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Any electronics (robotics) people here?

wct097

NPD Club President 2021-2022
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I may have some questions regarding sizing components (calculating voltage drops, current requirements, etc) for a circuit. It's been so long that this stuff really is confusing the shit out of me right now.

I'm looking at H-Bridge Motor Control circuits and trying to figure out what components would be required to drive a motor that has a stall current of 20A at 12VDC.

Loosely following this circuit from this website:


He's using IRF630 and IRF9630 MOSFETs. They're rated at (200V, 9A, 75W) and (200V, 6.5A, 74W) respectively. I think the circuit as drawn would basically be limited to a driving a motor that operates at 12V 6.5A max, right? Or is it closer to 6.1A since 12V @ 6.5A = 78W which is over the IRF9630's 74W rating?

So if I want to run a motor that will draw 20A max, then I'd need a P-channel and matching N-channel MOSFET rated at 12V/20A/240W minimum, right?

I'm thinking these would work:
N-Channel 55V/120A/300W: STP150NF55
P-Channel 55V/80A/300W: STP80PF55

Does that make sense? Am I understanding correctly? I'm assuming the other components in the circuit would need to be considered and the resistors matched to the gates on the MOSFETs and the capacitor enlarged to handle the larger motor, but the transistors are my current point of confusion/uncertainty.


The application, in this case, is me simply wanting to understand what goes into a motor controller to drive the motor linked above. I'm going to buy a small/cheap robot chassis in an attempt to play with controlling it using my Raspberry Pi. The motors on it are fairly small and can be controlled with a L293D H-bridge chip ($4). I was looking at other robot chassis that are much larger, and beyond what I'm willing to spend right now to play with, and am just trying to understand what really goes into controlling them.

The Mantis 6WD is a good example as it uses 6 motors that can each draw 20A @ 12VDC when they stall. They sell a dual channel motor controller for about $135, but that machine would take three of them, raising the cost to about $900, just for a chassis with motors and motor controllers.

I doubt I'll do more than play with the little $30 bot I'm planning to buy, but it's interesting and I like to figure out or understand things.
 

wct097

NPD Club President 2021-2022
Staff member
Super Moderator
VIP
The fun thing about Servo City's offerings is that I could take that small kit, buy a few extra components and use the parts to build a 6WD example (still using small motors, not fancy like the $500 version in the previous post.

I could use 3 L293D chips to run 6 wheels, but the next logical step would be to build a pair or three pairs of the H-bridge circuit capable of running the larger engines to experiment with. They'd still run the small 200mA motors, but I could at least prove they work before buying a larger and more expensive motor.
 

wct097

NPD Club President 2021-2022
Staff member
Super Moderator
VIP
I used to have some electronic stuff.... resistors, caps, LEDs, breadboard, etc. Not sure where it is.... probably in storage. I ordered the little robot kit from Servo City, then went a little overboard between DigiKey and Amazon ordering components to play with.

Going to start with the basic L293D driver using GPIO, then try implementing optoisolators (probably not needed with the L293D, but relevant to my testing). Next up, I'll try building an H-Bridge driver with transistors. Ordered the IRF630 and IRF9630s to work with.
 

wct097

NPD Club President 2021-2022
Staff member
Super Moderator
VIP
This is what I came up with for an optoisolated 4 motor driver circuit connected to a Pi:



Using GPIO, you're basically running each motor on three signal wires coming from the Pi. Two are used for forward/reverse signals and one is used for turning the motor on (enable) and potentially PWM (turning on and off really fast to cut the duty cycle). That means I'm using 12 connections to control the 4 motors, a power connection from the Pi, and 1 connection to turn power on/off to the entire circuit (think sleep mode for the bot while the Pi stays on). That's 14 circuits to isolate or 48 individual connections to optoisolator chips.
 

wct097

NPD Club President 2021-2022
Staff member
Super Moderator
VIP
I'm gonna make my Raspberry Pi into a robot to take over the world... Or at least the house or office.
 
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