Blowing power supplies



  • I recently got another Duet Wifi (v1.03, from the on special batch) and I've been having some trouble diagnosing a suspected short in my wiring. The printer has a 300x400mm 24v heated bed, and while heating the bed, 3 LED power supplies have died. The first power supply was a 400w model, and it blue the fuse on the power switch, before dying 2 minutes later after replacing the fuse. The second power supply was a 350w model, which blew this morning. I put the first down to the warning on the box "Install vertical direction", which I'd missed during installation. The second, I concluded must've been due to excessive draw from the heated bed.

    So naturally, I got my 800w supply, double checked all the wiring, and fired that up.

    With each power supply, I've been able to heat up to ABS temperatures and print a few times. The first two saw voltage drop to 22 in the Duet Wifi display, when everything was going. The 800w supply dropped from 24.5 to 24.2v, so I assumed I'd found the cause. But that blew about an hour ago.

    The 800w supply had been running a 300x300 heatbed printer of the same design, with a Duet Wifi as well for months. The board still works when I plug a USB cable in, and worked after each of the first two power supplies blew. What are the possible causes for this? Are any board related?



  • @mortarart

    you don't mention the details for the heated bed you are powering or if you are powering it direct or through a seperate SSR/mosfet.

    while the Duet can handle 18A from what i read the fuse is only 15A (nearest sise) so that means 360W of power for the heater.

    that you are moving from 350W to 400W then 800W suggest you think the heater is more than this?

    what make are the power supplies? while there are plenty of cheap ones out there they can be sadly lacking and rarely live up to the specs and fail when pushed hard - for a quality PSU 800W is often around £300.

    if you have access to a multimeter then a resistance reading would let you work out the heater power but its not likely to want to measure the power (most top out at 10A or 20A)



  • No SSR. Directly wired to the board. The 300x300mm heater was 300w, so I assumed the larger one was higher output, by at least 1/3, but it seems to heat up about twice as fast, so maybe even more. I'll measure the power and reply.

    What I find troubling is that the power supplies died. I would've thought that if they were over taxed, they would simply not supply enough current? The fuse on the Duet is fine.



  • Two of the power supplies came from RobotDigg. Same with the silicone heaters. The 800w model came from Ebay. They're all rather generic.



  • On further inspection. The first (400w) power supply, which initially blew a fuse in the IEC plug, has a blown input fuse, while the other two do not.



  • 0_1535795480901_SAM_2242.jpg

    And this photo shows a burned component in one of the power supplies. I suspect that's where this one blew.



  • 0_1535802111059_SAM_2244.jpg

    And finally, one of the DC capacitors on the 800w supply. So it seems I broke all three of them in completely separate places. I would assume that supports the notion that they just failed due to the load.

    What seems strange is that the bed heater was able to draw enough from the 800w supply, to bust it, without blowing the fuse on the Duet Wifi.



  • not knowing that supplier I wouldnt like to comment but I would suggest that those power supplies are at the bottom of the heap and the power specs are...... optimistic??

    have a look at the Meanwell sp-750-24 as an example of the size weight and cost of a "real" 750W PSU as a comparison.

    unfortunately the china supplies come with ratings that bear little resemblance to reality.

    in the computing world you were meant to size the PSU based on generic max values for each item in the computer and the numbers often came out 3 or 4 times higher than you needed so our friendly manufacturers realised people were looking for 750W or 1200W power supplies when in reality not everything runs at the same time and a 300W PSU was all that was really needed....... result? 300W PSU badged as 1000w LOL

    unfortunately this is common across most products where as consumers we continue to think the bigger the number the better the product.

    unfortunately for our use with a heater we need all of that power all of the time (certainly at heat up) so we need a PSU that can supply the full demand reliably.

    the drop in voltage is a giveaway as a quality supply will maintain the voltage right up to its full capacity - and as you suggested have protection built in to stop it failing - if I over power my Meanwell it shuts down to protect itself.... guess what else the china suppliers leave out to save costs!

    its our own fault as consumers as we want everything at 1/4 of the price but while oriental manufacturing is great for many things they also will build ANYTHING down to the price the market wants with unsurprising results.

    working on your old heater at 300W for a 300x300 bed then the extrapolation to 400x400 bed would be 533W

    first off this is way to much for the Duet so if you have not killed it then you have got lucky - seriously look to correct that before your luck runs out. also that would explain the death of the first 2 supplies and if an optimistic rating on the 800W then a fair load to push.

    given this I would find out the heater details before risking any further damage



  • on the not blowing the fuse you need to understand the a fuse rating is not absolute. there are time factors involved that are not quoted as this falls into the design and gets complex.

    to give a rough idea (overly simplified and generalised before anyone pokes holes 🙂 )....

    a 15A fuse in general will blow at 30A ( I know weird huh)

    the point is protection against faults e.g. short circuit.

    in that case the current will theoretically rise to infinity and the fuse will blow "instantly"

    however if it truly blew at 15A then you would have all sorts of problems as a 15A motor for example draws on average 6x current to start and most devices have an inrush current slightly higher when turned on. so in reality there has to be some headroom.

    most times a fuse will handle up to about 2x the rated load for an extended duration but at over twice the load will always blow

    you cant rely of a fuse for protection from slight overload, thats not what they are there for.

