2016-03-15 02:15 archang has quit [Ping timeout: 248 seconds]
2016-03-15 02:16 archang has joined #qi-hardware
2016-03-15 02:22 archang has quit [Remote host closed the connection]
2016-03-15 02:24 archang has joined #qi-hardware
2016-03-15 03:25 fengling has joined #qi-hardware
2016-03-15 04:37 DocScrutinizer05 has quit [Disconnected by services]
2016-03-15 04:37 DocScrutinizer05 has joined #qi-hardware
2016-03-15 04:38 sandeepkr__ has joined #qi-hardware
2016-03-15 06:04 GeorgeHahn has quit [Read error: Connection reset by peer]
2016-03-15 07:48 xiangfu has joined #qi-hardware
2016-03-15 08:54 FDCX has quit [Ping timeout: 244 seconds]
2016-03-15 09:08 FDCX has joined #qi-hardware
2016-03-15 09:17 icanicant_ has joined #qi-hardware
2016-03-15 09:24 pcercuei has joined #qi-hardware
2016-03-15 10:12 <DocScrutinizer05> >>I measured the gate of the MOSFET with a scope and it gives me a very flat 24V.<<  !!!!!
2016-03-15 10:18 <DocScrutinizer05> provide circuit diagram with testpoints you used for the measurement and I can tell you what exactly is wrong. So far I just can tell that either your probing or your circuit (or both) is flawed
2016-03-15 10:21 <DocScrutinizer05> for the FET we also need the particular type you used (either part number or at very least whether it's P or N, depletion or enrichment type)
2016-03-15 10:24 <DocScrutinizer05> shevek: ^^^
2016-03-15 10:25 <DocScrutinizer05> also please provide details about the PSUs you used
2016-03-15 10:39 fengling has quit [Quit: WeeChat 1.4]
2016-03-15 10:48 <DocScrutinizer05> searching Mouser I hardly found FETs with Vgs >25V (and that's ABS MAX). Vgs_th (recommended) maximum I found is 12V, usual are up to maybe 5V
2016-03-15 10:50 <DocScrutinizer05> most are 0.6 to 2V though
2016-03-15 10:52 <DocScrutinizer05> I hope you didn't connect drain to +24V and load from source to GND
2016-03-15 10:55 <DocScrutinizer05> otherwise odds are your FET autoadjusts to Vsource = Vgate - Vgs_th (up to 3V maybe), so when you tie gate to source level (24V) you have a autoadjusting voltage of Vds_th between drain and source. multiply that with the current of 4A and you're in the 2figure Watts dissipated by FET
2016-03-15 11:08 sandeepkr__ has quit [Read error: Connection reset by peer]
2016-03-15 11:10 sandeepkr__ has joined #qi-hardware
2016-03-15 11:12 <DocScrutinizer05> s/Vds/Vgs/
2016-03-15 11:18 <DocScrutinizer05> first datasheet I found. a tad to the high A and V side, but anyway. Also Vgs_max and Vgs_op are unusually high (15V, 18V)  http://www.mouser.com/ds/2/90/3m0065090d-838565.pdf
2016-03-15 11:34 <DocScrutinizer05> another arbitrary chosen one: http://paste.opensuse.org/17948781
2016-03-15 11:34 <DocScrutinizer05> oops
2016-03-15 11:34 <DocScrutinizer05> http://wstaw.org/m/2016/03/15/plasma-desktoprW2219.png
2016-03-15 11:37 <DocScrutinizer05> you see there's no need to go higher than 5-10V Vgs with the latter one, and when you have load in Source and tie Gate to Drain, you'll have a ~3V across the Drain-Source pins, no matter which current you draw
2016-03-15 11:42 <DocScrutinizer05> you also see that in a load-in-Source circuit, you need to drive the Gate with 4-10V *higher* voltage than Vdrain
2016-03-15 11:43 <DocScrutinizer05> yes, the last is Vdrain, not Vsource
2016-03-15 11:45 <DocScrutinizer05> there's honestly hardly any good reason to use load-in-Source circuit for on/off applications
2016-03-15 11:48 <DocScrutinizer05> the exact opposite applies for analog regulators etc, like Voltage regulators and audio amps, where you regularly use load-in-Source circuit - and you regularly expect the FET to get hot
2016-03-15 11:54 <DocScrutinizer05> another aspect: both of the quoted FET types would escape magic blue smoke with Vgs=24V, it's far beyond ABS MAX for both of them. No matter where the load
2016-03-15 11:55 <DocScrutinizer05> and even when you got your load in Source, in the moment you apply Vhigh (=24V) to Gate, your Source ist still 0V since the FET hasn't opened yet, so you'll blow the FET any way
2016-03-15 11:57 <DocScrutinizer05> to mitigate that effect, you need a series R in gate line, to get a sufficiently slow rising slope on gate's capacitance
2016-03-15 12:58 <wpwrak> yes, Vgs(max) tends to be quite limited. surprised me, too. in anelok, i have some FETs with the gate on VBUS. then i tried to vary them, just to notice that the new ones i was looking at had Vgs(max) = 5.0 V. not good :)
2016-03-15 13:00 <wpwrak> shevek: but in any case you should post a circuit diagram. hard to tell what's going on in your circuit. if the FET is intended to be either fully on or fully off, it shouldn't burn much power (if properly dimensioned). if it ends up somewhere in the resistive zone of operation, it may very well become an indian communication device (smoke signals)
2016-03-15 13:00 <wpwrak> also, how is the gate modulated ? just on/off at a low frequency ? PWM ? ...
