Should I make an external PCB with flyback diodes? - Page 1
Forum section: electronics / general

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 02:40:28 pm
content: I bought a DM556 Stepper driver from Ebay (99% sure it was actually just a DM542 with DM556 printed on the cover) and then I bought 2 DM860H drivers from Aliexpress.However, it turned out that the "DM556" and one DM860H had no flyback diodes whatsoever.The other DM860H had flyback diodes connecting to each motor lead and VCC, but not GND (there should be 8 diodes, not 4).I decided to make an external flyback diode board and wire it to the main filter cap of the DM860 (it contains its own rectifier) or the power input of the "DM556".I used these diodes:https://www.taydaelectronics.com/1n4937-fast-recovery-diode-1a-600v.htmlHowever, for various reasons, some of which are still not fully understood, I ended up breaking/damaging all the drivers and shorting most of the diodes in the board (presumably due to overcurrent).I have now purchased a new DM860H from Aliexpress that almost certainly has no flyback diodes.Should I bother to make a flyback diode board for it?Will it end disastrously like it did the first time?The DM860 has a 57VAC supply.The "DM556" has a 38.5VDC supply.I was thinking of these diodes:https://www.taydaelectronics.com/mbr20100ct-schottky-diode-20a-60v.htmlhttps://www.taydaelectronics.com/mbr20100ct-schottky-diode-20a-100v.htmlhttps://www.taydaelectronics.com/f16c20c-dual-fast-recovery-diodes-200v-16a.html

author: Kean
author_rank: Unknown
timestamp: March 04, 2025, 03:24:43 pm
content: I am not sure why you are playing around with adding exteral flyback diodes.I've never added any external diodes to my stepper drivers, and assumed they would be integrated. I generally have used Gecko drivers though, not cheap clones of the Leadshine ones. Certainly there may be clones around where the manufacturer has cut cost and eliminated components, but also they may have swapped out parts that are more robust and don't need external protection as per the original design.Did you actually test the drivers for proper operation before modifying and damaging them?Now they are damaged, have you tried to identify the fault and replace components?Are you sure you are using the right components, and wiring them up correctly?I saw your previous posts, but you never gave any component details or diagrams so no one could really help you. I recall you complaining about buying knockoffs, but then bought more cheap ones from AliExpress instead of paying more to buy from a reputable supplier.In the case of your DM556, it very likely shares a common PCB with the DM542. They are very similar designs just using different spec components to meet different requirements & price brackets. I believe the DM556 came out first, and the DM542 was likely a later revision to reduce cost. As far as I know Leadshine no longer make any of these, so you are buying old stock or clones.In my experience, stepper drivers rarely fail unless abused or old. The majority of failures I've seen are due to people unplugging motors while power is still applied to the driver, or poor/damaged wiring/connectors leading to similar problems. Older units could have electrolytic capacitors that have dried out, but MOSFETs failing is usually caused by miswiring or cable faults.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 03:40:27 pm
content: Quote from: Kean on March 04, 2025, 03:24:43 pmThe sense resistors on both DM860H drivers exploded.I replaced one with a 30 ohm resistor, but it turns out the sense resistor is actually 0.03 ohm!But when I try to power the driver with the wrong sense resistor, it just eats power and does not turn on.Possibly a tin whisker (I use lead-free solder).There is likely a pseudo-short somewhere.No motor was connected, yet it draws like infinite current.Perhaps it can be repaired.The fault LED on the DM556 is always on, regardless of connections or settings.Every time I draw a schematic, people says it's unreadable.Now I don't draw schematics anymore.

author: Kean
author_rank: Unknown
timestamp: March 04, 2025, 03:46:02 pm
content: If the sense resistors were damaged, then you must have had a dead short somewhere. And not just a tin whisker, that would surely burn out way quicker than the 30mΩ sense resistor.The MOSFETs may now also be damaged as well, but they are reasonably easy to check as they almost always fail short.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 03:48:19 pm
content: I tested the "DM556" and the DM860H with the half flyback diodes; they both worked fine.The DM556 could not work fast, but worked fine.The DM860 was overpowered and amazing!The stepper motors I am driving are 4.6A synchronous/stepping motors.The diodes were soldered with a large copper pad and lots of solder as a heatsink.The external flyback diodes on my PCB did not overheat or emit any smoke.The drivers are all 0% genuine.

author: langwadt
author_rank: Super Contributor
timestamp: March 04, 2025, 03:51:53 pm
content: Quote from: Langdon on March 04, 2025, 03:48:19 pmbut they worked just fine until you decide to randomly add stuff ?

