Quotedieterzar Hacking the power supply would be easy if I had a circuit diagram to work off. Thanks for exposing the insides of your Alpine PSU dietezar. In the topic Can the regulation of the ATX PSU be improved? I was looking for a solution that could be implemented without hacking the PSU. Your findings may help with that.by Radian - Ormerod
QuoteTreth @Radian, that's why I was suggesting grounding/earthing the aluminium plate (opening thread) as it seems to be easy to do and would provide a capacitive shield to the spikes in this area. My scope currently not working so can't investigate. I don't want to make things worse!!! Sure, I had your suggestion in mind - only there's a bit of distance between me and my Ormorod at the momentby Radian - Ormerod
Without suppression the flyback voltage peaks at around 50V with very high dV/dT. The combined surface area of the Heater Bed and ribbon cable carrying this spike are quite substantial providing plenty of scope for capacitive coupling. It just kind of gives me the creeps. WWND? I'm pretty sure they'd suppress it to prevent Feynman coming back (from Heaven) to pick holes in their designs.by Radian - Ormerod
Quotebobc I must admit to being slightly puzzled, the solution to ground loops and all the other problems on USB is Ethernet... and it is there on the Duet already. Ethernet is isolated, so there are no ground loop issues, the protocols are designed to be robust and recover from errors, and is probably faster than USB, depending on type. You can even get a telnet serial interface so it looks likby Radian - Ormerod
Thanks for all your thoughts on this. My take on it is that RRP already have to supply an ancilliary PCB and case with "watty" resistors which aren't zero-cost items in themselves. This item will always have to ship with ATX's so a re-design of the board to use semis instead of dumb resistors could keep pretty well within budget, but make the PSU behave more like an industrial one without the cosby Radian - Ormerod
The low-cost ATX supplies that have been shipped with Ormorod so far don't hold up their 12V rails very well under load, hence we hear the fan speed fall and rise as the heated bed switches on and off and some people struggle to get enough power into the heaters. As I understand it, the fixed resistor dummy loads on the 5V/3.3V rails are needed to keep the PSU from going into fault-shutdown butby Radian - Ormerod
As I understand it the ATX PSU is effectively running "open loop" as the opto-isolated feedback path to the transformer chopper driver is taken from the 5V/3.3V rail. Not having had the time to trace out the PSU I'm not sure, but reprap have had to artificially load those rails to get the PSU to play ball. Maybe an active dummy load could be substituted that takes its cue from the saggy 12V and aby Radian - Ormerod
Hi Treth, yes I know that there is no simple concensus answer on shield connection but I think this is simply because the most appropriate solution depends on many factors - for example, if the peripheral device has its own earth or not. Setting aside the full isolation solution (which might actually be a useful option for some if USB over CAT5 is used as it would also allow several tens of meterby Radian - Ormerod
Bed heater LED circled on Duet PCB (note that this one has green terminal blocks instead of pin headers like on yours) I also scribbled where you could connect a 10K resistor (if you had one) to bypass the thermistor . You would unplug the ribbon to remove the whole bed from the equation, and try fooling Duet to power the bed by pretending to have a reasonable temperature as per Ian's suggestiby Radian - Ormerod
QuoteMrCrispi No sign of life on the bed at all - didn't even realise there were led's will further investigate wiring... So no power to the bed. There's also a LED next to the transistor that switches power to the bed. It's tucked in between the transistor and the ribbon cable connector on the Duet PCB. It should light when you set a bed temperature above ambient. If this lights and the ones oby Radian - Ormerod
Hold on, is the bed actually being energised? I can't tell from reading the posts so far - it all seems to be about not being able to monitor the temperature. Simple question - do the bed LED's illuminate?by Radian - Ormerod
I've currently got my eye on the earthing: the supplied ATX PSU ties the mains earth to the 0V on one edge of the Duet PCB and the USB lead can connect to another mains earth (via connected PC) creating a potentially large enclosed loop. In the middle of this ring is the ground plane upon which the microcontroller sits. Ormorod is too power-hungry to connect to my PC supply which is derived fromby Radian - Ormerod
QuoteTreth @Forum readers, I'm interested in how many have modified the FET driver to remove spikes? Post a reply YES or NO Yes, for both bed and extruder loads. Before I was getting lock-ups during something like 2 out of 3 prints. After, I have had 2 lock-ups in I guess around 30 prints. These may be due to something else like mains-borne transients or ground loop current. It has become hardeby Radian - Ormerod
Quotedc42 I'm happy to report that RRP have now merged my changes into the Duet branch of the official firmware. The official 0.55 release build is here, I haven't tried it yet, but they seem to have accepted all of my changes. You still need to do the Pronterface patch to make USB printing and file upload work at full speed. That's great news dc.Official acceptance of the hard work done on thesby Radian - Ormerod
Quotedc42 Quotemarkbee Different times of day, no electronic mods (beside 12V stepdown not working on my board to be replaced) and both prints about the same time of print. Use of USB power is very marginal due to insufficient dropout margin in the 3.3V regulator, so it's probably best to get RRP to swap your board as soon as possible. Some of us (including myself) experienced print hangs untilby Radian - Ormerod