2016-03-15 13:06 <DocScrutinizer05> yeah, another possible failure mode I considered (but rejected as too little plausible) is (PWM) modulation with very high frequency, several MHz
2016-03-15 13:07 <DocScrutinizer05> anyway I wonder how an arduino(?) is driving a 24V gate voltage
2016-03-15 13:07 <DocScrutinizer05> without schematics that's all fortune telling
2016-03-15 13:12 <roh> well.. either not properly -> smoke signals, or with atleast one more transistor as driver i guess ;)
2016-03-15 13:12 <roh> what i dont quite get: why not just use classic low side switching and a logic level fet?
2016-03-15 13:13 <roh> its only a heating coil, isnt it?
2016-03-15 13:13 <DocScrutinizer05> yeah
2016-03-15 13:13 <DocScrutinizer05> source to GND, drain to coil, gate to arduino, done
2016-03-15 13:14 <DocScrutinizer05> basically
2016-03-15 13:14 <DocScrutinizer05> maybe use a normal transistor to control the gate
2016-03-15 13:15 <DocScrutinizer05> depending on VDD of arduino, and Vgs_th of FET
2016-03-15 13:17 <roh> 4A is 100W.. i guess one could even still switch that bipolar, with a tip121 or such
2016-03-15 13:17 <roh> i would always suggest using a base or gate resistor still...
2016-03-15 13:17 <DocScrutinizer05> sure, but a FET has better properties
2016-03-15 13:17 <DocScrutinizer05> Rds_on is lower
2016-03-15 13:19 <DocScrutinizer05> yes, base R is mandatory, particularly in common-emitter circuit. Gate R is never a bad idea too
2016-03-15 13:19 <roh> or use a schematic like this: https://github.com/Traumflug/Generation_7_Electronics/blob/Gen7Board-ARM-2.0/release%20documents/Gen7Board-ARM%202.0%20Schematic.pdf
2016-03-15 13:21 <DocScrutinizer05> you mean the part around Q1/Q2?
2016-03-15 13:22 solrize has quit [Ping timeout: 240 seconds]
2016-03-15 13:23 <DocScrutinizer05> and Q4 obviously
2016-03-15 13:24 <DocScrutinizer05> Q1, R5, R11, D1. Looks decent
2016-03-15 13:31 <roh> jup
2016-03-15 13:32 <roh> was just the first example schem which came up
2016-03-15 13:32 <DocScrutinizer05> sure
2016-03-15 13:32 <DocScrutinizer05> they are legion
2016-03-15 13:32 <DocScrutinizer05> always same
2016-03-15 13:33 <DocScrutinizer05> FET 101
2016-03-15 13:33 <roh> still.. i'd use better fet.. some irf stuff which i know from the ebike controllers.. those can do 72-96V peak at ~20A a piece
2016-03-15 13:34 <roh> having something speced for 4 or 5 A and 30V and use it at 24V 4A is not enough headroom
2016-03-15 13:34 <DocScrutinizer05> btw... roh you should look for better targets for your explosive experiments ;-)
2016-03-15 13:34 <roh> ?
2016-03-15 13:34 <DocScrutinizer05> massive traffic jam is not nice
2016-03-15 13:35 <roh> baah.. just regular crazies here in berlin.
2016-03-15 13:35 <DocScrutinizer05> hehe https://www.youtube.com/watch?v=D6mTn0qDKrw
2016-03-15 13:37 solrize has joined #qi-hardware
2016-03-15 13:38 <DocScrutinizer05> yeah, news tell about the killed guy being involved into drug dealing and fake money
2016-03-15 13:38 <DocScrutinizer05> pretty unusual execution method
2016-03-15 13:39 <DocScrutinizer05> so yes, cracy criminals
2016-03-15 13:40 <DocScrutinizer05> hard to believe some gangster boss spends thousands on C4 when a bullet for 50ct would do
2016-03-15 13:42 <DocScrutinizer05> I rather suspect the killed guy tried a new business: explosives or weapons
2016-03-15 14:14 <wpwrak> maybe the boss wanted to show off
2016-03-15 14:51 xiangfu_ has joined #qi-hardware
2016-03-15 15:12 <shevek> Yes, I was surprised by the 24V as well.  Hang on, I'll look for part numbers.
