Calling on the collective wisdom of the SDMB members:
tl;dr version: I have an Arduino Nano Extra that is sending step and direction pulses to an A4988 stepper motor driver board. I want to use a single 12 volt power source for both the boards and the stepper motor, but I can’t get the Arduino board to power up that way.
The details: My first attempt to power the Arduino board was with a repurposed PC tower power supply putting out 12.8 volts at the terminals. I connected the power supply negative post to the Arduino GND pin and the power supply positive post to the DC Vin pin on the board with a 100K ohm resistor in-line. The resulting voltage to the board was a respectable 3.7 volts, but the board did nothing. Measuring the voltage across the board’s GND and DC in pins with power applied, I measured only 1.68 volts, not enough to power up the board.
So, I tried a phone charger with a repurposed USB cable that put out 4.98 volts (rated 5V at 1.2A or 9V at 2A). That powered up the board, and I measured 4.98 volts across the board GND and DC in pins.
Next, I tried a 12 V, 1.2 A wall wart that I want to power the board and stepper motor with. It put out 14.8 V across the positive and negative lines, so I set it up the same way as the PC power supply, only with two 68K ohm resistors in series that brought the voltage down to 6V. With power applied, I measured 1.77V across the Arduino board GND and DC in pins, and the board did not power up.
Any ideas as to why two power supplies with adequate voltage would not power up the Arduino board, but a phone charger did?
The voltage regulator should be able to take 6–20 V if necessary, even if 7–12V are recommended, so, yes, just wire the 12V supply directly even if it is really 14.8 V or 12.8V and your board should still work.
Don’t ever use resistors in series to drop voltage unless you know for certain that the current draw is going to be very consistent. The voltage drop across the resistor will vary with the current which is going to give you a very un-regulated power supply voltage at your board. You are setting yourself up for oscillations and very weird behavior due to inrush current as the board powers up and as the Arduino’s processor varies in current due to different software loads.
PC power supplies also use switching regulators. While power supply designs vary, a typical switcher won’t regulate itself properly unless it has a minimum load, usually around roughly 10 percent of its rated load (unlike a linear supply which will usually regulate itself properly down to zero load).
Some PC power supplies also don’t have their own voltage regulation built into the 12 volt lines, and instead regulate themselves off of the 3.3 or 5 volt reference. Gotta do anything they can to save a few cents per power supply, I suppose.
If you looked at the power supply pins on your Arduino board with those resistors in series, you might have seen something very ugly instead of a constant DC voltage.
Note that your phone charger (the only one that worked) did not attempt to use inline resistance to attempt to regulate the voltage.
Do you mean the Nano Every? I haven’t heard of and can’t find the “Nano Extra”
The Nano Every (and most Arduinos, I think) is specced at 7-21 V. So any 12 volt-ish supply should be fine.
The board itself needs 5 V, and the regulator has a voltage drop. 6 might marginally work, but really I’d recommend the 7 for reliability.
I reemphasize ECG’s statement to not ever use resistors to drop voltage in a situation like this. Current-regulating resistors are fine for LEDs but not much else.
I was about to post the same thing: do note that your regulated 5.0V power supply needs to be connected to a different pin than the 6–20 V unregulated DC input. But, if I understood the OP, the point is to use some kind of 12V power supply, so the +5V pin is not going to be relevant.
Looks like the Nano Every has a switch-mode step-down regulator (the MPM3610). That means pretty high efficiency and no thermal problems at the 21 V limit. I’ve only skimmed the datasheet, but the lowest voltage input it shows for a 5 V output is an 8 V input. For 3.3 V output, it supports 5 V input, so there’s probably a bit of wiggle room here, but in any case the 7-21 V spec makes sense given the regulator properties.
Thank you all for the comments and suggestions. I promise not to drop voltage with resistors ever again. And yes, it is a Nano Every, not an Extra. I blame the bourbon for that mistake. I’ll hook up the 12V wall wart and see if that will power up the board.
These things are only a couple of bucks each. I use them all the time for Arduino projects. Or, you can build your own voltage regulating circuit, but why bother when these things are so cheap?
The Nano Every has a buck converter built-in. The MPM3610 is a pretty neat little chip actually; it has the inductor and various other components built-in.
Yeah, there is that. I’m just in the habit of starting with a Buck Converter because I can change to a different processor and use all kinds of different power supplies without worrying about power issues. But yeah, if the chip has a decent voltage regulator he should be able to just lose the inline resistors and the current supply should work fine.
It’s not a bad idea overall and it’s certainly an advantage for earlier Arduino devices, which used wasteful linear regulators. I’m not sure when they started using switch-mode regulators.
When playing with the electrical pixies, Ohm’s law is not merely your friend; it is your best buddy, your spouse, your mentor, your everything. V = IR so if you have a fixed resistor, the voltage dropped across it is clearly going to vary with current. No darn good at all.