I just set my first resistor on fire (not on purpose). I’m trying to apply what I’ve been reading about base and collector current relationships, hFE etc, in order to use a DC motor properly. However, I don’t want to burn the motor, so I’m testing different settings with resistors. I measure everything I can and see if I can predict the results.
I have a TIP41C (NPN) which gets +6V control voltage at the base (from a 1400mA max power supply) and a +9V supply voltage at the collector (from a 6A max supply). What I’m doing is basically trying different resistors at the base and at the collector, to see what I should do to control my 4A motor which isn’t connected yet.
The base resistor (Rb) drops a bit more than 5V most of the time, which seems to relate to the presumed 0.7V drop across the BE junction (i.e. both voltages add up to 6). With Rb = 1K and Rc = 10?, I got only 2.8V across Rc (which is the only load connected to a 9V supply), and I assume it is because the output current is limited by what shows up at the base (I measured Ib = 5mA). Ic was therefore 285mA.
I wanted to increase the current at the output further, so I replaced the base transistor with Rb = 100?. Ib became 52mA and the 10? resistor at the output burned immediately. Putting a 100? in its place I measured 92mA at the output (current gain less than 2). With a 1K at the output the gain actually became negative (Ic = 9.2mA).
What I’m trying to understand is how am I supposed to do these tests if the resistors burn when they get more than 2 amps… How can simulate a load such as a motor without actually connecting one (and without burning down my house).
Thanks a lot for your help.