#qi-hardware IRC log for Tuesday, 2016-03-15

DocScrutinizer05>>I measured the gate of the MOSFET with a scope and it gives me a very flat 24V.<<  !!!!!10:12
DocScrutinizer05provide 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 flawed10:18
DocScrutinizer05for 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)10:21
DocScrutinizer05shevek: ^^^10:24
DocScrutinizer05also please provide details about the PSUs you used10:25
DocScrutinizer05searching 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 5V10:48
DocScrutinizer05most are 0.6 to 2V though10:50
DocScrutinizer05I hope you didn't connect drain to +24V and load from source to GND10:52
DocScrutinizer05otherwise 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 FET10:55
DocScrutinizer05s/Vds/Vgs/11:12
DocScrutinizer05first 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.pdf11:18
DocScrutinizer05another arbitrary chosen one: http://paste.opensuse.org/1794878111:34
DocScrutinizer05oops11:34
DocScrutinizer05http://wstaw.org/m/2016/03/15/plasma-desktoprW2219.png11:34
DocScrutinizer05you 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 draw11:37
DocScrutinizer05you also see that in a load-in-Source circuit, you need to drive the Gate with 4-10V *higher* voltage than Vdrain11:42
DocScrutinizer05yes, the last is Vdrain, not Vsource11:43
DocScrutinizer05there's honestly hardly any good reason to use load-in-Source circuit for on/off applications11:45
DocScrutinizer05the 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 hot11:48
DocScrutinizer05another 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 load11:54
DocScrutinizer05and 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 way11:55
DocScrutinizer05to mitigate that effect, you need a series R in gate line, to get a sufficiently slow rising slope on gate's capacitance11:57
wpwrakyes, 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 :)12:58
wpwrakshevek: 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)13:00
wpwrakalso, how is the gate modulated ? just on/off at a low frequency ? PWM ? ...13:00
DocScrutinizer05yeah, another possible failure mode I considered (but rejected as too little plausible) is (PWM) modulation with very high frequency, several MHz13:06
DocScrutinizer05anyway I wonder how an arduino(?) is driving a 24V gate voltage13:07
DocScrutinizer05without schematics that's all fortune telling13:07
rohwell.. either not properly -> smoke signals, or with atleast one more transistor as driver i guess ;)13:12
rohwhat i dont quite get: why not just use classic low side switching and a logic level fet?13:12
rohits only a heating coil, isnt it?13:13
DocScrutinizer05yeah13:13
DocScrutinizer05source to GND, drain to coil, gate to arduino, done13:13
DocScrutinizer05basically13:14
DocScrutinizer05maybe use a normal transistor to control the gate13:14
DocScrutinizer05depending on VDD of arduino, and Vgs_th of FET13:15
roh4A is 100W.. i guess one could even still switch that bipolar, with a tip121 or such13:17
rohi would always suggest using a base or gate resistor still...13:17
DocScrutinizer05sure, but a FET has better properties13:17
DocScrutinizer05Rds_on is lower13:17
DocScrutinizer05yes, base R is mandatory, particularly in common-emitter circuit. Gate R is never a bad idea too13:19
rohor use a schematic like this: https://github.com/Traumflug/Generation_7_Electronics/blob/Gen7Board-ARM-2.0/release%20documents/Gen7Board-ARM%202.0%20Schematic.pdf13:19
DocScrutinizer05you mean the part around Q1/Q2?13:21
DocScrutinizer05and Q4 obviously13:23
DocScrutinizer05Q1, R5, R11, D1. Looks decent13:24
rohjup13:31
rohwas just the first example schem which came up13:32
DocScrutinizer05sure13:32
DocScrutinizer05they are legion13:32
DocScrutinizer05always same13:32
DocScrutinizer05FET 10113:33
rohstill.. i'd use better fet.. some irf stuff which i know from the ebike controllers.. those can do 72-96V peak at ~20A a piece13:33
rohhaving something speced for 4 or 5 A and 30V and use it at 24V 4A is not enough headroom13:34
DocScrutinizer05btw... roh you should look for better targets for your explosive experiments ;-)13:34
roh?13:34
DocScrutinizer05massive traffic jam is not nice13:34
rohbaah.. just regular crazies here in berlin.13:35
