#robotics Logs
May 31 2021
#robotics Calendar
12:00 AM rue_mohr: I wonder if they discovered that nobody ever watches the videos and nobody ever does the stuff.
12:07 AM rue_mohr: 124k views since 2013
12:07 AM flav0r: i cant find the sideburns guy
12:07 AM flav0r: on this board i am doing
12:07 AM flav0r: i want to have wifi on it …. so i am just gonna break apart a tiny usb wifi dongle
12:07 AM flav0r: and glue it to the board :)
12:07 AM flav0r: i have solder pads laid down for it
12:08 AM flav0r: CLOG !
02:14 AM flav0r: RUE !!
02:14 AM flav0r: help me understand the PCHANNEL MOSFET
02:14 AM flav0r: !!!
02:22 AM rue_bed2: heh
02:22 AM rue_bed2: to turn the screen on and offf?
02:22 AM flav0r: yessss
02:22 AM flav0r: im just getting back to it
02:23 AM rue_bed2: I can later, I'm falling asleep
02:23 AM flav0r: yah no worries
02:23 AM flav0r: what you been working on ?
02:24 AM rue_bed2: a dupont inert adapter for db0 shrouds
02:24 AM rue_bed2: oh I forgot to start the 3d print for the cnc
02:24 AM rue_bed2: oh well
02:41 AM flav0r: start it !
09:11 AM rue_mohr: it was midnight, I was falling asleep
09:33 AM polprog: its just like the N-channel, but the gate voltage is the other way around
09:34 AM polprog: and the current flows the other way around
09:49 AM rue_mohr: the VCO?
09:50 AM rue_mohr: oh mosfet, I'll get back from work
12:46 PM -!- #robotics mode set to +o by datapacket.freenode.net
12:46 PM flav0r: yaaaahhh
02:02 PM flav0r: back !
04:13 PM rue_mohr: hey
09:52 PM flav0r: DUDES !!
10:26 PM rue_mohr: hey
10:29 PM flav0r: whats shakin dude ?
10:35 PM rue_mohr: all sorts of toys in the mail
10:36 PM rue_mohr: composite to hdmi converter
10:36 PM rue_mohr: hdmi capture
10:36 PM rue_mohr: and 50 to 60 pin lcd adaters
10:56 PM flav0r: nice
10:56 PM flav0r: dude you wanna educate me on this mosfet at some point ?
10:56 PM flav0r: the penny refuses to drop
10:56 PM flav0r: i wanna understand it !
10:57 PM flav0r: also .. what are you up to with hdmi and lcds ?
11:10 PM rue_mohr: so, in one of these browser tabs
11:11 PM rue_mohr: there must be a thing for doing composite video with an esp32
11:11 PM rue_mohr: oh mosets
11:11 PM rue_mohr: so, mosfets are almost completely used an on-off siwtches
11:11 PM rue_mohr: these days
11:11 PM rue_mohr: mosfets power tersminals are the drain and the source
11:12 PM rue_mohr: which mean nothing, but the arrows point from negative supply to positive
11:13 PM rue_mohr: they are activated via gate, a votlage between the gate and the source (the one with the arrow)
11:13 PM rue_mohr: "normal" mosfets are gate-driven with about 10V, but can take up to 20V
11:13 PM rue_mohr: so 12 works
11:14 PM rue_mohr: "logic" mosfets have a lower rated turn on voltage, rated for 5 or 3.3
11:14 PM rue_mohr: so, a mosfet is a voltage-operated variable resistor
11:14 PM rue_mohr: in contrast to a bipolar that is a current operated current source
11:14 PM flav0r: oh i saw some vids on a guy doing composite video with esp32
11:14 PM flav0r: and also i think NTSC broadcast
11:15 PM rue_mohr: Rdson is the minimum resistance the FET can achieve (at its rated gate voltage)
11:16 PM rue_mohr: fets have an issue with high voltage, in order to make a high voltage fet, you cant give it a good rdson
11:16 PM rue_mohr: aka, a 20V fet will have an rdson of like 0.00001 ohms, but a 400V fet will have an rdson of maybe like 1 ohm
11:16 PM rue_mohr: this is why fets are not better than bipolars
11:17 PM rue_mohr: fets are usually just better with lower votlage stuff
11:17 PM rue_mohr: less loss
11:17 PM rue_mohr: at high votlages, bipolar is better for losses
11:17 PM rue_mohr: the gate of a fet has a fair bit of capacitance to the other leads of the fet, sometimes a few nF
11:18 PM rue_mohr: which means you have to drive a lot of current into/outof it to switch it fast
11:18 PM flav0r: ok so im reading soem of this stuff and im fuzzy on this gate threshold voltage
11:18 PM flav0r: so its negative .. but thats just relative
11:18 PM rue_mohr: well, for an N channel, you make the gate + with respect to the source
11:19 PM flav0r: so if at the gate i give it 3.3 and the source is also 3.3
11:19 PM rue_mohr: with a P channel, you make it negative with respect to the source
11:19 PM flav0r: so the difference is 0
11:19 PM flav0r: yah
11:19 PM rue_mohr: so it's be off
11:19 PM rue_mohr: the resistance varries with that votlage
11:19 PM flav0r: so whats the threshold number
11:19 PM rue_mohr: its not really linear
11:20 PM flav0r: does that mean the difference can exceed by that amount and it still works ok ?
