Techzone gen3 motherboard rs485 trouble.

I'm hooking up my spare set of tech zone gen3 for Adesina.
It is using stepsticks hooked up via a breadboard, and a seperate stepper controller for the extruder. My goal is to use teacup firmware on this.

The problem is, I can't get the motherboard to talk to the extruder controller. It only receives T:0.0, and can't start the heater.

I've tried loading the same fived firmware I use on my other gen3 set, to no avail.

The other motherboard can talk to both extruder controllers fine (with fived installed), but the "new" one talk to neither.

I do have a simple diy-scope if that helps, but I don't know how if, where or how to use it here.

Just like on my other set, I do run an extra grounding wire between the motherboard and extruder controller.

The only difference between the two sets now, are I'm not using ribbon cables to connect the x and y axis steppers, so there are fewer grounding wires going that way. Also I don't have endstop hooked up to x and y yet. (Don't see why that would matter anyway).

I now have teacup firmware back on there. (And yes, also on the extruder controller). I can jog all axis fine through pronterface, but no temperature readings.

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-Nudel
Blog with RepRap Comic

No takers?

Would be interesting to try and move the communctaion to other pins, to see if the pins are broken. Though the rs485 pins should be able to control 4 extruder controllers, no? Any suggestions for which one I could use is welcome. There is a spare 6 pin ISCP header, but is that for programming the chip only?

I'm also tempted to put in an arduino instead of the motherboard, and if it really is broken that seems to be the cheapest option anyway.

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-Nudel
Blog with RepRap Comic

As an overview, it can be anything really, either mb issues, either rs485 pins, or sn75176 ic, or there is no 180 ohm resistor or are 2 of them, afaik should be only one. I assume the firmware is exactly the same as the ones that work, so that can be ruled out, then it leave a hardware issue.
First, i would check that there is 5V supply, and a gnd on all the mcu vcc/gnd pins that should have either. Then i would check all rs485 lines connectivity one by one against the board layout on screen, on all points that i could. Fortunatelly if you have a working comparison setup, do compare the working setup with the broken setup. I would use the scope as a logic probe and see if something happens on each of the lines. If something is wrong probably one of the lines at least would be dead. If it is so, i would check before the last ic in each case, and that will tell if last ic is broken or not - if its not, then the line is dead before it, then should be the mcu - use another pin if possible, else replace.
I cant help much except this small overview and a little "push" like: "courage and good luck!".

I needed a push! Thanks
I'll poke around and make a thorough list, which may aid in my understanding of this...

A (kind of unrelated) thought just struck my mind, would it be difficult to hack the extruder firmware to control the temperature at 200deg C, without interference from the motherboard? It'd be kind of ugly, but a switch on the 12v line would make it easy to control. Though it would be kind of a bummer not to have the lovely temperature readings..

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-Nudel
Blog with RepRap Comic

It could be done, but if you are at that, why bother. See if the motherboard works, if it does, get 1 adc pin from that and make a small temperature or thermistor board (can be done on top of the '100 pins, with just the cap and resistor and other 2 pins after it, so no pcb at all). If you use a stepper from mb, and temp on mb also, then there is no need for extruder board at all. If you have a 2nd adc pin you could use it for HB even the same way with another thermistor board.

Absolutely, valid points. But no, I'm not that far yet. I'll poke at the motherboard a bit thoroughly first, and take it from there.

It always did seem strange to me how little work the extruder board actually does...

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-Nudel
Blog with RepRap Comic

I think extruder board itself is more like an remanant from the times when it was used (or planned) for other purposes. If you want to use a dc motor, it will need an encoder, and with encoder you need to "sense" the movements fast and to be able to react fast. I can only guess, but i bet that was the original purpose and it makes sense to use another free mcu just for that, an mcu that has no burden and it is able to react fast. Today, with a stepper extruder there is no control loop, so reaction time is irrelevant, hence extruder board is more or less obsolete.

The motherboard has to work at least a part of it since it reports something and it was programmed. The main suspect would be the SN75176 ic or the 180 ohm resistor (either missing or having 2 of them), or some interrupted tracks. The resistor can be seen visually, tracks can be checked with a meter. Check the resistance of that resistor also, maybe is the wrong one. Also you have a working motherboard, check some components in comparison if something looks different.

