In my previous post, Lessons from the trash can., I introduced the idea of creating the Planet Analog Lessons Learn Knowledge Collection . This time I will share a short story around what could probably be the most common mistake that waits out there for catching our fingers. This affair taught me more than one lesson, for this reason I have chosen to put it into context.
The wrong power supply polarity
This is as obvious, common, and easy to avoid as buffer-overruns in C language programs. But it is worth to mention it here, and the story behind it taught me several unforgettable lessons.
Two former co-workers were constructing one DC (12V) to AC (110V) converter. The original circuit was designed for using PNP germanium transistors and is shown in Figure 1.
DC/AC inverter [incomplete] schematic using PNP Germanium transistors.
My colleagues wanted to extract more output power and re-designed the circuit for using NPN transistor of type 2N3055. This transistor was very popular and easy to obtain at that time.
After powering the circuit, the result was no current, no buzz, no output voltage… and questions. I recommended this circuit to them based on my previous experiences. I had built one unit and it was working more than reasonably well, yet with very low efficiency.
All calculations they made were correct and the transformer construction was impeccable. The windings polarity and component values were also carefully checked. There was no apparent reason for it to fail to work.
At that point, my suggestion was simple: check the power supply polarity. But their answer was also straight and simple: the positive and negative cables from the power supply are connected to the nodes labeled +VCC and -VCC, respectively, in the circuit.
To their knowledge, it all was “undoubtedly” correct. My offer to serve as a third opinion and revise the assembled circuit was not exactly what they wanted to hear at that moment.
They quit after some time, checking and rechecking their notes and went for lunch. When they came back to the office, the inverter was working and powering one large fluorescent lamp. They asked me what was wrong with the circuit. “Nothing”, I said, “it was apparently related to some strange behavior of the electrons coming from the power supply, they were moving in the wrong direction with respect to the circuit.”
It is time now to speculate. Did you figure out why?
There are a few lessons we can extract from this affair. Failing to check the power supply polarity before powering our circuits can produce unpredictable results, or even smoke as it happens in most situations. We must assume checking our power supplies’ output voltages and polarities as an obligatory pre-flight task.
And probably of higher importance in this case is the need to be humble, and the absolute convenience of being open-minded.
Occasionally we overlook extremely simple details.
The top one is: Wrong power supply voltage
In the late 80’s we had one very skilled professor from the Electronics Department that was giving practices in the electronics lab using one big 0V-to-150V DC power supply. He configured the power supply to provide 5VDC for one unique and rare Zilog Z80 development kit. His very first comment before all his classes was check, check, check the power supply, its polarity and the rest of the wiring. But that day he was not aware that during the time elapsed between his first and second classes, another professor reconfigured and used the power supply. During the second class, during the first switch-on, the kit was destroyed instantly.
I thank this professor, and of course many others too, for the many lessons and advice given to us during our quest to become engineers. I also thank him, in particular, for his effort to educate us in one simple and essential principle that I have present every day, it is a Russian proverb that says:
“Measure seven times, cut once”
Once again, we must never fail to check the power supply voltage before proceeding to power our circuits. And furthermore, never take for granted that no one has altered the state of shared resources like power supplies, oscilloscopes and other tools and lab equipment in general.
Note: To be strictly honest, my former professors and colleagues also contributed to it, but I owe to my Father (“mi Viejo”), with no doubt of any kind about it, and up to its full extent, amongst many other things, every single bit of my passion for doing things and my love for my profession, and also one very important part of the person I am.