11:20 PM flav0r: or
11:20 PM rue_mohr: there is a max gate votlage, dont exceed that or it'll arc over
11:20 PM flav0r: just means that you need to be that many volts less for it to work
11:20 PM rue_mohr: when a fet fails, it usually shorts the D and S togethor, and also fuses in the gate
11:21 PM rue_mohr: so all 3 leads get shorted togethor
11:21 PM flav0r: ok so really this is simple in this case … i have 3.3 on gate and source
11:21 PM rue_mohr: yea, your just switching with it
11:21 PM rue_mohr: the datasheet says, when you have 3.3 on the gate, what resistance you get
11:21 PM flav0r: i see some diagrams where people put what seems to be a pullup between gate and source
11:22 PM flav0r: kinda to keep it fully off
11:22 PM rue_mohr: yea, auto-off
11:22 PM flav0r: so that sorta makes sure it FAILS to off then
11:22 PM flav0r: ok
11:22 PM rue_mohr: dont let the gate float, it'll slip thru all its linear regions and dissipate lots of power and cook
11:22 PM flav0r: 1k should be ok there i guess
11:22 PM flav0r: ok so what about a resistor on the gate
11:22 PM rue_mohr: even 100k
11:25 PM flav0r: https://www.youtube.com/watch?v=SSiRkpgwVKY
11:26 PM rue_mohr: hahah wait no, I just want composite
11:26 PM rue_mohr: no freaking wireless
11:27 PM flav0r: https://www.youtube.com/watch?v=5t1_XNc3vNw
11:27 PM rue_mohr: crap that can do a video card alright
11:28 PM flav0r: https://www.youtube.com/watch?v=-JXuwwXQh8c
11:29 PM flav0r: https://www.youtube.com/watch?v=yhUCM9N-OKc
11:29 PM rue_mohr: okokok
11:29 PM flav0r: i think that guy is also doing composite (looks like it from the wires going to tv)
11:30 PM rue_mohr: soemwhere I have a link to some source
11:30 PM flav0r: ok quick before you get sidetracked
11:30 PM flav0r: resistor on gate !
11:30 PM rue_mohr: ok so
11:30 PM rue_mohr: remember I said that the gate, source and drain all have capacitance between them and its kinda alot?
11:30 PM flav0r: yah
11:30 PM rue_mohr: so, take an N channel fet
11:31 PM rue_mohr: hook the source to ground, and put a big mean load on the drain
11:31 PM rue_mohr: when the fet is off, the source is 0V and the drain is, lets say 24V
11:31 PM rue_mohr: k?
11:32 PM rue_mohr: tell me you follow the picture so far...
11:32 PM flav0r: yah
11:32 PM rue_mohr: so to turn that on, you raise the gate to 12V
11:33 PM rue_mohr: the voltage on the drain is going from 24V to 0V as your gate is going from 0V to 12V
11:33 PM rue_mohr: follow?
11:33 PM flav0r: yah
11:33 PM rue_mohr: there is a capacitance between them
11:33 PM flav0r: kinda mapping 0-12 to 0-24
11:33 PM rue_mohr: this causes the voltage change on the drain to effect the voltage on the gate
11:34 PM rue_mohr: the source is trying to turn the gate off while your trying to turn it on
11:34 PM flav0r: why 0-12 ? just example characteristics of a particular mosfet ?