If all is ok then leaves the main suspect that ic. If you dont want to bother with it, just try see if you can get 1 adc pin, put the cap and resistor according to the thermistor board schematic, and make the temp reading on motherboard. I think teacup should even have that by default. You might need to re-check all the mcu pins in the firmware to make sure its correct, i dont know if teacup has a pin layout ready for gen6.

Edited 1 time(s). Last edit at 06/17/2011 07:43PM by NoobMan.

Forgot that you will need another pin to control the supply for heater, but that will be just any digital pin and there should be plenty of those to choose from, in comparison to adc pins. As for fet there should be plenty of choices too, best would be one that is logic level.

Also if you want to write the temp in extruder firmware, find a line that is controlling something like "heater_output", and resembles with:

void extruder::controlTemperature() {currentTemperature = internalTemperature(); if(targetTemperature > currentTemperature) digitalWrite(HEATER_OUTPUT, HIGH); else digitalWrite(HEATER_OUTPUT, LOW); }

Lets say you want to set it at 200 C
Change it to ..... if(200 > current temperature) ... writes output high (enabling the fet output) otherwise writes low (turns off).

Providing that the rest of the code is unchanged and temp reading is the same and working, cant figure a better place to make the change like this. Everything else remaining the same it then should also report back current temp if the rs485 comms would be working.

Good luck

Yes, the board runs fine. I can jog all axis and so forth. Having a little trouble with the z endstop, but I think that is firmware related.

Anyway, I took off the motherboard and took a close physical inspection, and guess what I found.

(Accompany the following picture with this video: [www.youtube.com] )
It's rather large, so I'll resort to linking: [localhostr.com]

It looks like it might connect, but if so only barely and with very, very little surface area.
One more picture of the badly soldered IC: [localhostr.com]
There was also a skew transistor near the comm pins, but it looks to have proper contact: [localhostr.com]
The rest of the board looks ok (apart from the bad lighting) and have the same resistor count as the other board: [localhostr.com]

So I guess I might as well try to get that IC seated properly now. Off to the temperature controlled solder station (which I don't have) to use my abundant knowledge and experience with SMD soldering (which I don't possess, at all). But that'll have to be later today.

As for running off the motherboard only, and the schematics for a little heater circuit, it would be good to have on the wiki. As it could be a great option for people who get into trouble with their extruder controllers (or rs485 connections).

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-Nudel
Blog with RepRap Comic

That could indeed be the source of trouble. I believe that would be the transciever that was the main suspect. If i fit the pictures correctly to the layout of the motherboard from the pdf here

I would double check the orientation too. De-soldering might be tricky without proper tools or spare ic. If orientation is right, it doesnt look too shifted out of position, so first i would try leaving it like that and only adding some solder. If its not a short between the first pin to the right and the trace leading to the second. Also i would check that the extruder and the other boards have the same type of ic for rs485.

Also, in general for smd soldering, the "standard" is to have solder touching the pad for at least 25% of the smd contact surface. So i would add a little more solder to the components in the other picture and all over the board where soldering looks poor like that.

Good luck again.

Edited 1 time(s). Last edit at 06/18/2011 05:35AM by NoobMan.

If you want more info for running just from mb, I am not sure the "schematic" is the right word for it, but the schematics for these are all over the place really. Just embeded into bigger schematics. If we look at almost any piece of electronics around reprap, all use the same stuff.

To read temp with a thermistor, every single schematic is around [hydraraptor.blogspot.com]. Check the [www.reprap.org] same schematic and same as in extruder controllers and any thermistor temperature reading across all electronics. My take for toner transfer is [www.reprap.org] and you can find schematic as picture inside zip. The led and led resistor can be dropped, and r1 can be 0, then it just leave 2 components, usual values a C1=10uf and R2=4k7. And further it needs +5v, gnd, and an adc pin. Dont even need a pcb to solder 2 components with 3 lines in and 1 out (second out is gnd). That adc pin ends up in firmware as "temperature pin" and the voltage on that pin gets read and translated to temperature.

