Discussion on GPIO and motor connections with Raspberry Pi.
Granted that I am building a motor control circuit for my own 3D scanner design, I realize that the FabScan Raspberry Pi design of course also needed to use a motor control setup. So, for my own sake, I might as well review what they used in detail and summarize the key points that are pertinent to me.
20160829/http://hci.rwth-aachen.de/materials/publications/lukas2015a.pdf
So, what do I have to say about the specifics of their design, things worth noting, and differences with my design?
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Overall, their design requires more complicated electronics design and manufacturing. More electronic components are called for all-around. The use of printed circuit boards scales well for mass production, but it is not very friendly toward the kind of use case of building a custom design by hand, using it for a while, then being able to easily disassemble it and reuse the parts in a new project piecemeal. By far and large, this is the one thing that has bugged me most about some older electronics boards I have: there are some components soldered onto a PCB that I have in my immediate physical vincity but am no longer using in the present. I would love to quickly and easily repurpose those components for new projects, but alas, removing them from the PCB is too difficult.
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The use of stepper motors requires more complicated motor control driver ICs than is required for plain DC motors.
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The design of the FabScan calls for more motors, of course.
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The FabScan still uses an Arduino-compatible microcontroller to control the motors directly. The Raspberry Pi is in addition to the Arduino microcontroller and connects via GPIO. My design, by contrast, doesn’t really need a microcontroller for motor control.
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The Arduino-compatible microcontroller is soldered onto a custom-designed PCB. This custom-designed PCB is also designed to conform to the Raspberry Pi HAT standard, namely that it provides identification information so that the Raspberry Pi can automatically identify the PCB and assign the GPIO pins correctly.
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The motors run off of 12 volts. Therefore, the FabScan Pi HAT is designed to have a 12 V DC.
Again, I reiterate, because this is important!
So, let’s review. What was the proper way to use the GPIO pins on Raspberry Pi? For LEDs, you need to wire a resistor in series. What value of resistor is required? The official Raspberry Pi guide says that anything 50 ohms or greater should be okay. Indeed, I’ve checked the schematics in my 200 in 1 electronics kit that wire 3V power to an LED, and they use the 100 ohm resistor. So, we’re on the same page with that detail.
Now, what about limits on current draw? I thought I read something like no more than 35 mA for the 3.3 V GPIO connectors, is that correct? Well, actually it’s just a rule of thumb. Another rule of thumb recommendation is to limit the current draw from a single 3.3 V GPIO pin to 16 mA. The exact details are actually a little foggy, as the maximum current draw from a GPIO pin is contingent on the maximum 3.3 V current supply available. Suffice it to say, the maximum 3.3 V power supply available is everything left over after you subtract 5 V power drains, the Raspberry Pi Zero board itself, and the camera module.
Now, if you’re wondering about current draw on the 5 V power supply pins on the Raspberry Pi. Remember that the Raspberry Pi’s current draw at the 5 V power supply is limited to 1 A. The full circuit will never draw more than 1 A of power at 5 V, including power for the board, all USB peripherals, and any directly connected 5 V devices. That means you can’t really do too much with connecting motors to the 5 V power supply: with about 250 mA for the Raspberry Pi Zero board itself when under high CPU load and 250 mA for an attached Camera Module, that doesn’t leave much current to spare for powering a motor. If you also want to use HDMI, then you subtract another 50 mA. Each connected USB device in turn subtracts more from the available current. One really low power 5 V motor, yes that might work, though, so long as you don’t carry around too many extra power draining doodads.
So, now let’s calculate some final numbers for available 3.3 V power supply pin current. If you are using 500 mA of 5 V current, that leaves you with 1 Watt - 5 V x 250 mA = 2.5 Watts of power left over to channel to to 3.3 V power sources. In 3.3 V terms, that is 2.5 Watts / 3.3 V = 758 mA of current available for all 3.3 V devices. Indeed, this is comparable to the Raspberry Pi StackExchange member that tested up to 800 mA of current can be drawn from the 3.3 V supply pin.
