Amateur Circuit vs. Professional Circuit: The LM74500

You can certainly build a circuit to do what you want for your personal projects, and never have to worry about the terms “reverse polarity protection”, “fused connection”, etc. But if you’re ever building circuits for the greater good of humanity, these terms I mentioned quickly become liabilities down the line if you don’t take precautions against them.

Enter the LM74500 by Texas Instruments. Now, I hadn’t known about this chip until a few days ago when I was adding some protection circuitry to my automotive battery charger circuit, but I thought this thing was awesome enough to write about, and help to spread some knowledge around what divides the amateur circuit designer from the professional.

This chip is for reverse polarity protection, meaning that if you were to connect, say, -24V to the input rails of your precious circuit instead of 24V, this chip will quickly work its magic and save any expensive components downstream.

Don’t let the eight pins confuse you, we only need five of them. For starters, we have a power, ground, and enable pin, which we could certainly expect. Then, we have a pin that connects to an external capacitor to run the internal charge pump for driving semiconductor gates. Finally, we have a simple gate pin, which signals to a switch to control the circuit downstream.

Now, what makes this circuit so magical, is that this acts as the lazy fix for when the lead hardware designer asks you to implement reverse polarity protection. However, this is far from the case. This chip is incredibly robust, and its simple operation is what engineering is all about. There is no current ratings for this chip, it outsources any powerful switching to another device, and the LM74500 can consume a large range of voltages, and retain it’s operation. Going to KiCad with the first Google result for a reverse polarity protection circuit will needlessly complicate your design, but dropping one of these in saves time and money.

Professional circuit design isn’t about having intricate and complicated design. It’s the concise additions to the circuit as a whole that improve the resilience to user error, and provide nothing more than that, for years

And that’s pretty cool!