The fet again, lots of it, all over the place, in any single electronic piece. On low side, that is it interrupts the gnd line, so it has a gnd_input and a gnd_output. It connects to the mcu digital pin as gate, and interrupts the drain to the source. So its one mcu digital pin that goes into the fet really, dont need pcb again. Solder the wires directly to the package pins, e.g. TO-220 pins, and isolate the pins with heat shrink tubing after the soldering, put kapton tape on the backing to avoid shorts with sink and screw that anywhere in a hole. The heater itself gets voltage+ independently psu from another line and gets it dumped on the gnd provided by the fet. That is when the fet is off, the heater will have voltage on it but no gnd line to drive it into. Almost any fet around reprap electronics is an example for this, look at heated bed fet, or open schematics for any fet (or BJT ) that are used to drive the heaters or fans, all are exactly the same thing. The pin that is driving the fet ends up in firmware as "heater output" and its driven high or low, e.g. 5v is on and 0v is off.

Examples
Mendel heated bed has "schematics" for both thermistor read and fet, there is a 47uf C2 cap but i guess its not really needed, and a 1000u C3 which is redundant with the psu output capacitors if psu isnt in the other room that is;
Heated bed see the big picture and the "bed heater" blue square, has a diode i guess for transil purposes but that one doesnt make sense to me as i would of linked it before the fet and not after, and the psu should have had that anyway so its rather redundant;

Dont know if i overexplained stuff or made a mess of all things, but i guess both, as usual. I am sure you can get the modding of the mb done just have to find the free pins and how to link to them, just that is 4 layer and the board source isnt. "very open" doesnt help, but at leas you have the hardware physically so you can check things out.

That's some good information you provided there, makes it much simpler for us beginners! As regarding to free pins, the enable pins to the different stepper motors can be used if you hardwire the steppers to be active. That should leave up plenty of pins, no?

As for my badly soldered IC, it turned out one entire side wasn't soldered at all, and was just to lift up. So I desoldered it and with a lot of solder flux paste and probably a huge amount of luck got it to stick. My respect for people who do smd soldering by hand just vent up exponentially, I really want a proper soldering station now, and no, you can't see pictures.

I hooked it up, and voilà I get temperature readings!

For some reason the readings are very low, and only read in the 5-7 degrees range when it should be closer to 20-30, so I have to check the thermistor table and config. As my soldering was pretty sketchy, I'll check with the working fived firmware first, to make sure everything is working properly.

Thank you so much for your kind assistance NoobMan! You really helped me out, so here, have a cookie.


Edited 1 time(s). Last edit at 06/18/2011 04:09PM by Nudel.

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-Nudel
Blog with RepRap Comic

Cheers m8, glad its sorted and it works.

About enable i think some stepper ics need enable low while others need enable high to allow the outputs. Either way could be permanently arranged on the stepper board itself as it has gnd and should have a logic supply too.
However on the motherboard the first generic pins that get re-assigned usually are the MAX-endstop ones, as most ppls dont use them while most electronics do provide the option. And these should have some connectors somewhere already so should be much easier to hack.

SMD soldering by hand is pretty easy tbh, i find it much easier than trough hole which needs constant turn-over the board. Only need some good tweezers and some iron station and a little exercise but its easy for sure. I use the regular solder wire, not paste, paste is more for things like reflow or hot air station.

P.S. Wow thats a big cookie - thanks alot - yummy

NoobMan Wrote:
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> Mendel heated bed has "schematics" for both
> thermistor read and fet, there is a 47uf C2 cap
> but i guess its not really needed, and a 1000u C3
> which is redundant with the psu output capacitors
> if psu isnt in the other room that is;

For high frequencies (and fast transients), even 5cm can be too far. Do not underestimate the importance of decoupling. Any circuit I make has a liberal sprinkling of 100nF capacitors, and anything that uses more than about 100mA has some form of onboard bulk storage, 100uF or more. These supply larger currents until the inductiveness of the power supply wires allow the capacitors in the psu to take over. They can't replace the smaller capacitors because larger capacitors are generally slower, and we need both speed and capacity so we need both large and small capacitors.

Anything involving digital logic generates lots of fast transients, and absolutely requires a decoupling capacitor as close as possible (both physically and electrically) to the power pins of each and every chip.

I have ICs here that specify in the datasheet that the decoupling capacitor cannot be more than 2.5mm away or the chip will experience problems due to the inductiveness of the traces on the PCB!

I personally believe that ramps doesn't have enough decoupling capacitors.

> Heated bed see the big picture and the "bed
> heater" blue square, has a diode i guess for
> transil purposes but that one doesnt make sense to
> me as i would of linked it before the fet and not
> after, and the psu should have had that anyway so
> its rather redundant;

Actually that diode is extremely important. It stops the fet being destroyed by inductively generated high voltage when it turns off.

