When it comes down to picking transistors for modern computer-grade electronics projects, the question is between BJT and MOSFET. When you know for sure that you will only be controlling low current signals and are looking for the cheapest transistor to fit the bill, BJT is the clear winner. But what kind of BJT transistor should you choose, NPN or PNP? Looking at various circuits online, you’ll see that people use both, one or the other, without really any rhyme or reason. Except… that NPN tends to be preferred because it is conceptually easiser to understand: positive voltage turns the transistor on, no voltage or negative voltage turns the transistor off.
Well, after thinking about this for a little while, I came to some enlightenment. Let’s review some of my recent projects to analyze the problems I’ve made and what I could have and should have done better, especially if they are going through a V2 revision.
First of all, I had a 4-digit 7-segment common cathode LED display that I needed to control. How should I control it? Well, I looked at an example circuit that drive all the contacts directly from a microcontroller, and they used NPN transistors to sink the currents from the cathodes. Sounds fair enougn, I can just take the analog regulation components and paste them right into my own design. Wrong.
In my own design, I ended up connecting the common cathodes to a demultiplexer chip rather than directly to a microcontroller. After the fact, searching around, I saw active low demultiplexer chips were way more abundant and available in more configurations than active high demultiplexer chips. I should have known this in advance of purchasing my NPN transistors, because then I could have switched to using PNP instead and all would be good. Instead, I had to make a few compromises on my selection of demultiplexer chip, but I was fortunate to get what I was looking for. Again, I reiterate, because this is important! The reason why active low demultiplexer chips are way more abundant than active high demultiplexer chips is because that has kind of become de facto in the industry for chip select, especially in the case of SPI.
Also, when you think about it, when you are switching the common cathode, in the event that your microcontroller is capable of sinking all the output current, you would configure pull-down GPIO outputs, not pull-up GPIO outputs. To control an NPN transistor, you need to use pull-up GPIO outputs (active high), but you use pull-down outputs (active low) to control a PNP transistor.
The second lesson to be learend with PNP versus NPN. What if you are using a transistor to switch current to a remote load? Now, this is key: with a BJT transistor (this is also true to some extent with MOSFETs), you always have two current loops flowing through the transistor, the current flowing between the base to the emitter/collector, and the current flowing the emitter and the collector. Of course your remote load’s current is going through the emitter and collector, but which way is your small base switching control current going? Is it taking the shortest path possible back to the device, or must it detour through the long wires to the remote device before it makes its way back to its home device? The path to the remote device being longer will cause more resistance to your small control signal, which will in turn multiply the decrease in current that can flow between the emitter and collector for the main power supply purpose.
Therefore, when using BJT transistors to switch remote loads, take care to select your transistor or position it so that the switching current takes the shortest path. For NPN transistors, this means you should switch the negative or ground return current path coming from your remote device. For PNP transitors, this means you should switch the positive current path heading out to your remote device. This way, both the positive and negative ends of your control current will be sourced and sinked directly to your local device, without needing to take a detour out to the remote device being power switched.