I'm using a picaxe to control the sequencing of LED candles from the dollar store, but they aren't quite bright enough. I'd like to build some LED candles that run an LED at about 100mA as compared to the usual 20mA.
I have some LEDs that can take 100mA and have a Vf of around 3V. My source voltage is from a rechargeable Li-Ion battery with a nominal voltage of 3.7V. If I just use a resistor to drive the LED I'll get a large current change as the battery depletes and the voltage changes.
I've been reading about using a Jfet as a current source, as I'd like to keep things cheap and simple, as I'll be building around 20 of these. I've been reading about the use of a Jfet as a current source, and I think it may be difficult with the low battery voltage I'll be using?
Here are a couple of the articles I have been reading:
http://socrates.berkeley.edu/~phylabs/bsc/PDFFiles/bsc4.pdf
http://www.vishay.com/docs/70596/70596.pdf
So I think I have the basic understanding down. The negative voltage developed over the source resistor is fed back to the gate to "close" the Jfet more as current increases, as current decreases the voltage will rise closer to zero, which will open the Jfet to more current flow.
So, I have found some Jfets with an Idd of 200mA:
http://www.fairchildsemi.com/ds/J1/J105.pdf
Using the formula provided for basic source biasing on the first page of the vishay document:
-4V Gs(off)
0.1A Id
0.2A Idd
2.0K
I end up with 11.7 ohms.
This however, doesn't make sense to me, since no current flows through the gate of the Jfet, it all must go through the 11.7 ohm resistor, but if I calculate what resistor I'd need to run the LED with no Jfet I end up with:
3.7V supply, 3Vf drop, and 100mA = 7ohms.
So I can't be using more resistance on the Jfet circuit, doesn't make sense to me, where am I misunderstanding? Is my battery voltage just not high enough to make this work properly? If that's the case should I look into cascading Jfets or running two with a lower Gs(off) in parallel?
Or maybe there's a much more simple way to achieve a 100mA current source from a 3.7V Li-Ion cell?
Thanks a bunch for any help offered.
I have some LEDs that can take 100mA and have a Vf of around 3V. My source voltage is from a rechargeable Li-Ion battery with a nominal voltage of 3.7V. If I just use a resistor to drive the LED I'll get a large current change as the battery depletes and the voltage changes.
I've been reading about using a Jfet as a current source, as I'd like to keep things cheap and simple, as I'll be building around 20 of these. I've been reading about the use of a Jfet as a current source, and I think it may be difficult with the low battery voltage I'll be using?
Here are a couple of the articles I have been reading:
http://socrates.berkeley.edu/~phylabs/bsc/PDFFiles/bsc4.pdf
http://www.vishay.com/docs/70596/70596.pdf
So I think I have the basic understanding down. The negative voltage developed over the source resistor is fed back to the gate to "close" the Jfet more as current increases, as current decreases the voltage will rise closer to zero, which will open the Jfet to more current flow.
So, I have found some Jfets with an Idd of 200mA:
http://www.fairchildsemi.com/ds/J1/J105.pdf
Using the formula provided for basic source biasing on the first page of the vishay document:
-4V Gs(off)
0.1A Id
0.2A Idd
2.0K
I end up with 11.7 ohms.
This however, doesn't make sense to me, since no current flows through the gate of the Jfet, it all must go through the 11.7 ohm resistor, but if I calculate what resistor I'd need to run the LED with no Jfet I end up with:
3.7V supply, 3Vf drop, and 100mA = 7ohms.
So I can't be using more resistance on the Jfet circuit, doesn't make sense to me, where am I misunderstanding? Is my battery voltage just not high enough to make this work properly? If that's the case should I look into cascading Jfets or running two with a lower Gs(off) in parallel?
Or maybe there's a much more simple way to achieve a 100mA current source from a 3.7V Li-Ion cell?
Thanks a bunch for any help offered.