Inductors try to keep the same current flowing so when the fet turns off, the current must flow somewhere, generating an enormous voltage if there's no easy path. This is how the ignition coil in a car works- it "charges" an inductor then switches off suddenly, forcing the current to strike an arc across the spark plug in order to keep flowing. This is also how boost switchers work, they "charge" an inductor from the low voltage, then switch off really fast so the only way the current can keep flowing is by jumping up to the output voltage and charging the output capacitor.

That diode gives the current a path back to the decoupling and bulk storage capacitors. If the current had to go all the way back to the power supply, the fet would be dead before the capacitors there started consuming it, and if it wasn't there at all the high voltage would punch a hole through the silicon in the fet long before it arced through the air. Even straight wires have some inductance (albeit only a small amount).

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Wooden Mendel
Teacup Firmware

Hi
Btw, /respect for all the contributions to reprap. We can engage in polemic around the matter just pls dont take it too serious pls. I dont think you read the whole context and just rushed in, and i believe this time the context makes all the difference. I agree on the decoupling and rest of them, and i can almost feel you right there, but you know, what i was talking about had absolutely nothing to do with digital logic. Its actually heated bed schematics and i commented them as such. No logic, unless you want to count the fet getting driven off ttl, but thats not involved with what i said.

The 1000uf cap: is for heated bed, and i cant think of any other appliance which would care less about its current. In fact, it would run on AC and probably even better. We could think without that cap HB will drain the smps psu ending caps empty which will increase the ripple for the rest of electronics. Very long shot imo. Also the rest of electronics use linear regulators and have a 5v output vs a 12v input and maybe ~2v dropout. Also linear regs have better quality output than switching equivalents, and on the overall the ripple would of need pretty huge to even get close to affecting them.

The diode: which i considered to be meant for transil purposes, tranzorb, transient voltage suppresor, protection diode, etc indeed. To clarify, I would imagine possible topologies of that diode there:

-across the load after the fet (like it is on the schematic i linked, its not always grounded, and in series with fet internal diode): here does not have direct ground because it is after the fet, in parralel with the load. So if, and when, a transient does pop up exactly at its tip and it starts reverse conducting, well, it would have to reach ground through the fet internal diode which will have its piv for that purpose. Basically both diode would conduct the same transient, in series. So much for fet protection. More interesting, when the fet is off, the diode sits both legs on V+ line as heater remains with V+, and weirdly, the topology is more like (sort of speaking): the diode is trying to clamp the voltage across the heated bed because its positioned across its leads, and not the fet (i wonder if voltage drop across the hb as inductor is > piv). This placement is like diode is trying to protect the heated bed from a transient that is coming from somewhere else. From all possible topologies i can think of for a transil diode in the context, this makes least sense to me. So this is why i dissed it to start with.

- across the fet: afaik if you want to protect a switching fet with a protection diode like that, then diode its actually actually across the fet and not in the position that is there. I guess its why most fets actually have that diode built in across internal pins and no doubt that fet has its own also. This position is across the fet, like a usual rc snubber, but that would be in parallel to the fet internal diode. And there is no chance in hell i could match the PIV of the two diodes so i would bet that in practice one of them would be pretty useless. This position only makes sense to me if the fet doesnt have internal diode on its own, just afaik most fets do, and should, for this usage.

- across the load *and* always grounded: the diode could be grounded before the fet, to be always grounded, but then it will just simply be just across psu outputs, would be like a generic psu feature, and could be redundant with something that should of been at the exit of the psu (output transil or snubber circuit). I think this makes more sense, but its something the psu should have on its own. Because if we put it there, then it severs the entire electronics agregated inductance, if we can speak of such. So its more of a psu feature than a specific heated bed feature. On the side at least the 3 high power psu i made all have both snubber and transil on their own, mostly because "generic" snubber is impossible notion. Also its hard to argue this for all the heated beds around, but assuming my LC meter is right, my heated bed made from pcb traces has -10 microhenries. As a personal choice i would not bother making a 100V/us snubber for atx psu and such a low inductance. Honestly i simply dont consider it significant enough, but that might be just me. But indeed that is a matter of choice and of how "purists" we want to be. Anyway for the diode at hand, this is what i meant by saying "i would of linked it before the fet". This position makes most sense to me in the context, if the psu doesnt have its own, and if its considered to be *really* required.

