diode series or array?

[[email protected]]

I need a series of led's (about 7) to use as a voltage reference. I
want to use led's because I need a certain tempco and using a series of
led's for the voltage reference will give me the right tempco. Ideally
I'd like a part that I can get cheap (naturally), maybe from one of the
surplus houses like allelectronics, goldmine or bgmicro. But I haven't
found anything from them, so I might have to go to a regular supply
house. I used digikey's excellent web site to search for diodes in
series or array but didn't get anywhere.
To make the prototype I soldered led's in series by hand, but I may end
up making a bunch more of these for friends and I'd like to find a part
that has led's (or for a distant second choice, ordinary silicon
diodes) in series, or perhaps array. For reasons that I don't want to
go into here, I won't use a single led and divide the voltage up
resistively.
Any info on availability of a part like this? Nothing fancy, just a
bunch of led's in series, and the smaller the better.

 

[Spehro Pefhany]

I need a series of led's (about 7) to use as a voltage reference. I
want to use led's because I need a certain tempco and using a series of
led's for the voltage reference will give me the right tempco. Ideally
I'd like a part that I can get cheap (naturally), maybe from one of the
surplus houses like allelectronics, goldmine or bgmicro. But I haven't
found anything from them, so I might have to go to a regular supply
house. I used digikey's excellent web site to search for diodes in
series or array but didn't get anywhere.
To make the prototype I soldered led's in series by hand, but I may end
up making a bunch more of these for friends and I'd like to find a part
that has led's (or for a distant second choice, ordinary silicon
diodes) in series, or perhaps array. For reasons that I don't want to
go into here, I won't use a single led and divide the voltage up
resistively.
Any info on availability of a part like this? Nothing fancy, just a
bunch of led's in series, and the smaller the better.

Maybe use some SMT LEDs. But there are many kinds of LEDs with
different forward voltages at a given current, different curves of
voltage vs. current and different tempcos. 7 x 3mm leds will probably
be pretty cheap and small if you have the height.


Best regards,
Spehro Pefhany

 

[kell]

Maybe use some SMT LEDs. But there are many kinds of LEDs with
different forward voltages at a given current, different curves of
voltage vs. current and different tempcos. 7 x 3mm leds will probably
be pretty cheap and small if you have the height.

You know, I'll probably just buy a cheap bag of surplus red led's,
which
will probably have a Vf a bit over 2 volts. They won't come with a
data sheet.
I'm wondering how much tempco varies among plain-vanilla red led's?
I thought it would be about 3 mv/degC.

 

[Franc Zabkar]

I need a series of led's (about 7) to use as a voltage reference. I
want to use led's because I need a certain tempco and using a series of
led's for the voltage reference will give me the right tempco. Ideally
I'd like a part that I can get cheap (naturally), maybe from one of the
surplus houses like allelectronics, goldmine or bgmicro. But I haven't
found anything from them, so I might have to go to a regular supply
house. I used digikey's excellent web site to search for diodes in
series or array but didn't get anywhere.

What about an LED bargraph?

http://rocky.digikey.com/WebLib/Fairchild/Web Photos/New Photos/MV53164,MV54164.jpg

- Franc Zabkar

 

[Pooh Bear]

I need a series of led's (about 7) to use as a voltage reference. I
want to use led's because I need a certain tempco and using a series of
led's for the voltage reference will give me the right tempco.

How did you establish this tempco ?

If you've selected a part why not simply say what the part number is ?

Graham

 

[Pooh Bear]

You know, I'll probably just buy a cheap bag of surplus red led's,
which
will probably have a Vf a bit over 2 volts. They won't come with a
data sheet.
I'm wondering how much tempco varies among plain-vanilla red led's?
I thought it would be about 3 mv/degC.

Any reason even to expect it's consistent ?

Graham

 

[Jasen Betts]

["Followup-To:" header set to sci.electronics.basics.]

I need a series of led's (about 7) to use as a voltage reference. I
want to use led's because I need a certain tempco and using a series of
led's for the voltage reference will give me the right tempco. Ideally
I'd like a part that I can get cheap (naturally), maybe from one of the
surplus houses like allelectronics, goldmine or bgmicro. But I haven't
found anything from them, so I might have to go to a regular supply
house. I used digikey's excellent web site to search for diodes in
series or array but didn't get anywhere.
To make the prototype I soldered led's in series by hand, but I may end
up making a bunch more of these for friends and I'd like to find a part
that has led's (or for a distant second choice, ordinary silicon
diodes) in series, or perhaps array. For reasons that I don't want to
go into here, I won't use a single led and divide the voltage up
resistively.

