Help with transistor selection, hFE and current limiting resistor calculation
Farmer Scott
Posts: 30
I'm looking for some help in selecting a transistor that will be used to switch on and off computer peripherals on an as needed basis, to be controlled by a BS2.· I'll be switching on and off between 4 and 16 devices, each of which has·a maximum current requirement of [url=mailto:1A@5V]1A@5V[/url], however will only draw between 20mA and 500mA during normal operation.
So, as a worst case scenario, I'm assuming that Ic will not exceed 1A, and using it for my calculations.
This would mean hFE would be 25 (5·x (1/0.2)), assuming the BS2 will be sinking 20mA.
Determining the current limiting resistor is where things start confusing me...I've figured it should be 25ohms ((5 x 25) / (5 x 1)), is this correct?· Just doesn't seem correct for some reason...
All of the schmatics I've seen appear to be relatively simple, with only·a current limiting diode (I'll use a 1N4001) and a current limiting resistor in addition to the transistor.· Should there be additional passive components to increase reliability?
I've also considered using some of the load switch components, but they don't seem to be able to meet my current needs...unless I'm missing something.· Does anyone have an opinion on using a load switch, or other component for this purpose?
Thanks in advance for the help...I've been searching the internet for hours every night all week to try and come up with an answer...
Scott
So, as a worst case scenario, I'm assuming that Ic will not exceed 1A, and using it for my calculations.
This would mean hFE would be 25 (5·x (1/0.2)), assuming the BS2 will be sinking 20mA.
Determining the current limiting resistor is where things start confusing me...I've figured it should be 25ohms ((5 x 25) / (5 x 1)), is this correct?· Just doesn't seem correct for some reason...
All of the schmatics I've seen appear to be relatively simple, with only·a current limiting diode (I'll use a 1N4001) and a current limiting resistor in addition to the transistor.· Should there be additional passive components to increase reliability?
I've also considered using some of the load switch components, but they don't seem to be able to meet my current needs...unless I'm missing something.· Does anyone have an opinion on using a load switch, or other component for this purpose?
Thanks in advance for the help...I've been searching the internet for hours every night all week to try and come up with an answer...
Scott
Comments
This link might help...
...If you use a darlington transistor configuration, don't forget to use a 1.2V drop as opposed to a 0.6V drop across the B-E junction of the transistor.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 1/19/2008 6:14:39 PM GMT