Please somebody point to a mistake i made. I tend to make one big mistake each day, and i would be glad to have it done here as its best place to do them. Most importantly it would mean i can relax for the remainder of today

Just thought again about diode position, and a strange thought, maybe it was actually intended to protect the heater from the fet. So at first i thought "diode is trying to protect the heated bed" is something funny, as i consider my hb rugged enough, but now that i am actually starting to have second thoughts about it and cant be ruled out. It *can* be there to actually protect the heater and not the fet. How is that for a turn of situation. I think it wants to protect the heater from fet switching transients because the heater is nichrome wire and simply wouldnt take too much stress. Not for fet protection at all, has nothing to do with the fet protection, as it cant do it from there, its more about protection from it. Not meant for generic psu transil, that seems kinda secondary effect with 2 diodes in series. Maybe it was really intended only to drive transients away from the nichrome wire itself. Simply like that. I find it very interesting, and it wouldnt first come to me that it could be about that. I guess i am starting to have too much prejudice. I see a fet and a transil diode and i assume they are meant together. This seems the case where they arent.

Either if the diode is across psu leads, or across the fet, the transients would have to travel through HB. The only way to protect the nichrome itself is from that placement. So i found the real justification for its placement there.

I think i found my mistake and corrected it. Therefore i can relax for the rest of the day.

Edited 1 time(s). Last edit at 06/20/2011 08:05AM by NoobMan.

You seem to be confused about the protection diode above the fet, and the fet's internal diode.

A fet's internal diode is a side-effect of its manufacture. Basically, a mosfet actually has 4 connections, gate, channel1, channel2 and substrate. It's the gate-substrate voltage that activates or closes the channel. In almost all fets, channel1 is connected to substrate internally forming the 'source' connection, and channel2 becomes the drain. The barrier between the substrate and the channel is a PN junction, like a diode. If channel2 drops to a lower voltage than the substrate, that intrinsic PN barrier reverses and allows current to flow from channel2 to substrate. Since substrate is connected to channel1, it pretends to be a reverse biased diode between source and drain.

This internal diode provides zero protection against overvoltage- it is the wrong way around to forward bias in an overvoltage condition, and hooking the fet up the other way would keep the diode forward biased all the time.

Inductive kickback from the load will try to pull the fet's drain terminal above V+. The fet's internal diode does not conduct when this occurs- the fet is in no way protected by it.

The fet requires protection against these voltage transients or it will be destroyed. A zener or TVS (transient voltage suppressor) across the fet itself would be ideal, but failing that a regular diode that shorts the transient and dissipates the energy as heat will do.

This is the purpose of the protection diode- to short out the inductive kick from the load that occurs when the fet turns off. It serves no other purpose, and this purpose alone is an important one.

The load doesn't need protecting, and if it did, that diode wouldn't protect it. It cannot make the fet turn on slower in that configuration which is what would help with thermal shock. If anything, it slows the decay of current when the fet turns off, which is an important consideration if you want a relay to turn off really fast or you're making a gauss gun, but a heater won't care in the slightest.

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Wooden Mendel
Teacup Firmware

I will take any chance to correct myself tbh, but please give me a hand to straighten me out. Because i am sure we talk about same thing.

One diode that i was talking about topology / placement: heated bed its the image with schematics, D1 RSFGL, which after like half of day i thought the explanation is that it seems to protect the heater, supposedly nichrome wire as it clamps it and not the fet (it is in series with the fet). The first time i mentioned to ground it before the fet, so it will clamp the psu, and on other side the load + fet together. Other place would be across fet and will protect fet only. But in the position that it is, it clamps the load, so it looks like it is protecting the load from the transients created by the fet - only reason i can think for it to be in that place. Am i wrong in this interpretation?

The internal fet diode i was talking about is the one i made a green circle in the picture. From my very limited understanding of diode schematic symbols, that symbol coincides with an asymmetrical (unidirectional) transil diode, and it seems to me that it is the right place to protect the fet, at least sort of looks like. I always thought it does - is that what i got wrong? Then what does "drain to source breakdown voltage = 30V minimum" from datasheet? And why is the symbol like that there?

Edited 1 time(s). Last edit at 06/20/2011 11:47AM by NoobMan.