Some other important notes relating to power supply specifics. Note that the Raspberry Pi Zero does not in fact have any voltage protection circuitry on the power supply, unlike the other Raspberry Pi modules, so “the use of a power safety diode is probably superfluous.” Also, the Raspberry Pi Foundation does make recommendations on how to power the Raspberry Pi from through the expansion header power pins for HATs, so this is a commendable way to power the Raspberry Pi. However, the info does note that “Under no circumstances should a power source be connected to the 3.3V pins.” Now, is this claim really backed by any evidence, or is it just a misinformed warning? Indeed, it is a sound claim: the voltage regulators that generate the 3.3 V, 1.8 V, and core 1.2 V voltages are sourced off of the 5 V power supply pin. If you supply power into the 3.3 V pin, there will be no power source for the core voltage, hence your Raspberry Pi will not boot. So, now you know that this is why the NumWorks calculator hack wired the 2.8 V input to the Raspberry Pi 5 V input.
Oh, and another important note. That earlier Raspberry Pi StackExchange link that you found about powering with batteries on 3.3 V? Looking at the in-depth details of the newer answer appear to show that the other answer was relating to the original Raspberry Pi board, not the B+, Zero, and newer board design friends.
Oh, and one last note this time. Just to confirm, indeed I checked, on Digikey, the PAM2306AYPKE IC that generates the 3.3 V and 1.8 V supply voltages from the 5 V supply, and this is strictly a buck switching regulator. So, yes, you do need a buck/boost switched mode power supply if you are powering off of batteries, especially for the case of a 3 V battery pack: the 2.8 V threshold requires the battery to be >93% of its original voltage, whereas you’d probably want to be able to run the battery down to 10% of its original voltage.
Ah, I see they must have removed the comment about an LED burning out after about 10 seconds if you use it without a resistor. That was a nice handy tidbit of information, though.
20181117/DuckDuckGo raspberry pi gpio led
20181117/DuckDuckGo raspberry pi gpio led burn out 10 seconds
20181117/DuckDuckGo raspberry pi led resistor
20181117/DuckDuckGo raspberry pi gpio
20181117/https://www.raspberrypi.org/documentation/usage/gpio/
20181117/https://projects.raspberrypi.org/en/projects/physical-computing/16
20181117/https://projects.raspberrypi.org/en/projects/physical-computing/4
20181117/DuckDuckGo raspberry pi gpio 5 v supply pin current draw limit
20181117/https://www.raspberrypi.org/forums/viewtopic.php?p=158827
20181117/https://raspberrypi.stackexchange.com/questions/9298/what-is-the-maximum-current-the-gpio-pins-can-output
20181117/https://raspberrypi.stackexchange.com/questions/51615/raspberry-pi-power-limitations
20181125/DuckDuckGo PAM2306AYPKE
20181125/https://www.digikey.com/product-detail/en/diodes-incorporated/PAM2306AYPKE/PAM2306AYPKEDICT-ND/7794266
Ah, so now you update me on this documentation aspect. H-bridge you mention. What exactly is an H-bridge. This. Also, as it turns out, the L293D motor controller IC is in fact a dual H-bridge circuit inside. Yes, I would have known that had I’d been looking at the spec sheet more carefully the first time, but I just glossed over it hoping I wouldn’t need to understand the internals too carefully.
20181117/DuckDuckGo h-bridge circuit
20181117/http://www.circuitstoday.com/h-bridge-motor-driver-circuit
20181117/https://en.wikipedia.org/wiki/H_bridge
20181117/https://en.wikipedia.org/wiki/Diode_bridge
What are the keys to the H-bridge circuit? The ability to reverse the voltage using 4 transistors, each of which can be wired to a GPIO pin, and the flyback diode to protect those transistors and the other sensitive circuits from the inductive kick-back. UPDATE 2019-11-11: Note that the actual configuration of the circuit is a diode bridge rectifier between the motor and the power supply, not merely flyback diodes to connect a loop to the motor itself. A large grounding plane probably helps increase the efficacy of such a setup, but if the power circuit is totally open, it could be that the diode bridge rectifier may provide no protection whatsoever. However, a similar design is used for the protective circuitry in the original Raspberry Pi audio connection, so the circuit design still does have some merit.