DocScrutinizer05hehe https://www.youtube.com/watch?v=D6mTn0qDKrw13:35
DocScrutinizer05yeah, news tell about the killed guy being involved into drug dealing and fake money13:38
DocScrutinizer05pretty unusual execution method13:38
DocScrutinizer05so yes, cracy criminals13:39
DocScrutinizer05hard to believe some gangster boss spends thousands on C4 when a bullet for 50ct would do13:40
DocScrutinizer05I rather suspect the killed guy tried a new business: explosives or weapons13:42
wpwrakmaybe the boss wanted to show off14:14
shevekYes, I was surprised by the 24V as well.  Hang on, I'll look for part numbers.15:12
shevekhttp://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.15:15
shevekThe 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.15:17
shevekSo 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?15:18
shevekI 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.15:21
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.)15:41
shevekIs 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?15:41
shevekOr 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?15:43
shevekThe schematic is here: https://github.com/mtu-most/filament_extruder15:56
shevekAnd I made a pdf for you, which I put at http://wijnen.dtdns.net/~shevek/extruder.pdf15:59
DocScrutinizer05shevek: 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 OFF16:32
shevekYes, I'm using 5V power on the Arduino, it's at 4.8V on the digital output.16:33
DocScrutinizer05is the output a push-pull?16:33
shevekWhen 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.16:33
DocScrutinizer05resp a internal pullup resistor configured to be enabled?16:34
shevekThe internal pullup can only be enabled when it is set to input, and that never happens.16:34
DocScrutinizer05ouch16:34
shevekIs that a problem?16:35
DocScrutinizer05so how is 4.8V going to happen on gate?16:35
shevekThe pin is set to output and I set it high.16:35
DocScrutinizer05is the output a pushpull?16:35
shevekI think so.  It's an atmega328p, just like on an Arduino Uno.16:35
DocScrutinizer05for many GPIO on SoC you can configure if pushpull or open-collector 16:36
DocScrutinizer05I'd add a 10k resistor from gate to +5V16:36
shevekAh 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.16:36
DocScrutinizer05when it's set to input, the gate is floating and FET in resistive mode16:37
DocScrutinizer05add a pullup or pulldown R16:37
shevekIf 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?16:37
DocScrutinizer05yes16:37
DocScrutinizer05this is also the explanation why it escaped magic smoke16:38
DocScrutinizer05keeping CPU in reset for a few seconds might already suffice16:38
shevekBut it shouldn't ever have been floating.  When 24V is applied, it is booting for a few milliseconds, after that the output is asserted.16:38
DocScrutinizer05^^^16:38
shevekIt's nowhere near seconds.16:39
DocScrutinizer05add a pulldown and you're safe16:39
DocScrutinizer05100k16:39
DocScrutinizer05or 22k, depending on what the internal pullup rresistor of SoC16:40
DocScrutinizer05it should be internal R * 0.116:40
DocScrutinizer05I dunno if arduino has weak or strong pullups16:41
shevekI'm not sure, but that pullup is never active.16:41
DocScrutinizer05ATMEGA328 has no PowerOn reset input?16:43
DocScrutinizer05aaa pin 2916:43
DocScrutinizer05so that'16:44
DocScrutinizer05s on P12, pin216:44
DocScrutinizer05pull that to GND and see your FET go BBQ16:45
shevekRight; I don't do that. ;-)16:45
shevekBut I am aware of that risk.16:45
DocScrutinizer05add a pulldown R to gate and you're safe16:45
shevekYes, I'm planning on that for that reason.16:46
DocScrutinizer05best EE practice16:46
shevekBut I don't understand why it blew up already; I don't think I did anything that was wrong.16:46
DocScrutinizer05never build circuits with self destruct function ;-)16:46
DocScrutinizer05your crystal might be faulty so the CPU fails for a while (or forever) to generate clock16:47
DocScrutinizer05seen that with AT89c51 chip16:47
DocScrutinizer05the circuit needed a finger snip to start up, literally16:49
DocScrutinizer05knock the PCB, so crystal starts mechanical vibration and suddenly the CPU booted16:49
DocScrutinizer05reproducable16:49