11:34 PM rue_mohr: because of the internal capacitance
11:34 PM rue_mohr: 12V is a handy drive votlage for most 10V "normal" mosfets
11:34 PM flav0r: ah ok
11:34 PM rue_mohr: so
11:34 PM rue_mohr: the source is trying to turn the gate off while your trying to turn it on
11:34 PM rue_mohr: and it causes an oscillation
11:35 PM rue_mohr: so, to not oscillate, you put a series resistor on the gate to control the turn-on speed
11:35 PM flav0r: ohhh
11:35 PM rue_mohr: which is typically 10 ohms
11:36 PM rue_mohr: lets see if I have a picture
11:36 PM flav0r: i wouldnt bet against it
11:37 PM rue_mohr: http://ruemohr.org/~ircjunk/tempimage/LM555IGBTturnoff.png
11:37 PM rue_mohr: thwangggggggggg
11:37 PM rue_mohr: and thats with a resistor :)
11:38 PM rue_mohr: the green trace is the gate the white trace is the load
11:39 PM rue_mohr: (drain)
11:39 PM flav0r: hold on
11:39 PM rue_mohr: there is some inductance in there too
11:39 PM flav0r: ok
11:39 PM flav0r: so its showing the thing turning on
11:39 PM rue_mohr: I think its a wire-wound load resistor
11:39 PM rue_mohr: off
11:39 PM flav0r: oops
11:39 PM flav0r: :)
11:40 PM flav0r: well wait !
11:40 PM flav0r: i mean the load turning on
11:40 PM rue_mohr: load goes to + in this case
11:40 PM rue_mohr: so its being turned off
11:40 PM flav0r: yah
11:40 PM flav0r: well wait
11:40 PM flav0r: arrrgh
11:41 PM flav0r: you mean the mosfet is turning off ?
11:42 PM rue_mohr: yes
11:42 PM rue_mohr: I can show you on if you want
11:42 PM flav0r: well yeah
11:42 PM rue_mohr: http://ruemohr.org/~ircjunk/tempimage/LM555IGBTturnon.png
11:42 PM rue_mohr: its much more civil
11:42 PM rue_mohr: thats with a resistor tho
11:42 PM flav0r: but yeah
11:42 PM rue_mohr: I'm looking for one without
11:43 PM flav0r: alright but lets say we are powering some device
11:43 PM flav0r: the first pic .. the device would be on
11:43 PM flav0r: the second pic .. the device .. off
11:44 PM rue_mohr: the waveforms might be a bit different, but basically yea
11:44 PM flav0r: ok
11:44 PM flav0r: thats what i mean when i sai beofre “its on”
11:44 PM flav0r: and you said its off (refering to the FET itself)
11:44 PM flav0r: we were on the same page i think .. just different viewpoints !
11:44 PM flav0r: cool tho
11:44 PM rue_mohr: those diagrams are an N channel fet
11:45 PM flav0r: oh fuck
11:45 PM rue_mohr: to ground
11:45 PM flav0r: dude !!!!!!
11:45 PM flav0r: lol
11:45 PM rue_mohr: so reverse what your doing
11:45 PM flav0r: ok
11:45 PM flav0r: ok so how is the calc done for the resistor
11:45 PM rue_mohr: ah
11:45 PM flav0r: or were you saying 10 ohms is good enough
11:46 PM flav0r: that doesnt seem liek it would be so simple !
11:46 PM rue_mohr: http://ruemohr.org/~ircjunk/tempimage/improvedTotem2-noload.png
11:46 PM rue_mohr: another N channel driver, but its got some ring
11:46 PM rue_mohr: its prettymuch always 10 ohms
11:46 PM flav0r: ok sweet
11:46 PM rue_mohr: if you take apart like 400 fet-based devices, you will prettymuch see 99% 10 ohms
11:47 PM flav0r: i decline that
11:47 PM flav0r: ill trust you !
11:47 PM flav0r: thanks dude
11:48 PM flav0r: this makes a LITTLE more sense
11:48 PM rue_mohr: play, learn
11:49 PM flav0r: trying !
11:49 PM flav0r: lol
11:55 PM flav0r: https://imgur.com/a/mJ86hCI
11:55 PM flav0r: good to go !
11:56 PM rue_mohr: imugr doens't support my browser, you have to give me the direct link to the image :)
11:57 PM flav0r: https://i.imgur.com/8YhKUW1.png
11:58 PM rue_mohr: nope
11:58 PM rue_mohr: source goes to power supply
11:58 PM rue_mohr: er drain
11:58 PM rue_mohr: I hate taht
11:58 PM rue_mohr: the arrow goes to the power supply!
11:59 PM rue_mohr: it should point from negative to postiive
11:59 PM rue_mohr: ignore the diode