Any info on availability of a part like this? Nothing fancy, just a
bunch of led's in series, and the smaller the better.

a bunch of SMD leds soldered end-to-end ?
a bargraph module?

Bye.
Jasen

 

[Rich Grise]

Any reason even to expect it's consistent ?

Graham

Well, aren't they generally made out of the same kind of stuff?

And isn't "band gap potential", or whatever you call it, kinda related
to the type of material itself?

I've never done any tests like this, I just sort of ass-u-me-d that
a bag of LEDs would have come from the same run, and be fairly consistent
from one to the other. I wouldn't be so confident from batch to batch,
but you get the idea.

Thanks!
Rich

 

[Jamie]

I need a series of led's (about 7) to use as a voltage reference. I
want to use led's because I need a certain tempco and using a series of
led's for the voltage reference will give me the right tempco. Ideally
I'd like a part that I can get cheap (naturally), maybe from one of the
surplus houses like allelectronics, goldmine or bgmicro. But I haven't
found anything from them, so I might have to go to a regular supply
house. I used digikey's excellent web site to search for diodes in
series or array but didn't get anywhere.
To make the prototype I soldered led's in series by hand, but I may end
up making a bunch more of these for friends and I'd like to find a part
that has led's (or for a distant second choice, ordinary silicon
diodes) in series, or perhaps array. For reasons that I don't want to
go into here, I won't use a single led and divide the voltage up
resistively.
Any info on availability of a part like this? Nothing fancy, just a
bunch of led's in series, and the smaller the better.

http://cache.national.com/ds/LM/LM3914.pdf
there chip. just google for a dealer to sell them..

 

[kell]

How did you establish this tempco ?

If you've selected a part why not simply say what the part number is ?

Graham

I haven't selected a part. That 3 mv figure is just something I saw
somewhere on the net.
It looks like I am going to have to do something ridiculous like dunk
led's in boiling water or something to measure their tempco, or just
give up on it, because I find basically no info about it.

 

[budgie]

I haven't selected a part. That 3 mv figure is just something I saw
somewhere on the net.
It looks like I am going to have to do something ridiculous like dunk
led's in boiling water or something to measure their tempco, or just
give up on it, because I find basically no info about it.

if it's just the tempco that you need, have a look at zeners. Their tempcos are
usually documented, and varies with the breakdown voltage. From memory, the 5v1
units have a nearly zero tempco. IIRC it's negative below 5v1 rating and
increasingly positive as you go up in voltage above.

 

[Pooh Bear]

Well, aren't they generally made out of the same kind of stuff?

And isn't "band gap potential", or whatever you call it, kinda related
to the type of material itself?

I've never done any tests like this, I just sort of ass-u-me-d that
a bag of LEDs would have come from the same run, and be fairly consistent
from one to the other. I wouldn't be so confident from batch to batch,
but you get the idea.

Since the OP won't reveal his real requirements I hardly know what to suggest.
I know I wouldn't trust the results from a few leds out of a bag.

Graham

 

[kell]

Since the OP won't reveal his real requirements I hardly know what to suggest.
I know I wouldn't trust the results from a few leds out of a bag.

Graham

Don't worry about it. I'll adopt a more effectual approach.

 

[carl0s]

http://cache.national.com/ds/LM/LM3914.pdf
there chip. just google for a dealer to sell them..

Oooh, that's interesting. I have been wanting a dash-mounted LED RPM & Speed
display for my car, since the needles are obstructed by the steering wheel
at my driving position. This looks like it would let me create a rev-counter
bargraph off of the back of the analog needle, somehow, although my
electronics knowledge is very poor. Do you have any tips for something that
would let me drive a three-digit speed (MPH) display off of the back of the
speedo (analog needle) voltage? I think calibrating the speed might be
difficult, whereas I could do the rev-counter on my driveway, but something
which would let me adjust for a scale of 0 - 200 over a voltage change of
0 - 5v or whatever the car uses would be handy.

thanks,
Carl

 

[Jasen Betts]

["Followup-To:" header set to sci.electronics.basics.]

Oooh, that's interesting. I have been wanting a dash-mounted LED RPM & Speed
display for my car, since the needles are obstructed by the steering wheel
at my driving position. This looks like it would let me create a rev-counter
bargraph off of the back of the analog needle, somehow, although my
electronics knowledge is very poor.

not quite that simply, a typical analogue tacho measures the "pulse rate" of
the cars ignition system externally.... I guess somewhere deep inside that
could be convertted to a voltage (but it might not be - typical moving coil
meters only give 90 degrees of motion, not the 300 degres seen on automotive
dials) sop there's probably some other drive mechanism....

there are bargraph tacho kits available.