2016-03-15 15:15 <shevek> http://www.digikey.com/product-detail/en/STD17NF03LT4/497-3155-1-ND/654484 is the one I'm driving directly from an Arduino.  I have no resistors at all, just digital pin to gate, source to ground and drain through heater to 24V.
2016-03-15 15:17 <shevek> The other is http://www.digikey.com/product-detail/en/STD17NF03LT4/497-3155-1-ND/654484.  It's on a chinese board with a controller to handle things.  But now I see the datasheet it isn't supposed to handle Vgs > 20V.
2016-03-15 15:18 pcercuei has quit [Quit: brb]
2016-03-15 15:18 xiangfu_ has quit [Quit: leaving]
2016-03-15 15:18 <shevek> So you're saying I need a resistor on the gate?  I don't understand what would be its purpose.  I thought there's never supposed to be any current going through the gate?
2016-03-15 15:21 <shevek> I was originally using PWM on them, and that worked without the parts getting overly hot.  Later I switched to full on/off because I thought this could be causing the heat.
2016-03-15 15:41 <shevek> (They started getting hot without me making any changes AFAIK.  So I wanted to solve that problem, but I have no idea why it wasn't happening originally.)
2016-03-15 15:41 <shevek> Is it possible that the lack of a resistor on the gate could break the FET?  Is there a current through the gate that I wasn't told about?
2016-03-15 15:43 <shevek> Or maybe the limited inductance of the heater coil (it's wrapped around a steel pipe, so there is some) could cause the FET to break?  Would a diode like in the schematic that was referenced fix it?
2016-03-15 15:43 pcercuei has joined #qi-hardware
2016-03-15 15:44 pcercuei has quit [Remote host closed the connection]
2016-03-15 15:45 pcercuei has joined #qi-hardware
2016-03-15 15:53 sandeepkr__ has quit [Read error: No route to host]
2016-03-15 15:56 sandeepkr__ has joined #qi-hardware
2016-03-15 15:56 <shevek> The schematic is here: https://github.com/mtu-most/filament_extruder
2016-03-15 15:59 <shevek> And I made a pdf for you, which I put at http://wijnen.dtdns.net/~shevek/extruder.pdf
2016-03-15 16:23 icanicant_ has quit [Quit: Leaving]
2016-03-15 16:32 <DocScrutinizer05> shevek: the first FET in source-to-GND circuit is absolutely OK. Recommended Vgs-on is 5V. You must make sure that arduino delivers an active 5V (or *at least* 3V) to gate for ON, and pulls gate to GND solidly for OFF
2016-03-15 16:33 <shevek> Yes, I'm using 5V power on the Arduino, it's at 4.8V on the digital output.
2016-03-15 16:33 <DocScrutinizer05> is the output a push-pull?
2016-03-15 16:33 <shevek> When it boots, the first thing I do is set the pin to output, and for off I set it low.  The gate is never floating.
2016-03-15 16:34 <DocScrutinizer05> resp a internal pullup resistor configured to be enabled?
2016-03-15 16:34 <shevek> The internal pullup can only be enabled when it is set to input, and that never happens.
2016-03-15 16:34 <DocScrutinizer05> ouch
2016-03-15 16:35 <shevek> Is that a problem?
2016-03-15 16:35 <DocScrutinizer05> so how is 4.8V going to happen on gate?
2016-03-15 16:35 <shevek> The pin is set to output and I set it high.
2016-03-15 16:35 <DocScrutinizer05> is the output a pushpull?
2016-03-15 16:35 <shevek> I think so.  It's an atmega328p, just like on an Arduino Uno.
2016-03-15 16:36 <DocScrutinizer05> for many GPIO on SoC you can configure if pushpull or open-collector
2016-03-15 16:36 <DocScrutinizer05> I'd add a 10k resistor from gate to +5V
2016-03-15 16:36 <shevek> Ah right.  Yes, the atmega is pushpull in output, and either floating or pullup in input.  I set it to output, so it is always pushpull.
2016-03-15 16:37 <DocScrutinizer05> when it's set to input, the gate is floating and FET in resistive mode
2016-03-15 16:37 <DocScrutinizer05> add a pullup or pulldown R
2016-03-15 16:37 <shevek> If the output is floating for some reason (because the chip is in reset), I'd prefer the channel to be closed, so pulling it to ground would seem better?
2016-03-15 16:37 <DocScrutinizer05> yes
2016-03-15 16:38 <DocScrutinizer05> this is also the explanation why it escaped magic smoke
2016-03-15 16:38 <DocScrutinizer05> keeping CPU in reset for a few seconds might already suffice
2016-03-15 16:38 <shevek> But it shouldn't ever have been floating.  When 24V is applied, it is booting for a few milliseconds, after that the output is asserted.
2016-03-15 16:38 <DocScrutinizer05> ^^^
2016-03-15 16:39 <shevek> It's nowhere near seconds.