wpwrakregarding 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.16:50
DocScrutinizer05hmm?16:50
wpwrakdo 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"16:51
DocScrutinizer05huh?16:52
DocScrutinizer05you seen the circuit?16:52
DocScrutinizer05http://wijnen.dtdns.net/~shevek/extruder.pdf16:52
wpwrakthis was a general remark16:52
DocScrutinizer05I don't see how a 24V get on gate there16:52
shevekNo, that's the other board.16:53
DocScrutinizer05forget the other board16:53
shevekThat'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.16:53
shevekYes, let's forget about it.16:53
wpwrakthe one with 24 V / FET looks quite right. low-side switching keeps this simple16:54
DocScrutinizer05yes16:54
DocScrutinizer05just lacks a pulldown for gate when GPIO = Z16:54
shevekThe load is 48 windings of nichrome around a steel pipe of d=2cm.  You think that could be a problem with inductance?16:55
DocScrutinizer05umm yes16:55
DocScrutinizer05add a snubber diode16:55
shevekOk.  If that's what went wrong, I can have some confidence that with a diode it won't happen again.16:56
DocScrutinizer0548 windings around a steel pipe sounds like a pretty decent inductivity16:56
shevekI'll also add a pulldown, of course.16:56
shevekBut I'm pretty sure that wasn't the problem.16:56
DocScrutinizer05:nod:16:56
shevekI 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?16:57
DocScrutinizer05yes, except for charging the gate capacitor16:58
shevekAh, ok.  With a capacitor it makes sense indeed.16:58
shevekThen it's just a low pass filter.16:58
DocScrutinizer05a FET has parasitic gate capacitance16:58
DocScrutinizer05some few pF16:58
shevekOk.16:58
DocScrutinizer05and yes, the R forms a low pass16:59
shevekI think I should be able to make this work now.  Thanks!16:59
DocScrutinizer05:-) welcome16:59
DocScrutinizer05shevek: see https://raw.githubusercontent.com/Traumflug/Generation_7_Electronics/Gen7Board-ARM-2.0/release documents/Gen7Board-ARM 2.0 Schematic.pdf Q1, R5, R11, D1.17:02
DocScrutinizer05https://github.com/Traumflug/Generation_7_Electronics/blob/Gen7Board-ARM-2.0/release documents/Gen7Board-ARM 2.0 Schematic.pdf17:03
DocScrutinizer05http://wstaw.org/m/2016/03/15/plasma-desktopyw2219.png17:05
shevekYes, I saw that.  Should D1 be large, to dissipate the energy, or doesn't that matter much?17:07
DocScrutinizer05actually I seem to recall FETs don't like too steep voltage slope on gate, thus the 10R series resistor17:07
shevekI can put that in as well, if it's useful.17:08
DocScrutinizer05it is17:08
shevekOk.17:08
DocScrutinizer05common best practice17:08
DocScrutinizer05at least for power mosfets17:08
shevekThat's what this is.17:08
DocScrutinizer05sure :-)17:08
shevekBut D1, can I just use a small SMD diode, or must it be large?17:09
DocScrutinizer05no, any semi-decent diode will do17:09
DocScrutinizer05needs to cope with 4A for a few milliseconds17:10
shevekHm, the ones I have here say they go up to 3A; is that a problem then?17:11
DocScrutinizer05no17:11
DocScrutinizer05absolutely fine17:11
shevekOk, good.17:11
shevek1A is good enough as well?17:12
DocScrutinizer05as long as the have 40V 17:12
DocScrutinizer05yes, 1A is also fine, when it's continuous rating17:12
shevekI have a set which goes from 1A to 3A and from 20V to 90V.17:12
DocScrutinizer05you need >30V17:13
shevekI'll just use 1A 40V then.17:13
DocScrutinizer05sounds fine17:13
DocScrutinizer05if you want to do even more to defeat the surge from inductance, you add a 1nF from gate to GND, after the series R17:25
DocScrutinizer05or a 4.7nF17:25
shevek"after" meaning on the side of the FET?17:25
DocScrutinizer05yes17:25
shevekWhy would that change anything?17:26
DocScrutinizer05it slows down the switching, so the FET takes a tad longer to go from on to off state17:26
DocScrutinizer05and vice versa17:26
shevekAh right.  That makes sense.17:27
DocScrutinizer05so you don't stop from 50mph err 4A to stop in 50cm but in 10m17:27
DocScrutinizer05you even should use a 10nF and make the series resistor a 100 or 500R17:31
wpwrak(gate resistor) if directly driven by the MCU, you already get some 100 R from the FET at the port17:32
DocScrutinizer05right17:32
shevekSo I can omit the external gate resistor, or is it a good idea to keep it anyway?17:32