Do you have any tips for something that
would let me drive a three-digit speed (MPH) display off of the back of the
speedo (analog needle) voltage?

if there's a voltage there get a three digit milivoltmeter unit and scale
the voltage apropriately.

I think calibrating the speed might be
difficult, whereas I could do the rev-counter on my driveway, but something
which would let me adjust for a scale of 0 - 200 over a voltage change of
0 - 5v or whatever the car uses would be handy.

Lift the driving wheels off the ground, calibrate to your existing
instrumentation. or buy some time on a chassis dynomometer(sp?)

 

[[email protected]]

Oooh, that's interesting. I have been wanting a dash-mounted LED RPM & Speed
display for my car, since the needles are obstructed by the steering wheel
at my driving position. This looks like it would let me create a rev-counter
bargraph off of the back of the analog needle, somehow, although my
electronics knowledge is very poor. Do you have any tips for something that
would let me drive a three-digit speed (MPH) display off of the back of the
speedo (analog needle) voltage? I think calibrating the speed might be
difficult, whereas I could do the rev-counter on my driveway, but something
which would let me adjust for a scale of 0 - 200 over a voltage change of
0 - 5v or whatever the car uses would be handy.

thanks,
Carl

Mechanical car speedos are usually driven by a rotating cable from the
gearbox, and are all mechanical.

Probably the simplest way to convert to digital would be to glue a pot
track on the instrument face, with a pot wiper on the needle. Run it
off a regulated 10v or whatever, and feed to a multimeter or voltmeter.
Bit crude, but youre going to have fun trying to do it other ways.


NT

 

[[email protected]]

I need a series of led's (about 7) to use as a voltage reference. I
want to use led's because I need a certain tempco and using a series of
led's for the voltage reference will give me the right tempco. Ideally
I'd like a part that I can get cheap (naturally), maybe from one of the
surplus houses like allelectronics, goldmine or bgmicro. But I haven't
found anything from them, so I might have to go to a regular supply
house. I used digikey's excellent web site to search for diodes in
series or array but didn't get anywhere.
To make the prototype I soldered led's in series by hand, but I may end
up making a bunch more of these for friends and I'd like to find a part
that has led's (or for a distant second choice, ordinary silicon
diodes) in series, or perhaps array. For reasons that I don't want to
go into here, I won't use a single led and divide the voltage up
resistively.
Any info on availability of a part like this? Nothing fancy, just a
bunch of led's in series, and the smaller the better.

yes bargraphs are available, but pricey. Same goes for high power led
arrays, with series paralell elements. If you want an array of leds,
buy the leds... what else is there to say


NT

 

[Hal Murray]

Mechanical car speedos are usually driven by a rotating cable from the

gearbox, and are all mechanical.

"All mechanical" is an interesting term. The one I took apart as a kid
(many years ago) had a magnet rotating on the end of the cable. That
was near a conducting disk on a spring loaded needle. Eddy currents
and such.

Probably the simplest way to convert to digital would be to glue a pot
track on the instrument face, with a pot wiper on the needle. Run it
off a regulated 10v or whatever, and feed to a multimeter or voltmeter.
Bit crude, but youre going to have fun trying to do it other ways.

The one I played with wasn't very strong. I don't think a scheme
like that would work. Maybe newer ones are different.

 

[Jasen Betts]

["Followup-To:" header set to sci.electronics.basics.]

Mechanical car speedos are usually driven by a rotating cable from the
gearbox, and are all mechanical.

Probably the simplest way to convert to digital would be to glue a pot
track on the instrument face, with a pot wiper on the needle. Run it
off a regulated 10v or whatever, and feed to a multimeter or voltmeter.
Bit crude, but youre going to have fun trying to do it other ways.

If there's enough torque behind the needle to move the wiper that could work.
but, most mechanical speedos work bt a rotating magnet driving an aluminium
disc (or cup) by induction, opposed by the force of a very weak spring.

a carefully placed hall effect sensor could detect the passes of the magnet

the rate of these pulses would need to be translated into the speed...
the sort of thing a small microcontroller excells at.

Bye.
Jasen

 

[Rich Grise]

"All mechanical" is an interesting term. The one I took apart as a kid
(many years ago) had a magnet rotating on the end of the cable. That
was near a conducting disk on a spring loaded needle. Eddy currents
and such.

So, you tear apart the speedo, and mount a coil or Hall thing next to
the rotating magnet, and voila! PFM!

Cheers!
Rich