    I hope this massive simplification makes some sense and helps going forward



  • @opentoideas said in Blowing power supplies:

    in the computing world you were meant to size the PSU based on generic max values for each item in the computer and the numbers often came out 3 or 4 times higher than you needed so our friendly manufacturers realised people were looking for 750W or 1200W power supplies when in reality not everything runs at the same time and a 300W PSU was all that was really needed....... result? 300W PSU badged as 1000w LOL

    Actually I'm familiar with this phenomenon. I used to work in the Animation and Games field, and we blew a lot of generic power supplies, rendering scenes and textures. I would buy the more reputable brands, for my workstations when I was contracting, because I didn't want the interruptions to my work. Unfortunately this lesson hasn't carried across. Ironically, the 350w supply, which lasted the longest of the three, actually is branded as WEHO, which turns out to be still around, and seemingly a bit more reputable.

    @opentoideas said in Blowing power supplies:

    I hope this massive simplification makes some sense and helps going forward

    Nono, this has all been very helpful. I'll be using an SSR from here on out, and if I'm lucky I'll be able to repair the 800w supply to use that as a dedicated brick for the heater, though I will take your advice first and figure out the rating for the heater. Another thing I found advised, is to lower the voltage on the power supply, to reduce it's total amps. My thinking was stupidly the other way around. More voltage, meant watts * higher voltage = more capacity. But the capacity of the power supply is fixed by it's hardware, and if you can reduce the amount of amps it has to push through, then it's not going to fail as often!



  • @mortarart

    glad it helped. on the SSR front be aware that they are different for AC and DC and again China made ones have a spec that is at best misleading and at worst a crappy mosfet unable to cope with 1% of the rated power!

    sorry but its another and in some ways worse minefield.

    I would think something like https://www.th3dstudio.com/product/high-amp-12v-24v-mosfet-heated-bed-or-hotend/ would be more than sufficient and little different in cost to possibly fake SSR's you could end up with

    not affiliated with them in any way was just the first one I found on google that would fit the bill.



  • yes lower voltage can help "tune" to what you have available.

    good old ohms law - V=IxR

    R is the resistance of the heater and is fixed (ish)

    I = V/R so for the same resistance lowering V lowers I

    at a guess your heater will have a resistance of about 1 ohm which makes the calculations simple as V~I

    so 24V will be about 24A or 576W

    20V will be about 20A or 400W

    please check though as your hater could be wildly different LOL



  • v1.03 doesn't have a bed heater fuse.



  • @elmoret LOL well it wouldn't blow then. well spotted

    very lucky not to damage the Duet in that case!



  • But there is a blade fuse on this board? And I bought it from the recent special?

    My other Duet doesn't have one. What's the fuse for?

    The SSR I have is from RobotDigg; and given their 350w supply performed better than the 'rated' 400w and 800w supplies, I'm a bit trusting. But I'll be doing extensive research before wiring all of this up again. And again; your advice is very helpful.

    I wonder how low you can set a 24v Power Supply? I was able to heat the bed on this printer in a surprisingly amount of time.



  • @mortarart its possible that they ran out of the 1.03 and you got lucky and have the 1.04

    Compare them here www.duet3d.com/DuetWifi

    As for power supplies it depends some have no adjustment and even if they do its only a couple of volts either way. Just be sure to check with a meter before connecting up.



  • Just checking the obvious... did you set the power supplies to the right voltage? Not putting 220v on a 110v supply or something?

    800w is an amount of power that you should notice if you’re actually drawing that. It’s a small space heater worth of heat going somewhere. I would expect smoking connectors and hot wiring and such to help you track down where the energy is going.

    Or, stop buying crap power supplies 😛



  • @rcarlyle said in Blowing power supplies:

    Just checking the obvious... did you set the power supplies to the right voltage? Not putting 220v on a 110v supply or something?

    Yes. Definitely set to 220v, on the two that are selectable. The other was sold as a 240v model.

    800w is an amount of power that you should notice if you’re actually drawing that. It’s a small space heater worth of heat going somewhere. I would expect smoking connectors and hot wiring and such to help you track down where the energy is going.

    Yeah, this is the bit that's weird for me. I was using 14AWG wire, and it was warm after powering the heater. But definitely not hot to the touch. 33c at the highest at the screw terminals via IR gun. And the heater wire, was cold. Room temperature.

    I really don't want to be plugging in new power supplies, to blow them, considering that.

    Or, stop buying crap power supplies 😛

    I'm now less convinced it was due to quality. They'd all lasted for a substantial amount of time on my other large printers.

    @opentoideas said in Blowing power supplies:

    @mortarart its possible that they ran out of the 1.03 and you got lucky and have the 1.04
    Compare them here www.duet3d.com/DuetWifi
    As for power supplies it depends some have no adjustment and even if they do its only a couple of volts either way. Just be sure to check with a meter before connecting up.

    Now I'm very confused. Mine has one large fuse. That image clearly has two fuses. The Duet 1.03 image has zero. I'm hoping I don't have some experimental model that is frying power supplies.



  • Definitely says 1.03 on the board.



  • well there are a few things. did you buy your duet from an authorised reseller or could it be clone? the clones are similar but can be slightly different eg. printing may say "based on v1.03 design"

    you also really need to measure the resistance of the heater. if you have for example a 12V heater then at 24V it will be 4x the power but in that case I think the Duet would have been unlikely to survive but without numbers its difficult as anything could be going on.

    do you have a multimeter?


 

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