DocScrutinizer05I'd keep it, make it 1k even17:33
shevekOk.17:33
DocScrutinizer05or make it 220R and use a 100nF17:33
wpwrakif 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 MCU17:33
DocScrutinizer05http://www.wolframalpha.com/input/?i=100nF+*+220+Ohm17:33
DocScrutinizer05still 22us17:34
DocScrutinizer05I'm not used to such long timing constants we should use here :-)17:35
DocScrutinizer05make that 220kR17:35
DocScrutinizer05;-)17:35
DocScrutinizer05and 100nF17:36
DocScrutinizer05wow, that sounds odd17:36
shevekIf 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. ;-)17:36
DocScrutinizer05FET should work infinitely17:37
DocScrutinizer05wpwrak: could you please check the timing constant for 220kR -- 100nF ?17:38
DocScrutinizer05is it really 22ms?17:38
DocScrutinizer05I'm not used to that stuff in this magnitude anymore17:39
DocScrutinizer05hmm, well, a 22s for 100uF * 220k sounds right again, so yes, it's prolly 100nF and 220kR you should use17:41
DocScrutinizer05shevek: you can use this softstart method _only_ when you don't use ('high' frequency) PWM to regulate the heater17:46
shevekAh yes, that's a good point.  I do actually want that.17:47
DocScrutinizer05when you use PWM with a frequency >1Hz, you can't use that softstart and you need a diode that has 4A continuous 17:47
shevekOk.  I'll do that then.17:48
DocScrutinizer05or 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 1ms17:49
DocScrutinizer05IOW 100kR and 10nF17:49
shevekRight.17:50
DocScrutinizer05I suggest slow PWM for a inductive heater17:50
DocScrutinizer05fast PWM has no benefits but brings lots of hassle with such stuff17:51
shevekThe induction is not its main feature; it's a resistive heater, that happens to have some induction.17:51
DocScrutinizer05yeah, I know17:51
DocScrutinizer05sorry for the fuzzy wording17:51
shevekIt's actually only 12 windings, I was mistaken before; it's 4 of those in parallel.17:52
DocScrutinizer05still on a magnetic steel rod17:52
shevekYes.17:52
DocScrutinizer0512 windings air would be negligible. Steel core changes that completely17:52
DocScrutinizer05also take care for proper isolation of the NiCr wire on the steel ;-)17:53
shevekYes, of course. :-)17:53
shevekIt has a glass fiber coating.17:54
DocScrutinizer05good17:54
DocScrutinizer05if you want short circuit protection, add a 0.25R/4W resistor between FET source and GND17:56
DocScrutinizer05or even as small as 0.1R17:57
DocScrutinizer05which then only needs 2W17:58
DocScrutinizer05will greatly improve the reliability of your design, avoiding FET to break even on shorts18:00
shevekThat might be useful, yes.18:03
wpwrakDocScrutinizer05: 22 ms sounds reasonable. this calculator gets tr = 50 ms: http://sim.okawa-denshi.jp/en/CRtool.php18:06
DocScrutinizer05for 90%, yes. 22ms is for iirc 63%18:07
DocScrutinizer05or 66?18:07
wpwraki 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.18:07
DocScrutinizer05as I said: max 1% of PWM freq18:08
DocScrutinizer05I strongly discourage to use high freq PWM with heaters of that type18:08
wpwrakput a cap across the heater ?18:09
DocScrutinizer05the 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 100Hz18:10
shevekDocScrutinizer05: I think you mean 63%, 1-1/e.18:15
DocScrutinizer05yep, 6318:21
DocScrutinizer05and yes, of course the FET gets a little warm with this18:23
DocScrutinizer05rough estimation: 100W * 1% * 2 (switch state changes) * 0.5 (very rough estimation for 'efficacy' between ramping FET and load) = 1W18:25
DocScrutinizer05probbaly the 0.5 are way too high18:26
DocScrutinizer05worst case is half of 24V * half of 4A18:27
DocScrutinizer05at 4A voltage is near zero, at 24V current is zero18:28
DocScrutinizer05so it's not 0.5 but rather the integral over one half sine * 0.518:29
shevekIntegral, or average?18:31
DocScrutinizer05plus 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 that18:31
wpwrakhmm, not for steady state. what's roughly the DC resistance of the heater ?18:32
DocScrutinizer0524v@4A18:32
shevek18:32
wpwrak60 Ohm ?18:32
shevek618:32
wpwrakah . 6 Ohm :)18:32
rohon heaters one ususally doesnt do 'fast' pwm... cylces with the length of up to a few seconds are normal18:32
DocScrutinizer05what I said18:33
wpwrakthe © looks like a zero here :)18:33