2016-03-15 16:39 <DocScrutinizer05> add a pulldown and you're safe
2016-03-15 16:39 <DocScrutinizer05> 100k
2016-03-15 16:40 <DocScrutinizer05> or 22k, depending on what the internal pullup rresistor of SoC
2016-03-15 16:40 <DocScrutinizer05> it should be internal R * 0.1
2016-03-15 16:41 <DocScrutinizer05> I dunno if arduino has weak or strong pullups
2016-03-15 16:41 <shevek> I'm not sure, but that pullup is never active.
2016-03-15 16:43 <DocScrutinizer05> ATMEGA328 has no PowerOn reset input?
2016-03-15 16:43 <DocScrutinizer05> aaa pin 29
2016-03-15 16:44 <DocScrutinizer05> so that'
2016-03-15 16:44 <DocScrutinizer05> s on P12, pin2
2016-03-15 16:45 <DocScrutinizer05> pull that to GND and see your FET go BBQ
2016-03-15 16:45 <shevek> Right; I don't do that. ;-)
2016-03-15 16:45 <shevek> But I am aware of that risk.
2016-03-15 16:45 <DocScrutinizer05> add a pulldown R to gate and you're safe
2016-03-15 16:46 <shevek> Yes, I'm planning on that for that reason.
2016-03-15 16:46 <DocScrutinizer05> best EE practice
2016-03-15 16:46 <shevek> But I don't understand why it blew up already; I don't think I did anything that was wrong.
2016-03-15 16:46 <DocScrutinizer05> never build circuits with self destruct function ;-)
2016-03-15 16:47 <DocScrutinizer05> your crystal might be faulty so the CPU fails for a while (or forever) to generate clock
2016-03-15 16:47 <DocScrutinizer05> seen that with AT89c51 chip
2016-03-15 16:49 <DocScrutinizer05> the circuit needed a finger snip to start up, literally
2016-03-15 16:49 <DocScrutinizer05> knock the PCB, so crystal starts mechanical vibration and suddenly the CPU booted
2016-03-15 16:49 <DocScrutinizer05> reproducable
2016-03-15 16:50 <wpwrak> regarding gpio input = Z: careful with clamp diodes. first, don't burn them. second, they can mess up the voltage. e.g. it's perfectly safe to have a NPN transistor (or an n-FET) to drive a p-FET gate at, say, 24 V, with an MCU at 3.3 V or such.
2016-03-15 16:50 <DocScrutinizer05> hmm?
2016-03-15 16:51 <wpwrak> do the same without transistor in the middle, you either burn the clamp diode or worse, or you get at least Vg clamped to the MCU's VCC. so the gate may be at ~3.6 V instead of the desired 24 V, when the FET is "closed"
2016-03-15 16:52 <DocScrutinizer05> huh?
2016-03-15 16:52 <DocScrutinizer05> you seen the circuit?
2016-03-15 16:52 <DocScrutinizer05> http://wijnen.dtdns.net/~shevek/extruder.pdf
2016-03-15 16:52 <wpwrak> this was a general remark
2016-03-15 16:52 <DocScrutinizer05> I don't see how a 24V get on gate there
2016-03-15 16:53 <shevek> No, that's the other board.
2016-03-15 16:53 <DocScrutinizer05> forget the other board
2016-03-15 16:53 <shevek> That's a chinese board with a controller on it; it takes a logic input and for some reason puts the high voltage on the gate.
2016-03-15 16:53 <shevek> Yes, let's forget about it.
2016-03-15 16:54 <wpwrak> the one with 24 V / FET looks quite right. low-side switching keeps this simple
2016-03-15 16:54 <DocScrutinizer05> yes
2016-03-15 16:54 <DocScrutinizer05> just lacks a pulldown for gate when GPIO = Z
2016-03-15 16:55 <shevek> The load is 48 windings of nichrome around a steel pipe of d=2cm.  You think that could be a problem with inductance?
2016-03-15 16:55 <DocScrutinizer05> umm yes
2016-03-15 16:55 <DocScrutinizer05> add a snubber diode
2016-03-15 16:56 <shevek> Ok.  If that's what went wrong, I can have some confidence that with a diode it won't happen again.
2016-03-15 16:56 <DocScrutinizer05> 48 windings around a steel pipe sounds like a pretty decent inductivity
2016-03-15 16:56 <shevek> I'll also add a pulldown, of course.
2016-03-15 16:56 <shevek> But I'm pretty sure that wasn't the problem.
2016-03-15 16:56 <DocScrutinizer05> :nod:
2016-03-15 16:57 <shevek> I didn't quite understand what you were saying about a resistor on the gate.  I understand it's not needed here, but I don't see how it would ever do anything.  Isn't the gate supposed to be zero current at all times?
2016-03-15 16:58 <DocScrutinizer05> yes, except for charging the gate capacitor
2016-03-15 16:58 <shevek> Ah, ok.  With a capacitor it makes sense indeed.
2016-03-15 16:58 <shevek> Then it's just a low pass filter.