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 03:59:59 pm
content: Here's the full, longwinded story:Once upon a time, a high school student in Ontario took shop class and computer technology(TEJ3M0) in grade 11.One day during a period of boredom in shop class, he came across an old TorCAM CNC machine.The next day, during computer technology, he asked his teacher to try to fix it, and brought it into the comptech room to clean out the dust that was almost certainly within.After much trial-and-error, he tries out the CNC machine with the existing 30-year-old stepper drivers and LinuxCNC (Mach 3 decided to not work and cause the computer to "hard reboot").However, to much confusion and dismay, he finds that the machine does not work.The PC sent out pulses, the PXDX-122 breakout board relayed them faithfully, but the motors did not spin!After surmounting hardware failure (the PC was obtianed from the IT guy; it was initially destined to be recycled), he manages to confirm that the original drivers are busted.To be continued...

author: Kean
author_rank: Unknown
timestamp: March 04, 2025, 04:01:45 pm
content: Quote from: Langdon on March 04, 2025, 03:48:19 pmNot sure if that is a typo or not. If not, then why would you buy another one from AlilExpress?Can you show a photo of your additions? Something isn't adding up.In any case, I would recommend just wiring the motor to the driver next time as per the documentation without anything extra.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 04:38:36 pm
content: He originally tries to buy Lin Engineering drivers from Digi-Key, but realises they are far too expensive.Wary of Aliexpress and unaware that a 4.6A stepper needs 6.5A peak current, not 5.6A, he ends up buying a DM556 from Ebay for about $20.After 2 months, it arrives.He opens it to find a PCB marked "DM542" and a bunch of SMD TO268? transistors with a thermal pad between them and the metal heatsink to which the PCB is affixed.It works!However, speed is poor at best.Highly unimpressed with Ebay, he buys 2 DM860H drivers from Aliexpress.After about a week, they arrive, but smell funny and look kinda cheap.He disassembles them, washes the heatsink, and uses good quality brass standoffs/spacers instead of just a lame screws to hold the PCB to the heatsink.He discovers that one of the drivers has pads for flyback diodes to be connected, but no flyback diodes!Upon further inspection, he finds the other driver only has half the flyback diodes it should have!He gets some 1N4937 fast diodes he has lying around and solders them to a proto board, bending the leads over a large area and apply lots of solder for maximum heat conduction and sinking using his FX-888D station with a big "flat head" tip and lead-free solder.The diodes are arrangd in 2 isolated groups of 8 diodes, each with 4 groups of 2 diodes connecting to both power supply buses.Each group of 8 has 2 0.33µF 100V ployesrter film cap across the 2 power supply lines as a filter cpa to absorb some of the inductive spikes.He then solders the sockets for 2 of the motors to the board and then solders Teflon wires to the same spot as the socket leads as connections to the drivers.After testing the board with his multimeter, he concludes that it is good and brings the remaining DM860H to school too.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 04:55:09 pm
content: He connects one half of the flyback diode board to the power plug of the DM556 driver, which ran just 2 days earlier.And connects the motor connection plug to the Teflon wires, ensuring that polarity is correct.When he turns on the power, the driver's fault light goes on.After turning off the power, he unplugs the motor connector, the control connector, and attemps to reconnct the driver o the motor directly, but the fault LED stays on no matter what he does.The next day, he does the same with the DM860H driver with preexisitng half flyback diodes, but accidentally reverses the power wires so the driver's power goes right through the flyback diodes, blowing the fuse.He then connects the other half of the flyback diode board to the DM860H with no preexisiting flyback diodes with the driver on, blowing that fuse as well.After bypassing the fuses with 18AWG wire (and later a 5A breaker) at home, he goes to school the next day and tries again, with the power wires correctly connected.The DM860 with half flyback diodes is connected to the flyback diode board and the stepper motor.When the power is turned on, the sense resistor explodes violently.Being an idiot, he then tries to plug in the other DM860 without any flyback diode board or motor attached; the sense resistor blows up and much of the solder mask covering the PCB traces leaving the rectifier flakes off (presumably due to heat).Brings all the drives home to attempt repair and makes a 30Ω reisitor out of 20 12Ω resistor is hybrid series-parallel configuration.After soldering the resistor to the pads where the sense resistor was once connected, he attempts to plug the driver into the output of the rectifier board he originally made for the DM556.The driver does not light up, and the rectifier and transformer (a 40VA, ~24VAC transformer for the in-floor heating thermostats) overheat.There's a short somewhere...It then becomes apparent that an R030 resistor has a reisitance of 0.03Ω not 30Ω!

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 05:43:01 pm
content: While he is at home...After further testing via applying voltage and making sure voltage is not conducted wrong, he discovers that one of the diodes had a short across it, he initially assumes it's a tin whisker his multimeter didn't pick up. (which was a dumb assumption in retrospect)Luckily, he has a spare 24V 40VA transformer wired to an electrical box with a plug for temporary use.However, finding that neither the rectifier board nor the transformer is powerful enough due to the short, he decides to combine 6 24V and 12V switching power supplies to carry some of the load, using the flyback diode board to ensure power does not flow backwards into the power supplies and blow them up or otherwise damage them.However, he notices that, for some reason, power is flowing backwards across the diode into one of the power supplies, even though the diode worked perfectly just minutes prior.Finally, the output cap of a Delta 12V power supply connected via the diode board to aid the rectifier/transformer blows!Another diode has shorted, alowing reverse voltage to explode the cap (which is only rated for 16V for a 12V power supply that can be adjusted to as high as 14V).He puts the power supplies in an IKEA SAMLA bin and gives up (temporarily).