Input voltage limit, V+ as output, and actuator power supply options

The official pyboard running MicroPython.
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kwiley
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Input voltage limit, V+ as output, and actuator power supply options

Post by kwiley » Tue Apr 13, 2021 5:52 pm

The spec diagram (pybv11-pinout.jpg) and the MicroPython webpage (https://store.micropython.org/product/PYBv1.1H) and the SparkFun page (https://learn.sparkfun.com/tutorials/py ... -guide/all) all say V+ can be 3.6-16V, but the docs say it can only take up to 10V (https://docs.micropython.org/en/latest/ ... intro.html). Which description is correct? I realize high voltages are inefficient and dump a lot of heat, but what are the board-destruction limits?

Side-gripe: Why hasn't the incorrect description, whichever that is, been corrected yet after all these years? MicroPython isn't responsible for SparkFun of course, but the pinout diagram, the store webpage, and the docs webpage should all be consistent. 12V is a popular voltage and straddles the difference between these two figures. A person could easily be inclined to utilize a readily available 12V supply. 12V is common with motors for example. So one might be inclined to use 12V for the overall design, trusting the board to operate on the same voltage as the motor, but if the limit is actually 10V -- as documented in one location shown above -- then, well, that would be a serious design error.

I'm confused about V+ (which I think is identical to VIN when I see VIN in other descriptions and threads). I believe it can be used as a power input to power the board, but I also believe it can be used as a power output to power externals, such as sensors or actuators. Is that right? It's both input and output? Hows does it "know" (I'm sure it's a naive question, but I don't get it)? I mean, can you plug a battery pack or nonUSB source (perhaps a broken out barrel connector or something) into one of the three V+ pins and use one of the other V+ pins as the power supply to, oh say, a 6V servo? Can you use one V+ pin as an input power supply and another as an output power supply to other devices?

If so, what is the V+ current limit as an output? I am well aware of the voltage and current limits on all the other lines: 3.3V, 250mA (I presume that means 250mA summed across all pins at once). But what are limits on powering a peripheral from V+, and again, is it really that simple? Can I put power into the board on one V+ and then connect another V+ to a servo's power supply? And are there any current limits on such a design?

Since servos are frequently 6V, it would be convenient to put 6V into V+, have the board run off its internally converted 3.3 and then draw 6V off another V+ to power a servo. Is that a reasonable design or do I have to power the servo separately?

Thanks.

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pythoncoder
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Re: Input voltage limit, V+ as output, and actuator power supply options

Post by pythoncoder » Wed Apr 14, 2021 12:14 pm

I'd never spotted that discrepancy, believing 16V was the limit on a Pyboard 1.x. The voltage regulator is rated to 16V and has thermal shutdown, so nothing will go bang, but whether the regulator will start to shut down at 16V depends on factors such as whether you draw power from 3V3, on PCB layout and on maximum operating temperature. If I were planning to run from 12V I would give the regulator an easier life with a series resistor to dissipate some of the power. 39Ω 1W would drop 4V if the Pyboard drew 100mA, leaving the regulator to dissipate 0.5W.

As to Vin, if you power a Pyboard from a USB connection, Vin can be used as an output. USB 2 should provide 500mA, so about 400mA at nearly 5V should be available. You can power a Pyboard by supplying power to Vin. A diode ensures that if you supply more than 5V, power will not be fed into the USB connection.
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Re: Input voltage limit, V+ as output, and actuator power supply options

Post by kwiley » Wed Apr 14, 2021 5:58 pm

Thank you for the reply. What about my confusion about using V+/VIN for input and output at the same time? If I don't provide USB power, but rather supply battery power to V+ or perhaps utilize a DC wart that goes to a barrel instead of USB and break the barrel apart to supply a V+ pin, can I then still use the other two V+ pins as power supply to peripherals? Or does it not work both ways at the same time?

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Re: Input voltage limit, V+ as output, and actuator power supply options

Post by pythoncoder » Thu Apr 15, 2021 7:37 am

The two V+ pins are connected together so what you're suggesting would work. However that is not how I'd wire it as the current for your peripherals would pass through the Pyboard's PCB traces. If high currents are involved - and you mentioned actuators - this is not a good idea. Better to power the peripherals directly from the source, and run a separate wire from the source to Vin. The same applies to the Gnd connection.
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Re: Input voltage limit, V+ as output, and actuator power supply options

Post by kwiley » Thu Apr 15, 2021 8:11 am

Fair enough. Thanks for the excellent advice.

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