are seeing
might not be from the white fluorescence output of the LED, but rather
from the UV/blue LED pump's output. The white output might be
providing a DC bias to the detector, rather than providing an AC signal.
The best clue, I would think, would be to observe the output from a
white LED that is being switched fully on and off, and watch the depth
of the modulated signal coming out of your photo transistor. It too
should be fully on and fully off. I think it will never come even close
to off.... just fully on, and fully on - k, where k is a modulation
depth... which I think will be small if the modulation frequency gets
above 50Hz.
The white LED's I have worked with, though tiny, glow for a long time
after being run at full brightness. That persistence should make them
poorly suited for use as a pulse modulated white light source.
The old GaAs LED's used to modulate well into the GHz region. I doubt
that you can modulate the white light output of a white LED with much
depth into the 60Hz region.
-Chuck Harris
Mark Sims wrote:
It should work fine. The phototransistor does have some sensitivity in the blue
region. It uses a fast comparator circuit (with a little hysteresis). One input
is the average/filtered phototransistor output, the other is the raw signal. It
can detect the PWM signal from a small flashlight (a couple of lumens) in a room
lit by 10,000+ lumens of overhead LED lighting. It can also detect the light
output fluctuations caused by DC-DC converter switching. ---------------------
What would your PWM sensor circuit do if all of the AC was in the blue, and the
white light appeared as a DC bias on the phototransistor?
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Notice that many white leds are specifically engineered to have long
persistence, so that they can be driven directly from 50/60 Hz without
flicker.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Chuck,
After searching for the wave length of NE2 neon bulbs, I found that the
spectrum is between 600-650nm. So I ordered red-org at 615nm. The white
LED's I've have do have the characteristics you mentioned, Thank you for
all your input, maybe the results will show some improvement.
Dallas
I've just noticed that TI and Linear's specs for 'Long Term Stability'
(typical) are different. TI state 20ppm/1000Hr while Linear state
8ppm/SQRT(kHr). That's a big difference - is this likely to be a real
difference or just specmanship?
I note that Linear (in Note 4) also state that "Devices with maximum
guaranteed long-term stability of 20ppm/SQRT(kH) are available."
Presumably they would be a special order as there doesn't appear to be a
unique part no. Would they be likely to be much more expensive?
Then on page 4 Linear show a graph of long term performance of 44 units
(rather cheekily starting the graph at 2 months or approx 1500 hours!).
To reproduce something approaching the mean curve using the formulae
(drift ppm/SQRT(kHr)) x SQRT(month * 24 * 30.5/1000), requires me to use
2.2ppm/SQRT(kHr). That is way less than the typical 8.5ppm value.
To get a curve that resembles the 3-sigma curve requires a value of
5.7ppm/SQRT(kHr) which is still better than that 8ppm typical figure.
I'm not sure how to interpret this; what value would you use if you were
designing a reference that isn't going to be re-calibrated after the
initial calibration and you don't intend to burn in for several months?
Assuming the equipment is expected to have a 15 year life, operating in
a range of 0 to 40C, what maximum total drift would you be comfortable
specifying? I'd prefer it to be less than 100ppm, but that would require
a drift of < 9ppm/SQRT(kHr), but that assumes that the SQRT(KHr) drift
characteristic is valid for periods much longer than 12 months.
Are there any other references, at similiar or lower cost, that could be
reasonably guaranteed to have a total drift of < 100ppm after 15years?
Is it reasonable to assume that there are some types of voltage
reference will always drift, albeit noisily, in one direction allowing
the original calibration to be offset to some extent to reduce the
maximum error over its lifetime?
Having looked at several application notes and lots of datasheets, in
those that include graphs of drift over 1kHrs or so of several 'typical'
examples, I have not been able to see any meaningful correlation between
the specified typical 1k drift figures and the graphs. Eg. in Linear's
Design Note 229 (Don't Be Fooled By Voltage Reference Long-Term Drift
and Hysteresis" the graphs of drift for the LT1461S8 and the LT1790SOT23
show very different drift after 1600 hours - in the range 50 to 130 for
the former and approx -5 to +45 for the latter, yet the LT1461 is spec'd
at 60ppm/SQRT(1kHr) and the LT17910 at 50ppm.
I realise that I would probably need to contact the manufacturers for
real answers but its been my experience that they aren't often
interested if you're not buying large volumes, and I know that a lot of
people here have a lot of experience in this area.
Thanks,
Tony H
On 9/10/2014 7:00 PM, Tony wrote:
I've just noticed that TI and Linear's specs for 'Long Term Stability'
(typical) are different. TI state 20ppm/1000Hr while Linear state
8ppm/SQRT(kHr). That's a big difference - is this likely to be a real
difference or just specmanship?
I note that Linear (in Note 4) also state that "Devices with maximum
guaranteed long-term stability of 20ppm/SQRT(kH) are available."
Presumably they would be a special order as there doesn't appear to be a
unique part no. Would they be likely to be much more expensive?
Isn't 8ppm/SQRT(kHr) better than 20ppm/SQRT(kH)? Why would the latter be
more expensive? Or is it the difference between "typical" and "guaranteed?"
--
Mike
On Wed, Sep 10, 2014 at 08:34:11AM -0400, Chuck Harris wrote:
The white LED's I have worked with, though tiny, glow for a long time
after being run at full brightness. That persistence should make them
poorly suited for use as a pulse modulated white light source.
The old GaAs LED's used to modulate well into the GHz region. I doubt
that you can modulate the white light output of a white LED with much
depth into the 60Hz region.
