Hi,
I am a hobbyist with mainly digital experience. I am trying to improve my analog knowledge.
As a follow-on from my hijack at:http://forum.allaboutcircuits.com/showthread.php?t=31738&page=2
I have experimented with a couple of current regulating circuits to drive 6 bright white LED's in this schematic:
Through experimentation, I found that I get 40mA, that is 20mA through each of the strings of LED's, when Vgs is 4.2V which is what the diode and the 3.6V zener is there for. This all works fine, but at 10V the current drops to about 25mA and at 14V it goes up to 50mA. This doesn't seem like my circuit is regulating the current very well.
Now, my question is, what level of current regulation can be achieved in practice? I understand that there will be limits, but I am not sure when to judge whether my current regulation is as good as it can be. All I know is that my circuit sucks at it.
Secondly, what role does the 100 Ohm resistor play in the effeciency of the regulator? Surely, if the current regulator does it's job properly, then it is not needed? Bill Marsden commented in the other thread, that the maths would explain everything, I've done all of that, but clearly I am missing something fundamental. Here's what I've got:
1. The 100 Ohm resistor drops 2v at Vcc of 12V.
2. The 3 LED's drop 9.6V (@3.2V each) altogether.
3. That leaves a Vds on the jfet of 0.4V
4. This would leave me with a lower Vcc limit of 10,6V before the jfet starts shutting down, right?
5. The reason for the current increasing with Vcc is because Vds on jfet is increasing which lowers it's resistance, right?
According to mu understanding a jfet is an excellent current regulator, what am I missing?
Thanks in advance.
I am a hobbyist with mainly digital experience. I am trying to improve my analog knowledge.
As a follow-on from my hijack at:http://forum.allaboutcircuits.com/showthread.php?t=31738&page=2
I have experimented with a couple of current regulating circuits to drive 6 bright white LED's in this schematic:
Through experimentation, I found that I get 40mA, that is 20mA through each of the strings of LED's, when Vgs is 4.2V which is what the diode and the 3.6V zener is there for. This all works fine, but at 10V the current drops to about 25mA and at 14V it goes up to 50mA. This doesn't seem like my circuit is regulating the current very well.
Now, my question is, what level of current regulation can be achieved in practice? I understand that there will be limits, but I am not sure when to judge whether my current regulation is as good as it can be. All I know is that my circuit sucks at it.
Secondly, what role does the 100 Ohm resistor play in the effeciency of the regulator? Surely, if the current regulator does it's job properly, then it is not needed? Bill Marsden commented in the other thread, that the maths would explain everything, I've done all of that, but clearly I am missing something fundamental. Here's what I've got:
1. The 100 Ohm resistor drops 2v at Vcc of 12V.
2. The 3 LED's drop 9.6V (@3.2V each) altogether.
3. That leaves a Vds on the jfet of 0.4V
4. This would leave me with a lower Vcc limit of 10,6V before the jfet starts shutting down, right?
5. The reason for the current increasing with Vcc is because Vds on jfet is increasing which lowers it's resistance, right?
According to mu understanding a jfet is an excellent current regulator, what am I missing?
Thanks in advance.
Attachments
-
538.2 KB Views: 26
-
11.2 KB Views: 173