low current flashing LED

[joecool85]

Hi
Heres one that claims 30uA average current from a 3V supply... https://www.electro-tech-online.com/custompdfs/2006/03/3vledfs1-2.pdf

Wow, thats perfect! Where can I get that transistor?

**edit**
I think this is it
**broken link removed**

 

[hyedenny]

You dont have to use those exact transistors. A 2n3904 and a 2n3906 will work fine... There are tons of possibilities. and YOURE WELCOME!!!

 

[joecool85]

I didn't even see the 2n2222. Thats sweet it will work with 2n3904 and 2n3906. Which would be which in this schematic?

 

[hyedenny]

The '3904 is the NPN (2n2222) and the '3906 is the PNP (2n2907). You can run it off of a cap with the appropriate current limiting resistors, but the relationship between how long a cap with 10 times the farads of a smaller one will last isnt linear, for several reasons.

 

[joecool85]

Any way to roughly figure out how long such a circuit would last on a 1farad cap? max voltage of 5v in the cap.

 

[hyedenny]

Hi,
SURE! The discharge time is not linear, its exponential... Use this formula:
Vf = Vo(e^-t/RC)
where Vf = final voltage
Vo = initial voltage
t = time in seconds
R = resistance in ohms
C = capacitance in FARADS (not microF)
e = "natural log base" = approx. 2.718

 

[Roff]

Hi,
SURE! The discharge time is not linear, its exponential... Use this formula:
Vf = Vo(e^-t/RC)
where Vf = final voltage
Vo = initial voltage
t = time in seconds
R = resistance in ohms
C = capacitance in FARADS (not microF)
e = "natural log base" = approx. 2.718

If you go back and read the whole thread (a daunting task, admittedly), you will find that he wants to power the circuit from a big cap, and is wondering how long the circuit will continue to flash as the big cap discharges through the circuit.

 

[hyedenny]

If we use 2.2k ohms as the resistance of the circuit (you have to discharge the monster cap through a limiting resistor), and charge the 1 Farad cap to 12 volts, and stop the clock at 5 volts, I get about 1926 seconds, or just over half an hour.
Use the formula I mentioned before, solving for t using natural logs:

ln(Vf) - ln(Vo) = -t/ RC
ln5 - ln12 = -t/2200
1.609 - 2.484 = -t/2200
-.875 = -t/2200
t = 1926 seconds

how'd I do, teacher?

 

[joecool85]

I think you did well, thanks. I don't think I will end up having a useful purpose to run this off a cap, but its fun to play with anyway. I appreciate all the help for such a meaningless project. Although, I have learned a lot, so I guess its not wasted.

 

[Roff]

If we use 2.2k ohms as the resistance of the circuit (you have to discharge the monster cap through a limiting resistor), and charge the 1 Farad cap to 12 volts, and stop the clock at 5 volts, I get about 1926 seconds, or just over half an hour.
Use the formula I mentioned before, solving for t using natural logs:

ln(Vf) - ln(Vo) = -t/ RC
ln5 - ln12 = -t/2200
1.609 - 2.484 = -t/2200
-.875 = -t/2200
t = 1926 seconds

how'd I do, teacher?

Joecool, it gets better.
Hyedenny, where did you come up with 2.2k, 12V, and 5V? Last I heard, he wants to use a 1 Farad cap in place of a battery as the POWER SUPPLY for the flasher, starting with a 5V charge.
I think this would be hard to come up with analytically. I simulated it on SwitcherCAD, and it oscillated for 1000 minutes. It might not be useful for that length of time, because, while the period (2.2 sec) and pulse width (10ms-20ms) remained relatively constant, the LED current, as you might conclude, did not. The peak current was only about 3.5ma just before it stopped oscillating.

 

[hyedenny]

hi,
I said IF we use 2.2k, 12v, 5v, 1F, etc..... It was to show an example of how to use the formula. Plug in any values you want!

 

[Roff]

hi,
I said IF we use 2.2k, 12v, 5v, 1F, etc..... It was to show an example of how to use the formula. Plug in any values you want!

Well, it was a nice lesson, but I suspect Joe thought it was the answer to his question, when in reality it had very little relevance. The flasher is a nonlinear load. While running the sim, I fiddled around with coming up with an equivalent load. A 31k resistor in series with an LED matched the discharge curve of the flasher circuit almost exactly.