shevekThat makes sense; they don't respond fast anyway.18:33
rohanything faster does only heat the fet more, and not help stabilize the temperature much, since there is lots of thermal/mechanical integration happening18:33
DocScrutinizer05inertia I called it18:33
rohi think our pwm runs on something like a single digit frequency in hz ;)18:34
shevek1Hz seems reasonable, I think.18:35
DocScrutinizer05microwave ovens have a PWM at ~0.1Hz or less18:35
DocScrutinizer05;-)18:35
shevekThey have a very slow startup.18:35
DocScrutinizer05yeah, sort of18:36
DocScrutinizer05the magnetron anode heating takes some time18:36
DocScrutinizer05err kathode18:37
wpwrakworst-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 return18:37
DocScrutinizer05huh?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) = 1W18:38
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 1ms18:39
wpwrakthat'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 resistor18:40
wpwrakso that's what you want to avoid18:40
DocScrutinizer05^^^^^18:40
DocScrutinizer05I said 10Hz, _not_ 10kHz18:41
wpwrakyour low-pass filter is around 7 Hz18:41
DocScrutinizer05*sigh*18:42
DocScrutinizer05[2016-03-15 Tue 18:49:58] <DocScrutinizer05> IOW 100kR and 10nF18:42
wpwrakah, you made it smaller. good. now it's 160 Hz18:43
DocScrutinizer05http://www.wolframalpha.com/input/?i=10nF+*+100+kOhm = 1ms18:45
DocScrutinizer051000ms/1ms = 0.1%18:46
DocScrutinizer05100ms (aka 10Hz)/1ms = 1%18:46
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 18:47
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 that18:48
DocScrutinizer05that's significantly less than the 63% for one T18:49
DocScrutinizer05or even the 90% your calculator seems to use18:49
DocScrutinizer05so no smoke signals, the FET will just get a little warm18:50
wpwrak(my calculator) tr is now 2.3 ms18:51
DocScrutinizer05yes, for 90%18:52
DocScrutinizer05not for 2.2V to 2.8V, out of 0 to 5V18:52
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.18:53
DocScrutinizer05yet you avoid transients of 4A resp 24V @ <1us18:53
wpwrakwith fc = 160 Hz and a switching frequency around 1 Hz, that would be fine, though18:54
DocScrutinizer05we're assuming frequency << timing constant of the low pass filter18:55
wpwrakyes. i was worried about the frequency you had before.18:55
DocScrutinizer05you should read the complete backscroll18:55
wpwrakit's sometimes hard to spot things in those walls of text ;-)18:56
DocScrutinizer05the point is that you must not 'spot things' since everxy part of info is equaly relevant18:57
wpwraknot if it supersedes previous information :)18:57
DocScrutinizer05in that case it's even more relevant18:58
wpwrakexactly. hence it's important to spot those bits. qed :)18:58
shevekAll info is equally relevent.  Some info is more equally relevant than others. :-p18:58
DocScrutinizer05no, since my and roh's suggestion is still to use sub-1Hz PWM frequency18:59
DocScrutinizer05thus the original suggestion still is valid18:59
DocScrutinizer05the 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 10Hz19:00
wpwrakyes, but i was warning about a dangerously near "danger zone". it's not a "suggestion" but a fairly hard speed limit19:01
DocScrutinizer05ohmy, we all got that meanwhile19:01
wpwraksee, it's simple ;-)19:01
DocScrutinizer05Ffilter<<Fpwm != Ffilter >> Fpwm19:02
DocScrutinizer05you seen my initial: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 19:03
DocScrutinizer05[2016-03-15 Tue 18:48:23] <shevek> Ok.  I'll do that then.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 1ms19:03
DocScrutinizer05it'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 wall19:05
DocScrutinizer05let's rather evaluate which snubber diodes would work for a heavily inductive load at 24V@4A, for freq of <1Hz and for 50Hz19:12
DocScrutinizer05obviously a 4A diode with a Wattage of 4*0.7W should always suffice19:13
DocScrutinizer05but I guess we can go significantly 'cheaper'19:13
DocScrutinizer05particularly since our inductivity has a ESR of 6 Ohms19:14
DocScrutinizer05((would always suffice)) not true, for frequencies of several kHz and way above, you need a schottky (low paraistic capacitance)19:18
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