A number of steps can be taken to increase the effectiveness of this method of protection. A resistor across Vcc and GND can help. Matter of fact, that is almost how the Raspberry Pi Zero is designed, specifically having both a resistor and a capacitor to ground to act as a low-pass filter, better called a decoupling capacitor. This provides a clear path for high frequency kick-back to flow, but low-frequency “kick-back,” literally the case where the motor is being turned like an electric generator, does not have a return path when the circuit is switched off. Again, however, this is probably acceptable since the audio circuit is wired up the same way, and naturally the low-frequency current will be low voltage since it will be related to maximum natural operating voltage of your motor. Essentially, a low-pass filter circuit is a form of electrostatic discharge (ESD) protection. But, to be on the safe side when designing complex circuits, it never hurts to add additional decoupling capacitors to your system.
- 5 V protection: 100 K resistor + 100 nF capacitor, 2 x 10 uF capacitors, 47 uF capacitor
- 3.3 V protection: 10 uF capacitor, 220 nF capacitor
- 1.8 V protection: 10 uF capacitor, 220 nF capacitor
All of this being said, one final word of warning is not to try to insert a “variable supply voltage” circuit in front of the power to the motor, since this would tantamount to eliminating the kick-back voltage return path that goes through the power supply’s ESD protection bridge, unless you also design that circuit to have an ESD return path.
For simple low-voltage micro motors, the cheap commodity silicon diodes (1N540x and 1N400x) can be used. Note that in my very simplest of cases of motor control, where I just have a motor and a laser wired directly to a power supply and the slide switch on the power supply is what I use to “control” the motor, the motor control circuit can be further simplified under some circumstances.
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First of all, only a single flyback diode needs to be used for motor control if the motor can never be controlled in the reverse direction.
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Second, in place of transistors on both sides of the motor, you can use two linear voltage regulators. Literally, a simple Zener diode achives the desired effect: if the motor is mechanically rotated outside of electronic control, it will generate a voltage and a current. The voltage depends on the direction speed that the motor is turned. The “diode” function of the Zener diode prevents a negative voltage from flowing through the wrong direction on sensitive electronics. The “voltage regulator” function of the Zener diode prevents the flow of too high of a voltage that could damage the sensitive electronics.
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Third, you may note that it is not necessary to have a voltage regulator on each side of the motor. If there are no other circuits in series with the motor and the power supply, only a single voltage regulator is needed to prevent current loops through sensitive electronics.
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Fourth, if you know for sure that external forces will not cause over-voltage or wrong polarity currents to come from motor, then you may even eschew with the single linear voltage regulator and use only the cheap commodity silicon diode to protect against inductive kickback.
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Finally, another note worth considering. If you think additional circuitry is needed to protect the laser against the user installing the battery backwards, you might opt to instead place the voltage regulator on the laser power path rather than the motor power path.
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Now, with all that was previously said about GPIO current draw limits, this means that unfortunately you probably can’t power a motor from GPIO pins directly. But, if you could, then even when you have electronic motor control, you could still make a drastically simpler circuit.
20181117/https://en.wikipedia.org/wiki/Flyback_diode
Also, that circuitstoday site noted some other important motor
control ICs to try: L298, TC4424, and LM2941. Indeed, there is a 3 V
dual H-bridge motor control IC available, the L298, you don’t need to
step down to only a single H-bridge if you want to do this. Alas, the
L298 still requires 5 V power for its logic.
20181123/http://www.circuitstoday.com/h-bridge-motor-control-circuit-using-l298
Other notes about the flyback diode. A flyback diode of a sort is in fact used in the design of a switched mode power supply. Well, that being said, there are switched mode power supplies in integrated circuit packages that reverse power protection.
20181123/https://en.wikipedia.org/wiki/Schottky_diode