2016-03-15 16:58 <DocScrutinizer05> a FET has parasitic gate capacitance
2016-03-15 16:58 <DocScrutinizer05> some few pF
2016-03-15 16:58 <shevek> Ok.
2016-03-15 16:59 <DocScrutinizer05> and yes, the R forms a low pass
2016-03-15 16:59 <shevek> I think I should be able to make this work now.  Thanks!
2016-03-15 16:59 <DocScrutinizer05> :-) welcome
2016-03-15 17:02 <DocScrutinizer05> shevek: see https://raw.githubusercontent.com/Traumflug/Generation_7_Electronics/Gen7Board-ARM-2.0/release documents/Gen7Board-ARM 2.0 Schematic.pdf Q1, R5, R11, D1.
2016-03-15 17:03 <DocScrutinizer05> https://github.com/Traumflug/Generation_7_Electronics/blob/Gen7Board-ARM-2.0/release documents/Gen7Board-ARM 2.0 Schematic.pdf
2016-03-15 17:05 <DocScrutinizer05> http://wstaw.org/m/2016/03/15/plasma-desktopyw2219.png
2016-03-15 17:07 <shevek> Yes, I saw that.  Should D1 be large, to dissipate the energy, or doesn't that matter much?
2016-03-15 17:07 <DocScrutinizer05> actually I seem to recall FETs don't like too steep voltage slope on gate, thus the 10R series resistor
2016-03-15 17:08 <shevek> I can put that in as well, if it's useful.
2016-03-15 17:08 <DocScrutinizer05> it is
2016-03-15 17:08 <shevek> Ok.
2016-03-15 17:08 <DocScrutinizer05> common best practice
2016-03-15 17:08 <DocScrutinizer05> at least for power mosfets
2016-03-15 17:08 <shevek> That's what this is.
2016-03-15 17:08 <DocScrutinizer05> sure :-)
2016-03-15 17:09 <shevek> But D1, can I just use a small SMD diode, or must it be large?
2016-03-15 17:09 <DocScrutinizer05> no, any semi-decent diode will do
2016-03-15 17:10 <DocScrutinizer05> needs to cope with 4A for a few milliseconds
2016-03-15 17:11 <shevek> Hm, the ones I have here say they go up to 3A; is that a problem then?
2016-03-15 17:11 <DocScrutinizer05> no
2016-03-15 17:11 <DocScrutinizer05> absolutely fine
2016-03-15 17:11 <shevek> Ok, good.
2016-03-15 17:12 <shevek> 1A is good enough as well?
2016-03-15 17:12 <DocScrutinizer05> as long as the have 40V
2016-03-15 17:12 <DocScrutinizer05> yes, 1A is also fine, when it's continuous rating
2016-03-15 17:12 <shevek> I have a set which goes from 1A to 3A and from 20V to 90V.
2016-03-15 17:13 <DocScrutinizer05> you need >30V
2016-03-15 17:13 <shevek> I'll just use 1A 40V then.
2016-03-15 17:13 <DocScrutinizer05> sounds fine
2016-03-15 17:25 <DocScrutinizer05> if you want to do even more to defeat the surge from inductance, you add a 1nF from gate to GND, after the series R
2016-03-15 17:25 <DocScrutinizer05> or a 4.7nF
2016-03-15 17:25 <shevek> "after" meaning on the side of the FET?
2016-03-15 17:25 <DocScrutinizer05> yes
2016-03-15 17:26 <shevek> Why would that change anything?
2016-03-15 17:26 <DocScrutinizer05> it slows down the switching, so the FET takes a tad longer to go from on to off state
2016-03-15 17:26 <DocScrutinizer05> and vice versa
2016-03-15 17:27 <shevek> Ah right.  That makes sense.
2016-03-15 17:27 <DocScrutinizer05> so you don't stop from 50mph err 4A to stop in 50cm but in 10m
2016-03-15 17:31 <DocScrutinizer05> you even should use a 10nF and make the series resistor a 100 or 500R
2016-03-15 17:32 <wpwrak> (gate resistor) if directly driven by the MCU, you already get some 100 R from the FET at the port
2016-03-15 17:32 <DocScrutinizer05> right
2016-03-15 17:32 <shevek> So I can omit the external gate resistor, or is it a good idea to keep it anyway?
2016-03-15 17:33 <DocScrutinizer05> I'd keep it, make it 1k even
2016-03-15 17:33 <shevek> Ok.
