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Transistor emitter collector base11/6/2022 ![]() Once equilibrium is reached (just microseconds after applying power), no current flows through the PNP transistor, no current flows through the LEDs, and no current flows through either the 10 k? or 1 k? resistor. Under these conditions the PNP transistor could be described as “off,” since it does not conduct current. Together, the 10 k? resistor (connected to +12 V) and the 1 k? resistor (which are in series) act as a single 11 k? “pull-up” resistor, and pull the base of the PNP transistor up to +12 V. And, as the output of the ‘7407 is connected directly to the (“open”) collector of the internal transistor, no current flows into or out of the SN7407N’s output.īecause no current can flow into or out of the SN7407N’s output, to the rest of the circuit it is very much like the ‘7407 the chip is simply not there. Under these conditions the NPN transistor could be described as “off,” since it does not conduct current. Since the output of the NOT gate is connected (through a resistor) to the base of the internal NPN transistor, the base of the transistor is held low. When a logical input to the SN7407N is high, the output of its internal NOT gate is low. Let’s go over this case in more detail, starting with a reminder about what is in each of the six channels of that SN7407: ![]() No current flows through that location, regardless of whether there is a resistor there. In that case, no current flows through the PNP transistor, and the 1k resistor does not affect anything. But, let’s take a look in a little more detail, and see what would happen if we didn’t have that there.įirst off, and as Jon does, let’s consider the case when the SN7407N input is high and its output is “high” (i.e., not pulled low). The short answer is that this is a “base resistor” that we use to limit the maximum current that flows through the base of the PNP transistor. So basically, what would the difference be if there was no 1 kilo-ohm resistor at all?Īnd, that’s actually an excellent question, about something that we usually gloss over. One question I had was – what is the purpose of the 1 kilo-ohm resistor that is connected to the base of the PNP transistor? Because when the open collector is ‘high’ then the base of the transistor is at 12 V and it appears the 1 kohm resistor didn’t affect anything, and then when the open collector goes ‘low’ then the base is connected to ground through the output of the SN7407. I really appreciated the tutorial, and I was able to follow along and understand it very well. ![]() Our reader Jon wrote in with a question about our open collector tutorial: ![]()
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