I have an Arduino R4 Wifi. I have only just bought it so I need to check to make sure I do not burn it out. I obviously cannot run a 5v power relay directly from the board (resistance is only 70 ohms!) I have therefore done this: [switch for arduino] I know I missed off the diode for the power re…

Yes, that was also what I was referring to when replying to siebe. I really should have checked the datasheet, rather than a post on StackExchange.

[image] siebe: Arduino's have a build in overload protection That depends on the Arduino. The protection circuit if any is to protect what is connected to the Arduino not to protect the Arduino itself.

[image] siebe: Arduino's have a build in overload protection According to the schematic , the only polyfuse on the Arduino UNO R4 WiFi is on the VCC pin of the Qwiic connector.

What is the general gist of what you are saying? Are you stating that it is still highly likely that if you overload GPIOs (above about 8ma) it can still break them?

In the data sheet the manufacturer tells us the maximum safe current the GPIO can provide. Taking more than this maximum current, can stress the GPIO causing it to die. :cry:

Right OK. This was why I was so careful to use a high resistor on the base. I thought that it was better to simply operate the transistor switch, rather than potentially draw too much current. There is a laziness element on my behalf, because I should actually be concentrating on the math. In future…

Always, do the math. 8 ma, 5V means what min resistance, placed between pin and either 5V or GND? 625 ohms. Given that you are driving an NPN transistor, the voltage across the resistor will be less, so you will draw even less current. So 1K is more than safe. No need to "worry", these things a…

Say we have a controller that the GPIO max is 40mA. It would be reasonable to derate this to say 20-30mA. For a BJT, we will design for GPIO current is 20mA when it is sitting at a HIGH (5V). 5V - .7(Vbe) / 20mA = 215Ω (for a BJT base resistor) . A standard resistor is 220Ω. 5v -.7(Vbe) / 2…

OK, thank you. I was also considering making sure I did not wreck the power transistor (PN2222) as well. I will endeavor to do better math, in more detail, for future projects.

Oh right, yes. I was going to mention that my UNO R4 wifi is actually 8ma maximum on the GPIO pins, but you are actually giving an example, so it is all fine.

Is it correct to connect the gnd of the Arduino to the negative terminal of other supplies, like batteries

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LarryD June 13, 2025, 10:33pm 29

This post, discussing making a soldered prototype PCB (with 2 PDFs), might be interesting to you.

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Is it correct to connect the gnd of the Arduino to the negative terminal of other supplies, like batteries

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