This is 2009.
ASCII made schematics 18 years ago.
The 9V battery (7.39V) is dead and should be replaced.
Uncle $crooge
To think of this another way, consider the homes in your neighborhood to all be powered in parallel off of the same power distribution source. Each home has the same voltage at the wall outlets and each appliance operates independantly in parallel within your home from the breaker or fuse box. There is no disruption in any branch by the usage of another. All homes have equal power, all appliances operate to specification, all lights burn brightly. Turning off lights at your residence has no effect on your neighbors, and turning off lights in your living room has no effect on the lights in your bedroom. The only exception would be when an appliance is operating which exceeds the current capacity of the circuit, for instance when an AC unit compressor or fan motor becomes rotor locked and the circuit shorts, then the current capacity of that circuit is exceeded because the voltage has dropped, robbing from the other circuits in your home and dimming the lights, and if the condition is severe enough, dragging down the neighbors supply as well. This is what happens in your LED circuit if the battery is insufficient for your load.
Whatever. That was a quick and dirty visual aid. I'm not interested in uploading a file tonight.Originally Posted by audioguru
Yes his battery is low. He doesn't care about the battery. He is interested in learning the math and theory.
I'm trying to help.
Send him some new batteries and we'll all be happy
For ascii schematics use code tags and change the font to fixedsys, they preserve spacing.
Code:- At -- least --- I'm ---- pretty -----sure.
"Because I be what I be. I would tell you what you want to know if I
could, mum, but I be a cat, and no cat anywhere ever gave anyone a
straight answer, har har."
OK, may as well fix it then.
______________________________________ 9 volt rail
|.............................|................... .........|
|.............................|................... .........|
|.............................|................... .........|
\/............................\/...........................\/
--...........................--...........................--
|.............................|................... .........|
R.............................R................... .........R
|.............................|................... .........|
_______________________________________ Ground rail
Well, according to the preview it didn't preserve the spacing, oh well. I cleaned it up with some periods as spacers.
Not perfect, but better. Should convey the idea.
ke5 you didn't understand my whole post.
Or just type code in square brackets to start then type /code in square brackets at the end, just like HTML tags. PHP is basically just an extension of the HTML specification. The code tags provide the ability to save the white spaces, and fixedsys is the only commonly available fixed width font available on most systems. If you'll notice in my posts the text with the preserved whitespacing was in the little box.Code:You only changed the font to fixedsys. In order to preserve spacing you have to use the PHP code tags as well. Select the text in the advanced editor and click the icon that looks like the pound sign. #
It's quick, it's dirty, and it works really well. Until electromaster embeds a paint program =)
Last edited by Sceadwian; 14th November 2009 at 04:56 AM.
"Because I be what I be. I would tell you what you want to know if I
could, mum, but I be a cat, and no cat anywhere ever gave anyone a
straight answer, har har."
I wonder if the battery is 7.39V without a load then its voltage drops to only a couple of volts when it tries to light an LED.
Uncle $crooge
Brand New Battery!
To count out any inconsistencies due to my woefully dead battery, here's a new experiment with a brand new 6V lantern battery, with a 100 ohm and a 16 ohm resistor removed:
1) Measured the battery source voltage 6.47Vs
2) Measured the new resistor 215 ohms
3) Hooked up the circuit, LED alight and measured 4.01Vresistor
4) Measured the LED voltage 2.39Vled
5) Calculated the current and LED voltage drop:
a) Current:
I = Vresistor / R
I = 4.01 / 215
I = 0.0186511627906977A
I = 18.65mA
b) LED voltage drop:
Vf = Vs - Vresistor
Vf = 6.47 - 4.01
Vf = 2.46
(see attached filmstrip of the experiment)
From the previous experiment with the same LED:
Vf = Vs - Vresistor
Vf = 7.82 - 5.09
Vf = 2.73
I then repeated the experiment with a 12 volt wall wart power supply and a 330 ohm resistor added (I went out and bought 100 pieces 330 ohm pack the other day) to give 536 ohms:
1) Measured the source voltage 12.15Vs (fluctuated a few decimal points up and down)
2) Measured the voltage across resistor 9.77Vresistor
3) Calculated LED forward voltage:
Vf = Vs - Vresistor
Vf = 12.15 - 9.77
Vf = 2.38
Given the fluctuations in the source voltage I'm more inclined to trust the 6V battery answer - the LED Vf = 2.46. This value is consistent with the 12V supply results due to the fluctuations in Vs.