I found it useful to switch off debug messages by addding the line: ser.write('M111 S0\n') So this is my reference plot using silver foil: I'm amazed at how the tiny Kapton tape gaps stand out! It's not like they're even as big as 0.1mmby Radian - Ormerod
Of course dc42, you're right - with an unrealistic parasitic capacitance of Zero the sim really struggles with the "Gate resistor only" model. I only have to put on10pF to get a sensible response: Increasing Cparasitic lengthens the time spent ringing, so by comparing with the real scope traces 10pF seems to be in the right ball-park. It has no discernable affect on the flyback diode/bypass capby Radian - Ormerod
100R Gate: 200R Gate: Not what you were expecting? Avalanching doesn't appear to be being modelled. Not sure why not. I thought it was. BTW LTspice is a freebie from Linear Technologies 1uF Capacitor between Drain & Source showing Drain current in blue: Note that the peak current due to the relatively slow (2uS) switch-on is virtually the same as the steady Heater current (by design )by Radian - Ormerod
Hi dc, I didn't know that Dennis had also slugged the gate. Without that additional tweak the current drawn from the 12V would drop 10Amps in a few tens of nS rather than uS which is somethig I've been trying to avoid imposing on the "less than ideal" PSU we're working with. I ran a pair of sims in LTspice to illustrate the difference between the capacitor snubber and the freewheeling diode (greby Radian - Ormerod
QuoteRory166 Dmould Whilst not disagreeing with you. There is a post somwhere here that I have replicated Radians work using a battery scope and Duet powered from Laptop USB ie no mains at all, not even laptop charger. There was a suggestion about using two scope probes in difference mode AC coupled with the two earth clips joined but not connected. I have yet to try this but I believe Radianby Radian - Ormerod
Just to add that I've started a dedicated thread to discuss the Heater switching transients issue.Unfortunately this particular thread about the quality of the 3.3V supply contains an awful lot of discussion about the heater switching transients. The two are indeed related, but there are parallel issues to do with the "not so LDO" regulator and noise immunity that might be better continued here.by Radian - Ormerod
Just to add that I've started a dedicated thread to discuss the Heater switching transients issue.by Radian - Ormerod
There are already several threads discussing issues with the power supply and they frequently include discussion about the heater switching transients. To avoid too much fragmentation it seems appropriate to isolate this particular topic in its own thread. Power supply problems, Power supply regulation, etc. can remain the main topic in those threads. My other topic regaring the quality of the 3by Radian - Ormerod
Quotechriscain To have a baseline for the sensor before any mods I wrote a quick script to check the bed level + sensor readings Excellent idea! I'm running it now to check my own readings. Hehe... how's about turning the X,Y,Z's into a .stl and printing it back onto the bed !?! (maybe not at 1:1)by Radian - Ormerod
QuoteDennisCowdery d) wire a 1A 30v (piv) schottkey diode across the heater bed -observe direction!! Hi Dennis, do you have a way to post up a scope trace of the Drain voltage and the 12V PSU using the diode? Just curious to compare it with the capacitor snubber. I'm being a bit lazy now, as I could do it myslef but I've lost count of the number of times I've pulled the Duet PCB out to tinker.by Radian - Ormerod
@dc42 I was working with 3.5uH as the bed plus cable inductance (which I had measured with an LCR meter) and 12A which my 25A bench supply indicated. BTW, Basics_about_switching_loads_with_MOSFETs on the reprap.org site discusses the use of a freewheeling diode and laments its absence on a number of printer controllers! The article seems to be more concerned for the health and safety of the MOSFby Radian - Ormerod
I personally enjoyed the challenge of construction (and would enjoy doing it again) - it was an ideal project over the festive break. I was fully aware that it was a kit that had to be built - I only wish I could remember how I knew that. I don't think it said as much on the RS site when I placed the order. Also, given that I was encountering problems at an awkward time for support, the support wby Radian - Ormerod
Hi Dennis, one concern I have with using a diode to quench the back-EMF is the current it has to handle. There's about 250uJ stored in the Bed and it's released in a couple of uS. Isn't that around 10A peak? If the diode fails, it will almost certainly go short and may then cause some real grief. Suppression Capacitors are designed to fail O/C and take this kind of thing in their stride. BTW, doby Radian - Ormerod
So dc, now I find myslef wondering what the subtle difference is placing the capacitor across the load versus across the switch... both provide a path for the back-EMF and equally well prevent the rise above 12V at the Drain terminal. From a theoretical POV the two networks are identical AC wise due to the bulk capacitance of the PSU. In practical terms I think the difference is the series inductby Radian - Ormerod
Quotedc42 So I'm thinking that a capacitor of 0.22uF would probably be enough to prevent avalanching, saving 40uJ dissipation at turn-off, but only add 16uJ of dissipation at turn-on. 0.22uF certainly would prevent avalanching, but it would definatley "ring" all the way down. I most recently played around with values on the Extruder heater MOSFET which switches roughly half as much current thanby Radian - Ormerod