2016-03-15 17:33 <DocScrutinizer05> or make it 220R and use a 100nF
2016-03-15 17:33 <wpwrak> if the FET burns up, is there a failure mode where gate could get shorted to high voltage ? if yes, a gate resistor would protect the MCU
2016-03-15 17:33 <DocScrutinizer05> http://www.wolframalpha.com/input/?i=100nF+*+220+Ohm
2016-03-15 17:34 <DocScrutinizer05> still 22us
2016-03-15 17:35 <DocScrutinizer05> I'm not used to such long timing constants we should use here :-)
2016-03-15 17:35 <DocScrutinizer05> make that 220kR
2016-03-15 17:35 <DocScrutinizer05> ;-)
2016-03-15 17:36 <DocScrutinizer05> and 100nF
2016-03-15 17:36 <DocScrutinizer05> wow, that sounds odd
2016-03-15 17:36 <shevek> If the FET burns up, I think I'll throw away the board and make a new one, so protecting the controller isn't really a priority in that case.  Not unless FETs burning up is supposed to be a regular thing, but in that case I want to make them easily swappable. ;-)
2016-03-15 17:37 <DocScrutinizer05> FET should work infinitely
2016-03-15 17:38 <DocScrutinizer05> wpwrak: could you please check the timing constant for 220kR -- 100nF ?
2016-03-15 17:38 <DocScrutinizer05> is it really 22ms?
2016-03-15 17:39 <DocScrutinizer05> I'm not used to that stuff in this magnitude anymore
2016-03-15 17:41 <DocScrutinizer05> hmm, well, a 22s for 100uF * 220k sounds right again, so yes, it's prolly 100nF and 220kR you should use
2016-03-15 17:44 pcercuei has quit [Quit: leaving]
2016-03-15 17:46 <DocScrutinizer05> shevek: you can use this softstart method _only_ when you don't use ('high' frequency) PWM to regulate the heater
2016-03-15 17:47 <shevek> Ah yes, that's a good point.  I do actually want that.
2016-03-15 17:47 <DocScrutinizer05> when you use PWM with a frequency >1Hz, you can't use that softstart and you need a diode that has 4A continuous
2016-03-15 17:48 <shevek> Ok.  I'll do that then.
2016-03-15 17:49 <DocScrutinizer05> or you at least must use a softstart timing that's shorter than 1% of your PWM frequency. So when you use 10Hz PWM, softstart should have a timing for 1ms
2016-03-15 17:49 <DocScrutinizer05> IOW 100kR and 10nF
2016-03-15 17:50 <shevek> Right.
2016-03-15 17:50 <DocScrutinizer05> I suggest slow PWM for a inductive heater
2016-03-15 17:51 <DocScrutinizer05> fast PWM has no benefits but brings lots of hassle with such stuff
2016-03-15 17:51 <shevek> The induction is not its main feature; it's a resistive heater, that happens to have some induction.
2016-03-15 17:51 <DocScrutinizer05> yeah, I know
2016-03-15 17:51 <DocScrutinizer05> sorry for the fuzzy wording
2016-03-15 17:52 <shevek> It's actually only 12 windings, I was mistaken before; it's 4 of those in parallel.
2016-03-15 17:52 <DocScrutinizer05> still on a magnetic steel rod
2016-03-15 17:52 <shevek> Yes.
2016-03-15 17:52 <DocScrutinizer05> 12 windings air would be negligible. Steel core changes that completely
2016-03-15 17:53 <DocScrutinizer05> also take care for proper isolation of the NiCr wire on the steel ;-)
2016-03-15 17:53 <shevek> Yes, of course. :-)
2016-03-15 17:54 <shevek> It has a glass fiber coating.
2016-03-15 17:54 <DocScrutinizer05> good
2016-03-15 17:56 <DocScrutinizer05> if you want short circuit protection, add a 0.25R/4W resistor between FET source and GND
2016-03-15 17:57 <DocScrutinizer05> or even as small as 0.1R
2016-03-15 17:58 <DocScrutinizer05> which then only needs 2W
2016-03-15 18:00 <DocScrutinizer05> will greatly improve the reliability of your design, avoiding FET to break even on shorts
2016-03-15 18:03 <shevek> That might be useful, yes.
2016-03-15 18:06 <wpwrak> DocScrutinizer05: 22 ms sounds reasonable. this calculator gets tr = 50 ms: http://sim.okawa-denshi.jp/en/CRtool.php
2016-03-15 18:07 <DocScrutinizer05> for 90%, yes. 22ms is for iirc 63%
2016-03-15 18:07 <DocScrutinizer05> or 66?
2016-03-15 18:07 <wpwrak> i wonder if adding massive filtering to the gate won't make the FET operate a lot as resistor, thus heating up. kinda not what you want with PWM.
2016-03-15 18:08 <DocScrutinizer05> as I said: max 1% of PWM freq
2016-03-15 18:08 <DocScrutinizer05> I strongly discourage to use high freq PWM with heaters of that type
2016-03-15 18:09 <wpwrak> put a cap across the heater ?
2016-03-15 18:10 <DocScrutinizer05> the system is very inert, and you get all sorts of trouble, from switching loss like the one we discuss here, to noise generated by the heater itself, wehn you go beyond 50 or 100Hz
2016-03-15 18:15 <shevek> DocScrutinizer05: I think you mean 63%, 1-1/e.