So I guess you were correct audioguru - the dead battery was leading us in the wrong direction because it was unstable?
The current with the 12V wall-wart and the 536 ohm resistor was 9.77V/536 ohms= 18.22mA. The current with the 6V battery and the 215 ohm resistor was 18.65mA.
The forward voltage of an LED increases slightly when its current is increased but yours dropped a little maybe because the LED warmed up.
I don't know if you measured the battery and wall-wart voltages when they were loaded. They usually drop a little when loaded.
Uncle $crooge
...
I measured the source voltages without load, I guess, just using the multimeter and the battery/wall wart.
Glad to see my calculation of current and yours match up with the 6V battery. I guess I'm getting closer to an understanding of this subject.
Thanks very much for your help, it is most appreciated.
One more question: if there are these variances in voltage due to heat and possible mismatches because of the source having no load when measured etc... is this a reliable way to measure the forward voltage of an LED? Would it be better to maybe base calculations for the required current limiting resistor on the supplier's theoretical forward voltage from a datasheet (assuming you have one)?
An LED made by a manufacturer has a range of forward voltage. It might be 2.5V, 3.0V or 3.5V. If you have a few volts extra then the variation won't make much difference because the current-limiting resistor will moderate the change of current beween the different forward voltages.
That is why it is not recommended to connect LEDs in parallel unless their forward voltages are matched. The LED with the lowest voltage will hog all or most of the current and will soon burn out. Then the LED with the next lowest voltage will burn out then they all burn out. Like in cheap Chinese LED flashlights.
Uncle $crooge
so the traffic lights idea...
Ahh so, my traffic light idea with having the three colors in parallel with one another has some problems.
What would you recommend for that - how could you isolate each color LED so that this issue would not occur?
What I actually want to do is:
Use three tri-color LEDs and a blue LED to represent a combination of states in the system:
LED1 represents Power:
- Red = powering up
- Orange = battery low
- Green = powered up and working
LED2 represents Communications:
- Red = No connection
- Orange = Network available
- Green = Host found and communication established
LED3 represents Sensors:
- Red = No sensors located
- Orange = Sensors located but no data collected
- Green = Sensors located and data streams established
LED4 represents Sensor data:
- When data is collected from the sensors the blue LED will glow brighter. It will be dim when no data is being collected.
From what you're saying I think this will be a very complex system, requiring isolated power supplies for each color LED used. I think I have lots to do!
If you have any pointers for how to go about the above I'd be very grateful.
You can use your micro-controller to light the LEDs individually through current-limiting resistors.
The max allowed output current from a PIC is 25mA. You can have an output that is high at about 4.5V drive a 2.6V LED through a 100 ohm resistor for a current of 19mA. Or use 120 ohms for a current of 15.8mA. Or use 150 ohms for a current of 12.7mA.
I have never seen a red, orange and green LED. An LED with red and green makes yellow-orange when both the red and the green are lighted at the same time.
Uncle $crooge
Still Wrong
I just re-read my blog post and it is still wrong
pure robot.com: Voltage Regulation
I now think, with the experience of this thread that my whole premise of running a couple of LEDs in parallel to demonstrate the voltage regulator was off track, and for two reasons:
1) You shouldn't run two LEDs of different characteristics in parallel using the same resistor value for each, because the LED with the lower voltage drop (in this case the green/orange state tri-color one) will take up far more current in the circuit and die more quickly.
2) I'm using too low a voltage to drive these LEDs, in particular the blue LED. Blue LEDs will typically require 3.5V and there's not enough voltage supplied to meet the LEDs needs at 3.3Vs.
My numbers and measurements are incorrectly stated. It's apparent looking back at the numbers on the post that there is a Vs-Vf (or Vresistor) 0.54 in the blue LED case, supplying it with 5.4mA (not 27.6mA as stated). The green LED actually has a Vs-Vf (or Vresistor) of 1.43, supplying it with 14.3mA (not 18.7mA as stated).
The post confuses Vf (which is what is shown being measured in the blog pictures) with Vs-Vf (or Vresistor) which would have been a more useful measurement to take to calculate the current. The blue LED was a mistake because there's not enough voltage supplied by the voltage regulator to drive it.
Could someone confirm my mistakes above?
In all three diagrams your photo clearly shows that the meter is measuring Vf (across the LED) yet the captions always say Vs-Vf.
de KI6RWX
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