DjinnG
Newbie level 5
I have already posted some questions and received help for building a power supply based on an LM338. I now would like to modify this supply to provide higher current output.
I have found two similar circuits for doing just that:
**broken link removed**
**broken link removed**
I've successfully used TIP32C bypass transistors to enhance the current output of my power supply, but I am still missing some info.
The circuits shown above use a 7812 which can pass only low current levels. I want to use the full potential of the 338. To achieve that I use a smaller series resistor (~0.3 ohm) so that the transistors will only kick in when 2-3A of current are flowing through the 338.
I would like to know how to calculate the series resistor's value in order to obtain the following properties:
1. Get the most out of the 338 before switching on the transistors,
2. Use the current limiting ability of the 338 to limit the current through the transistors (TIP32C Iec-max = 3A). I am aware that this will probably affect (1) and lower the "kick-in" current (TIP32C Ieb-staturation = 1.8V)
Since the base resistors rely on how much voltage will be applied to the base across the entire current range, I am not sure how to calculate their value. The TIP32C datasheet shows that Ieb-max = 3A, but I should not even get near that much....
Thanks.
p.s. - Following is my evaluation of how the circuit behaves when the load keeps drawing more and more current:
While Vr7 < 0.6V (i.e. Veb < 0.6V) current is supplied by the 338.
When the load draws enough current for Vr7 = ~0.6V, the transistors start conducting.
When the load draws more current, Vr7 > 0.6V and the additional current is provided by the transistors.
what I am not sure of is whether Veb (and I338) keep rising as the current being drawn continues to rise. It makes sense that the transistors cannot provide all the current at Veb = 0.6V, and that they climb up toward saturation to provide more current, but that also means that current through the 338 also keeps rising. If that is true, then by how much? and going back to my original questions, how can I make sure that the 338 current limit coincides with the transistors limit so that the transistors are protected?
I have found two similar circuits for doing just that:
**broken link removed**
**broken link removed**
I've successfully used TIP32C bypass transistors to enhance the current output of my power supply, but I am still missing some info.
The circuits shown above use a 7812 which can pass only low current levels. I want to use the full potential of the 338. To achieve that I use a smaller series resistor (~0.3 ohm) so that the transistors will only kick in when 2-3A of current are flowing through the 338.
I would like to know how to calculate the series resistor's value in order to obtain the following properties:
1. Get the most out of the 338 before switching on the transistors,
2. Use the current limiting ability of the 338 to limit the current through the transistors (TIP32C Iec-max = 3A). I am aware that this will probably affect (1) and lower the "kick-in" current (TIP32C Ieb-staturation = 1.8V)
Since the base resistors rely on how much voltage will be applied to the base across the entire current range, I am not sure how to calculate their value. The TIP32C datasheet shows that Ieb-max = 3A, but I should not even get near that much....
Thanks.
p.s. - Following is my evaluation of how the circuit behaves when the load keeps drawing more and more current:
While Vr7 < 0.6V (i.e. Veb < 0.6V) current is supplied by the 338.
When the load draws enough current for Vr7 = ~0.6V, the transistors start conducting.
When the load draws more current, Vr7 > 0.6V and the additional current is provided by the transistors.
what I am not sure of is whether Veb (and I338) keep rising as the current being drawn continues to rise. It makes sense that the transistors cannot provide all the current at Veb = 0.6V, and that they climb up toward saturation to provide more current, but that also means that current through the 338 also keeps rising. If that is true, then by how much? and going back to my original questions, how can I make sure that the 338 current limit coincides with the transistors limit so that the transistors are protected?