2016-03-15 18:21 <DocScrutinizer05> yep, 63
2016-03-15 18:23 <DocScrutinizer05> and yes, of course the FET gets a little warm with this
2016-03-15 18:25 <DocScrutinizer05> rough estimation: 100W * 1% * 2 (switch state changes) * 0.5 (very rough estimation for 'efficacy' between ramping FET and load) = 1W
2016-03-15 18:26 <DocScrutinizer05> probbaly the 0.5 are way too high
2016-03-15 18:27 <DocScrutinizer05> worst case is half of 24V * half of 4A
2016-03-15 18:28 <DocScrutinizer05> at 4A voltage is near zero, at 24V current is zero
2016-03-15 18:29 <DocScrutinizer05> so it's not 0.5 but rather the integral over one half sine * 0.5
2016-03-15 18:31 <shevek> Integral, or average?
2016-03-15 18:31 <DocScrutinizer05> plus the FET going from off to on not during capacitor charging up from 0 to 5V but rather between 2.2V and 2.6V or sth like that
2016-03-15 18:32 <wpwrak> hmm, not for steady state. what's roughly the DC resistance of the heater ?
2016-03-15 18:32 <DocScrutinizer05> 24v@4A
2016-03-15 18:32 GeorgeHahn has joined #qi-hardware
2016-03-15 18:32 <shevek> 6Ω
2016-03-15 18:32 <wpwrak> 60 Ohm ?
2016-03-15 18:32 <shevek> 6
2016-03-15 18:32 <wpwrak> ah . 6 Ohm :)
2016-03-15 18:32 <roh> on heaters one ususally doesnt do 'fast' pwm... cylces with the length of up to a few seconds are normal
2016-03-15 18:33 <DocScrutinizer05> what I said
2016-03-15 18:33 <wpwrak> the Ω looks like a zero here :)
2016-03-15 18:33 <shevek> That makes sense; they don't respond fast anyway.
2016-03-15 18:33 <roh> anything faster does only heat the fet more, and not help stabilize the temperature much, since there is lots of thermal/mechanical integration happening
2016-03-15 18:33 <DocScrutinizer05> inertia I called it
2016-03-15 18:34 <roh> i think our pwm runs on something like a single digit frequency in hz ;)
2016-03-15 18:35 <shevek> 1Hz seems reasonable, I think.
2016-03-15 18:35 <DocScrutinizer05> microwave ovens have a PWM at ~0.1Hz or less
2016-03-15 18:35 <DocScrutinizer05> ;-)
2016-03-15 18:35 <shevek> They have a very slow startup.
2016-03-15 18:36 <DocScrutinizer05> yeah, sort of
2016-03-15 18:36 <DocScrutinizer05> the magnetron anode heating takes some time
2016-03-15 18:37 <DocScrutinizer05> err kathode
2016-03-15 18:37 <wpwrak> worst-case FET load would be around 2 A, where it would have to burn 24 W. so if the PWM uses the corresponding range, the smoke may return
2016-03-15 18:38 <DocScrutinizer05> huh?
2016-03-15 18:38 <DocScrutinizer05> [2016-03-15 Tue 19:25:37] <DocScrutinizer05> rough estimation: 100W * 1% * 2 (switch state changes) * 0.5 (very rough estimation for 'efficacy' between ramping FET and load) = 1W
2016-03-15 18:39 <DocScrutinizer05> [2016-03-15 Tue 18:49:27] <DocScrutinizer05> or you at least must use a softstart timing that's shorter than 1% of your PWM frequency. So when you use 10Hz PWM, softstart should have a timing for 1ms
2016-03-15 18:40 <wpwrak> that's if you quickly ramp through the resistive zone. if you switch fast enough (just a few Hz, it seems, with the large filter), you can hold the gate voltage such that the FET becomes a ~6 Ohm resistor
2016-03-15 18:40 <wpwrak> so that's what you want to avoid
2016-03-15 18:40 <DocScrutinizer05> ^^^^^
2016-03-15 18:41 <DocScrutinizer05> I said 10Hz, _not_ 10kHz
2016-03-15 18:41 <wpwrak> your low-pass filter is around 7 Hz
2016-03-15 18:42 <DocScrutinizer05> *sigh*
2016-03-15 18:42 <DocScrutinizer05> [2016-03-15 Tue 18:49:58] <DocScrutinizer05> IOW 100kR and 10nF
2016-03-15 18:43 <wpwrak> ah, you made it smaller. good. now it's 160 Hz
2016-03-15 18:45 <DocScrutinizer05> http://www.wolframalpha.com/input/?i=10nF+*+100+kOhm = 1ms
2016-03-15 18:46 <DocScrutinizer05> 1000ms/1ms = 0.1%
2016-03-15 18:46 <DocScrutinizer05> 100ms (aka 10Hz)/1ms = 1%
2016-03-15 18:47 <DocScrutinizer05> [2016-03-15 Tue 19:25:37] <DocScrutinizer05> rough estimation: 100W * 1% * 2 (switch state changes) * 0.5 (very rough estimation for 'efficacy' between ramping FET and load) = 1W. probbaly the 0.5 are way too high. worst case is half of 24V * half of 4A
2016-03-15 18:48 <DocScrutinizer05> [2016-03-15 Tue 19:31:42] <DocScrutinizer05> plus the FET going from off to on not during capacitor charging up from 0 to 5V but rather between 2.2V and 2.6V or sth like that
2016-03-15 18:49 <DocScrutinizer05> that's significantly less than the 63% for one T
2016-03-15 18:49 <DocScrutinizer05> or even the 90% your calculator seems to use
2016-03-15 18:50 <DocScrutinizer05> so no smoke signals, the FET will just get a little warm
2016-03-15 18:51 <wpwrak> (my calculator) tr is now 2.3 ms
2016-03-15 18:52 <DocScrutinizer05> yes, for 90%
2016-03-15 18:52 <DocScrutinizer05> not for 2.2V to 2.8V, out of 0 to 5V
2016-03-15 18:53 <wpwrak> (smoke) what i mean that there is/was a fairly low frequency limit before Vg becomes "DC" and keeps the FET operating in resistive mode.