My understanding is that most "white" LEDs contain phosphors
excited by actual UV LED UV light drive (or at least used to) and so this
is not surprising at all.
A blue LED might, on the other hand, be mostly LED light directly.
--
Dave Emery N1PRE/AE, die@dieconsulting.com DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."
Yes, that is true. Blue LED's are GaN, or InGaN, and are direct
emitters. UV LED's are AlGaN, or AlGaInN, and also are direct emitters.
However, most white LED's are a phosphorescent (fluorescent) material that
is pumped with a blue or UV LED, and glows white.
There are also some white LED's that are a combination of red, blue and
green LED's, but they are expensive, and require special drivers as they
have 4 leads... one for each color, and a common anode (cathode).
-Chuck Harris
David I. Emery wrote:
On Wed, Sep 10, 2014 at 08:34:11AM -0400, Chuck Harris wrote:
The white LED's I have worked with, though tiny, glow for a long time
after being run at full brightness. That persistence should make them
poorly suited for use as a pulse modulated white light source.
The old GaAs LED's used to modulate well into the GHz region. I doubt
that you can modulate the white light output of a white LED with much
depth into the 60Hz region.
My understanding is that most "white" LEDs contain phosphors
excited by actual UV LED UV light drive (or at least used to) and so this
is not surprising at all.
A blue LED might, on the other hand, be mostly LED light directly.
Hello,
many questions I will keep it short:
All ageing specs are "typical" if you want to have "guaranteed" values
you will have to measure it over a reasonable time. (I recommend min 6
months).
Every treatment (soldering, mechanical/temperature shock) of a reference
may create a new ageing cycle with different slope.
So 100ppm/15 years outside of "lab conditions" (23 deg , constant
humidity) is something that I would not guarantee without re-calibration.
Although typical drift of pre-aged + selected references will be in the
1-2ppm/year range if properly treated.
Also its meaningless if you want to have LT or National (TI) parts since
LT is the only manufacturer which still produces them.
With high demands you will also have to sort out the "noisy" references.
Some "typical" LM399 (all from NS) ageing data can be found on web:
http://www.gellerlabs.com/LM299AH-20_Case_Study.htm
http://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg478496/#msg478496
With best regards
Andreas
Am 11.09.2014 um 01:00 schrieb Tony:
I've just noticed that TI and Linear's specs for 'Long Term Stability'
(typical) are different. TI state 20ppm/1000Hr while Linear state
8ppm/SQRT(kHr). That's a big difference - is this likely to be a real
difference or just specmanship?
I note that Linear (in Note 4) also state that "Devices with maximum
guaranteed long-term stability of 20ppm/SQRT(kH) are available."
Presumably they would be a special order as there doesn't appear to be
a unique part no. Would they be likely to be much more expensive?
Then on page 4 Linear show a graph of long term performance of 44
units (rather cheekily starting the graph at 2 months or approx 1500
hours!). To reproduce something approaching the mean curve using the
formulae (drift ppm/SQRT(kHr)) x SQRT(month * 24 * 30.5/1000),
requires me to use 2.2ppm/SQRT(kHr). That is way less than the typical
8.5ppm value.
To get a curve that resembles the 3-sigma curve requires a value of
5.7ppm/SQRT(kHr) which is still better than that 8ppm typical figure.
I'm not sure how to interpret this; what value would you use if you
were designing a reference that isn't going to be re-calibrated after
the initial calibration and you don't intend to burn in for several
months?
Assuming the equipment is expected to have a 15 year life, operating
in a range of 0 to 40C, what maximum total drift would you be
comfortable specifying? I'd prefer it to be less than 100ppm, but that
would require a drift of < 9ppm/SQRT(kHr), but that assumes that the
SQRT(KHr) drift characteristic is valid for periods much longer than
12 months.
Are there any other references, at similiar or lower cost, that could
be reasonably guaranteed to have a total drift of < 100ppm after 15years?
Is it reasonable to assume that there are some types of voltage
reference will always drift, albeit noisily, in one direction allowing
the original calibration to be offset to some extent to reduce the
maximum error over its lifetime?
Having looked at several application notes and lots of datasheets, in
those that include graphs of drift over 1kHrs or so of several
'typical' examples, I have not been able to see any meaningful
correlation between the specified typical 1k drift figures and the
graphs. Eg. in Linear's Design Note 229 (Don't Be Fooled By Voltage
Reference Long-Term Drift and Hysteresis" the graphs of drift for the
LT1461S8 and the LT1790SOT23 show very different drift after 1600
hours - in the range 50 to 130 for the former and approx -5 to +45 for
the latter, yet the LT1461 is spec'd at 60ppm/SQRT(1kHr) and the
LT17910 at 50ppm.
I realise that I would probably need to contact the manufacturers for
real answers but its been my experience that they aren't often
interested if you're not buying large volumes, and I know that a lot
of people here have a lot of experience in this area.
Thanks,
Tony H
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Hi Dallas,
Let us know if it helps. Neons are very fast light sources, and
can switch way faster than the CdS photo resistors can respond.
A nice red/orange LED should work well with the original CdS
photo resistors.
-Chuck Harris
Dallas Smith wrote:
Chuck,
After searching for the wave length of NE2 neon bulbs, I found that the spectrum is
between 600-650nm. So I ordered red-org at 615nm. The white LED's I've have do have
the characteristics you mentioned, Thank you for all your input, maybe the results
will show some improvement.
Dallas
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