#robotics Logs

Mar 20 2019

01:54 PM s34n: polprog: I found another STM Eval board for seemingly the same purpose as the one I looked at yesterday. https://www.mouser.com/datasheet/2/389/steval-ihm041v1-955744.pdf
01:58 PM s34n: it has a very similar power supply
01:58 PM s34n: Apparently you can buy these boards from mouser
01:58 PM polprog: let me look
01:58 PM polprog: STM you say
01:59 PM s34n: I mean ST Micro, makers of STM32
02:00 PM s34n: and makers of this board, and the paper from yesterday
02:00 PM polprog: yeah. so you know that STM8, the CPU on that board you were asking about yesterday and STM32 are three totally different things
02:00 PM s34n: ye
02:00 PM s34n: yes
02:00 PM polprog: good
02:01 PM s34n: I'm studying the ideas
02:01 PM polprog: it does have a similar power supply, as i said it works for this sorta thing
02:01 PM polprog: and you have to isolate it from mains with an isol. transformer to be able to probe it safely
02:02 PM s34n: speaking of isolation, here's something I don't understand...
02:02 PM s34n: It seems to me there are two paths from mains to the motor.
02:03 PM s34n: One goes through the L7805, which will be 5V
02:03 PM s34n: the other goes through the two resistors to do zero-crossing, then into the MCU
02:04 PM polprog: i can see your confusion
02:04 PM polprog: and i see why
02:04 PM polprog: you see P2, thats the mains in, right?
02:04 PM s34n: right
02:04 PM polprog: so pin 1 of P2 goes straight to the motor
02:05 PM polprog: pin 2 of the motor goes through a triac (like yesterday), then to a line which seemingly connects multiple things...
02:05 PM polprog: that line is ground
02:05 PM s34n: ah. I see the bypass
02:06 PM s34n: going around the L7805
02:06 PM polprog: yes
02:06 PM polprog: again, a more modern schematic would have less common busses and more ground symbols but thats really up to the designer
02:06 PM polprog: and this being rotated doesnt help ;)
02:07 PM polprog: so same thing again, a CPU that is driving a triac to periodically connect and disconnect the motor from the mains
02:08 PM polprog: there is also a MOV (metal oxide varistor) across the triac, this MOV will conduct when the voltage goes above a certain value (like say 300V? depends on the MOV but somewhere in this area)
02:08 PM polprog: part no in BOM wil ltell you exactly, they didnt specify that
02:08 PM polprog: in the schem
02:09 PM polprog: the power supply is virtually identical
02:09 PM polprog: a zener diode and a sillicon diode (Z1 & D1) that charge a cap up, this time to 10V and then the 7805 makes 5V for the CPU
02:10 PM polprog: as i said, the only doenside of this supply is lack of isolation
02:10 PM polprog: zero crossing is done in the same way, two resistors for failure safety
02:11 PM polprog: rest is MCU stuff, leds, buttons, stuff like that
02:12 PM s34n: the wire coming off the ain4 pin of the MCU goes into a resistor.
02:12 PM polprog: yeah, it comes out of a potentiometer
02:12 PM s34n: but that's not a vanilla resistor, is it?
02:12 PM polprog: its a pot
02:12 PM polprog: adjustable resistor
02:13 PM s34n: yeah. that's what I thought
02:13 PM polprog: that resistor has one end at 5V and another at ground so it selects some voltage in that range, no idea what for
02:13 PM polprog: its probably described later in the datasheet
02:15 PM s34n: there is no real description of anything in the datasheet
02:15 PM polprog: shame
02:15 PM s34n: just the scematic
02:15 PM s34n: schematic
02:15 PM polprog: oh lol, right
02:15 PM polprog: i guess that sets some reference voltage or something
02:15 PM polprog: may be a crude analog input
02:15 PM polprog: if you were to choose between this and yesterdays board, id go for this
02:15 PM s34n: there is a photo, though
02:16 PM polprog: unless you find an STM32 based one. STM8 are neat but there isnt much docs about them
02:16 PM polprog: so far the best 8-bitters, community and toolkit wise are AVRs
02:16 PM polprog: [this post was sponsored by the avr community]
02:16 PM polprog: :^)
02:18 PM s34n: what are the TP1/2 things?
02:18 PM polprog: test points
02:19 PM polprog: if you look at the picture you can just see them , top right corner
02:19 PM polprog: soldered as wire o-s for easy probe attachment
02:19 PM s34n: ah
02:22 PM polprog: lol they didnt even attach a BOM in this document
02:24 PM s34n: polprog: it looks like this is meant to use the pot to dial in a set speed for the motor, then leave it alone
02:29 PM polprog: most likely
02:29 PM polprog: since there isnt any other input
02:29 PM polprog: (the comms header only has TX broken out)
02:29 PM polprog: why wouldnt they put RX on it as well, lol
02:30 PM Tom_L: they're talkers not listeners
02:31 PM polprog: hehe
02:33 PM s34n: I guess there are the push buttons
02:34 PM s34n: What does the MCU do to get the triac to change motor speed?
02:34 PM polprog: hmm
02:35 PM polprog: i think it changes the duty cycle or the drive frequency, i dont know much about it
02:35 PM polprog: someone else her surely will
02:35 PM polprog: here*
02:35 PM s34n: It's a universal motor, so it should have to change freq should it?
02:36 PM s34n: *shouldn't
02:36 PM polprog: ive no idea really
02:36 PM polprog: i think variable duty cycle should be enough, i only used DC and stepper motors tho
02:39 PM s34n: polprog: what I meant was, pins ain4, ain3, and pc7 look like they lead to the triac. What do they do to the triac?
02:40 PM s34n: I don't think ain4 does anything to the triac, it just reads the pot?
02:41 PM s34n: it's pc7 that controls the triac, right?
02:44 PM s34n: polprog: isn't ain4 taking in ac?
02:46 PM s34n: I guess I'm confused about the pot. It is 5VDC on two legs and 115VAC on one leg?
02:57 PM polprog: no, the pot is between 5Vdc and the ground
02:57 PM polprog: the ground is imaginary, remember that
02:57 PM polprog: its just a common point
03:29 PM SpeedEvil is now known as Guest15966
03:37 PM s34n: polprog: help me out with that pot. One leg goes to the MCU. One leg goes to 5V. One leg goes back to main.
03:38 PM s34n: polprog: The main leg is what you are calling ground, right?
03:38 PM polprog: follow the line from the pot to the 7805
03:39 PM polprog: one of the mains input is referenced to ground
03:39 PM polprog: thats like saying, "whatever happens, i will measure my voltages with reference to this point"
03:40 PM polprog: you always measure the voltage across two points, because you measure the voltage difference
03:40 PM polprog: when we say that something is 5V, we mean that there is 5V of difference between that point and some other point that we decided to call ground, the reference
03:41 PM polprog: ttyl
05:20 PM BitEvil is now known as SpeedEvil
11:34 PM rue_mohr: things are happening here, this is good
11:38 PM zhanx: https://www.amazon.com/Vilros-Raspberry-Starter-Power-Premium/dp/B0748MPQT4