2016-03-15 18:53 <DocScrutinizer05> yet you avoid transients of 4A resp 24V @ <1us
2016-03-15 18:54 <wpwrak> with fc = 160 Hz and a switching frequency around 1 Hz, that would be fine, though
2016-03-15 18:55 <DocScrutinizer05> we're assuming frequency << timing constant of the low pass filter
2016-03-15 18:55 <wpwrak> yes. i was worried about the frequency you had before.
2016-03-15 18:55 <DocScrutinizer05> you should read the complete backscroll
2016-03-15 18:56 <wpwrak> it's sometimes hard to spot things in those walls of text ;-)
2016-03-15 18:57 <DocScrutinizer05> the point is that you must not 'spot things' since everxy part of info is equaly relevant
2016-03-15 18:57 <wpwrak> not if it supersedes previous information :)
2016-03-15 18:58 <DocScrutinizer05> in that case it's even more relevant
2016-03-15 18:58 <wpwrak> exactly. hence it's important to spot those bits. qed :)
2016-03-15 18:58 <shevek> All info is equally relevent.  Some info is more equally relevant than others. :-p
2016-03-15 18:59 <DocScrutinizer05> no, since my and roh's suggestion is still to use sub-1Hz PWM frequency
2016-03-15 18:59 <DocScrutinizer05> thus the original suggestion still is valid
2016-03-15 19:00 <DocScrutinizer05> the 10Hz was an example why we need to take care, since even at 10Hz the filter timing gets relevant, then I gave an example how to calculate IF the PWM was at 10Hz
2016-03-15 19:01 <wpwrak> yes, but i was warning about a dangerously near "danger zone". it's not a "suggestion" but a fairly hard speed limit
2016-03-15 19:01 <DocScrutinizer05> ohmy, we all got that meanwhile
2016-03-15 19:01 <wpwrak> see, it's simple ;-)
2016-03-15 19:02 <DocScrutinizer05> Ffilter<<Fpwm != Ffilter >> Fpwm
2016-03-15 19:03 <DocScrutinizer05> you seen my initial:
2016-03-15 19:03 <DocScrutinizer05> [2016-03-15 Tue 18:47:53] <DocScrutinizer05> when you use PWM with a frequency >1Hz, you can't use that softstart and you need a diode that has 4A continuous
2016-03-15 19:03 <DocScrutinizer05> [2016-03-15 Tue 18:48:23] <shevek> Ok.  I'll do that then.
2016-03-15 19:03 <DocScrutinizer05> [2016-03-15 Tue 18:49:27] <DocScrutinizer05> or you at least must use a softstart timing that's shorter than 1% of your PWM frequency. So when you use 10Hz PWM, softstart should have a timing for 1ms
2016-03-15 19:05 <DocScrutinizer05> it's really not helping to replace a percieved wall of text with a 10 times larger wall of discussion text just to pick up the lost details in that first wall
2016-03-15 19:12 <DocScrutinizer05> let's rather evaluate which snubber diodes would work for a heavily inductive load at 24V@4A, for freq of <1Hz and for 50Hz
2016-03-15 19:13 <DocScrutinizer05> obviously a 4A diode with a Wattage of 4*0.7W should always suffice
2016-03-15 19:13 <DocScrutinizer05> but I guess we can go significantly 'cheaper'
2016-03-15 19:14 <DocScrutinizer05> particularly since our inductivity has a ESR of 6 Ohms
2016-03-15 19:18 <DocScrutinizer05> ((would always suffice)) not true, for frequencies of several kHz and way above, you need a schottky (low paraistic capacitance)
2016-03-15 21:00 sandeepkr__ has quit [Read error: Connection reset by peer]
2016-03-15 21:00 sandeepkr__ has joined #qi-hardware
2016-03-15 22:46 sandeepkr__ has quit [Ping timeout: 248 seconds]
2016-03-15 22:52 solrize has quit [Ping timeout: 250 seconds]