D1 protects the FET by preventing inductive loads from taking the drain above the 12V rail thus limiting the maximum source drain voltage to 12.7V.

The specified HEXFET probably does not need it because instead of the normal body diode it has one with a defined breakdown voltage that will clamp a load to something around 30V. Most MOSFETs would need the external diode though.

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[www.hydraraptor.blogspot.com]

I am trying to explain stuff to myself as i usually do trying to understand it, and i believe its a peculiar way and uses stupid terms but pls bear with me.

Perhaps its my preconception wrong when trying to figure transient path. Because my preconception is that a transil diode like that doesnt "dissipate" the transient on itself (not most of it anyway although ofc will dissipate some heat), but more like "conducts" the transient away (is that wrong?). Am i confusing a transil diode with something like a generic protection diode? - for example something that would just protect against a capacitor discharge by re-routing or by-passing it away from its protection objective?

Anyway, lets assume each switch creates a 1.8kV transient. Diode D1 (rsfgl) needs 400V to conduct in reverse.
a) we put diode across fet => 1.8 kv travels through nichrome wire
b) we put diode like is in that picture => max voltage "travelling" through nichrome wire is ... 400V (right?)


Sorry if i am too buggy i would just saw some willingness to explain things and i want to profit from the situation because honestly, its a rare situation to me. My learning curve was and still is in pretty bad shape so you know, i do appreciate the help and i am trying to take in all i can get.

The picture is here [www.reprap.org]

I assume the transient made by Lp will be made on the point of 12V line as it tries to pull more voltage from it until its filled up with energy. Transient goes mostly through the D1 then fet to ground, untill its less than 400V, at which point D1 is not reverse-conducting anymore, so the path goes through heater and then trough fet.

Or is the transient created in another place, the point between the fet, diode, and heater, when the fet switches on as this point changes from 12V to gnd. In which case the diode could forward conduct to 12V line (btw if its not "eaten" then where does it go from there?). Except probably the internal fet reverse conducts untill 30V and puts this transient to gnd. Which path would the transient rather go from there: D1 in forward conduction or internal fet in reverse condition, or both?

Edited 1 time(s). Last edit at 06/20/2011 04:27PM by NoobMan.

D1 is not a Transil diode. It is an ordinary fast recovery rectifier. It is connected across the load such that it is reversed biased when the FET is on, so does not conduct, but will if the inductance of the load tries to take the drain above 12V plus its forward voltage ~ 0.7.

If the diode was not there the body diode of the FET would break down at some value more than 30V and clamp the back emf from the load to that voltage. The body diode behaves like a Zener or a Transil / TVS diode. The datasheet shows the waveform you get with an un-clamped inductive load. Note this is not what a plain MOSFET would do (they would be destroyed by the BEMF) but more modern ones tend to include this protection.

The position of D1 is entirely normal for suppressing BEMF with an inductive load. The bed heater will be largely resistive, but it will have some inductance and this can be enough to destroy a MOSFET.

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[www.hydraraptor.blogspot.com]

how about a walk-through?

1) fet is off -> drain voltage is 12v via load
2) fet turns on -> drain voltage drops to ~0.1v or less, load inductance prevents instantaneous current, but current quickly builds
3) fet remains on -> maximum current flows through load

4a) fet turns off (D1 in place) -> load inductance tries to "throw" drain voltage to a few thousand volts, but D1 forward biases and conducts, shorting out this spike. Actual drain voltage only reaches VCC+1v or ~13v
5a) fet turns on -> load current builds

now compare with:

4b) fet turns off (no diode) -> load inductance "throws" drain voltage to a few thousand volts. fet undergoes avalanche breakdown from overvoltage, silicon is damaged.
5b) fet turns on -> damage causes hotspots, fire ensues, fet is totally destroyed


This assumes a non-protected fet, which is a healthy and careful assumption to make.

Perhaps a read about Boost converters will help, since they exploit this principle to give an output voltage higher than the input voltage.

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Wooden Mendel
Teacup Firmware

Thank you both so much for explanations, i started to see light when i saw "bemf".

I understand now it tries to increase its load gnd side to be more positive in respect to its V+ terminal so D1 was actually meant to conduct forward and not in reverse like a transil (which i was so fixed upon for some reason).

Thanks again. I probably have lots of other bad representations in my mind, but now its one less