Should I just suck it up and buy a good stepper driver? - Page 1
Forum section: electronics / general

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 06:09:18 pm
content: I am repairing a CNC machine at my high school.It has toroidal trasformer with 3 isolated 28.5VAC windings.It is a 3 axis XYZ machine with each axis controlled by a 4.6A stepping/synchronous motor.It is connected to a PC running LinuxCNC via a parallelport and PMDX-122 breakout board.The machine works, but the original 30-year-old stepper drivers are all totally broken.I bought some DM860H and DM556 drivers off Ebay and Aliexpress, respectively; they worked, but have no flyback diodes.I accidentally broke all of them.I ordered 1 new DM860 from Aliexpress.Should I just suck it up and buy good, genuine drives for the remaining axes?All the axes?I can make a rectifier, if needed.What brands are good?I am reluctant to spend like $300 ($435 in Canada, where both the machine and I reside).Canada's potential future retaliatory tarrifs on USA in response to Trump's potential future tarrifs on Canada might affect stuff too.The computer works fine, so does everything else, all I need is stepper drivers at this point (I need 1-2 THK LM guides for th Z axis, but that's a problem for future me).

author: langwadt
author_rank: Super Contributor
timestamp: March 04, 2025, 08:04:51 pm
content: Quote from: Langdon on March 04, 2025, 06:09:18 pmso the drivers where not the problem, you messing with them was ...I've had zero issues with DM860Hs/DM556s

author: Langdon
author_rank: Frequent Contributor
timestamp: March 04, 2025, 09:44:00 pm
content: Yeah, but I only messed with them because they were missing crucial components (a stepper driver needs flyback diodes, a complete set).And they should have output short protection and recovery.They should not just die forever due to one mistake!

author: thm_w
author_rank: Super Contributor
timestamp: March 04, 2025, 10:38:15 pm
content: Critical components according to you. Next time just use the product as is and don't mess with it.They should be using FETs so flyback diodes aren't needed:https://www.cnczone.com/forums/stepper-motors-drives/404966-mosfet-replacements-dm860h-stepper-drivers.htmlIf you can try to keep the same topics in one thread:https://www.eevblog.com/forum/mechanical-engineering/should-i-make-an-external-pcb-with-flayback-diodes-a-good-idea/https://www.eevblog.com/forum/mechanical-engineering/why-does-this-knockoff-dm860h-have-only-half-the-flyback-diodes/

author: Kean
author_rank: Unknown
timestamp: March 05, 2025, 01:36:42 pm
content: Quote from: Langdon on March 04, 2025, 06:09:18 pmI am not sure if I should ask... but what does that statement mean? I hope it doesn't mean you've been powering the drivers off 28.5VAC.And please stop opening new threads for the same topic. It creates clutter, and disconnects readers from prior details and discussion.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 05, 2025, 01:59:56 pm
content: The DM860H drivers have an internal rectifier.I already made a rectifier to power the DM556(6x680uF 63V caps that I got for free and a rectifier bridge+heatsink from an iMac PSU)

author: Kean
author_rank: Unknown
timestamp: March 05, 2025, 02:36:18 pm
content: Quote from: Langdon on March 05, 2025, 01:59:56 pmAh, OK. I've played with the DM542 but never touched the DM860H that I can recall.I've personally stuck to Gecko drivers for all of my serious CNC builds.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 05, 2025, 06:20:10 pm
content: 2 problems with gecko drives:1. They may be subject to Canada's 25% tarrifs (not yet, as far as I can see, but they may be in future)2. They accept up to 80VDC, but I can only build a rectifier for about 40VDC or about 80VDC.Isn't running them at exactly their max input voltage a bad idea?I can configur the 3 independent 28.5VAC windings of my transformer to supply 28.5VAC, 57VAC, and/or 85.5VAC.These rectify to about 40VDC, 80VDC, and 120VDC

author: thm_w
author_rank: Super Contributor
timestamp: March 05, 2025, 10:06:02 pm
content: Was the machine using the three 28V windings as one for each axis? If so just leave it as is, no need to make rectifiers, or rewire or screw with anything else.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 06, 2025, 01:09:55 am
content: This was the original config:3 28.5VAC (mislabled as 40VAC) windings in parallel feeding an old, highly inefficient (and slightly unstable) full-wave rectifier chip, and one huge 13000µF 40V cap.(They used a 40V cap for a 38.5VDC supply!)Only 1.5V of headroom!It had a max surge rating of 50V, but there was no surge protector on the input.A lightning strike could have caused a catastrophic explosion!The original stepper drivers were basically space heaters.All this, along with the spindle/router (no idea what originally existed), was originally switched by a single 15A 250V/ 125V coil relay (like the relay swithed and was powered by line voltage).However, the relay contacts actually got welded together by a spark, likely due to the huge inrush current of the toroidal tranformer.

author: eugene
author_rank: Frequent Contributor
timestamp: April 11, 2025, 03:52:51 pm
content: Quote from: Langdon on March 04, 2025, 06:09:18 pmIf you added flyback diodes then that's probably what broke them. Bipolar steppers (4 wires) are typically driven with H-bridge drivers that reverse the polarity back and forth during each cycle. There's no place for flyback diodes. If you did install them across a motor winding, which way would you orient them? They would necessarily short the driver during one output polarity or the other.Even if your motors happen to be unipolar (6 wires) you'll likely want to run them as bipolar (using just 4 wires) as commonly available drivers like the ones you mentioned will perform best as bipolar. The center tap of unipolar steppers goes back to the days when complex driver electronics was more expensive.The DM860H is a reasonably good and reliable stepper driver when used as intended. Use it unmodified. Just make sure to set it up properly, especially paying attention to the current limit setting.

author: Langdon
author_rank: Frequent Contributor
timestamp: April 13, 2025, 06:17:21 pm
content: The flyback diodes directed voltage above VCC to VCC and voltage below GND to GND.I think the diodes shorted b/c they were too small.Overcurrent apparently causes diodes to fail short, even if they ae prevented from overheating.

author: Infraviolet
author_rank: Super Contributor
timestamp: April 13, 2025, 07:49:07 pm
content: "but I can only build a rectifier for about 40VDC or about 80VDC"If you're doing a significant rebuild of a machine and not trying to keep it exactly to the original design, why not get new power supplies for the stepper drivers? You can buy laptop-power-brick-style supplies able to do 36V or 48V with output currents of up to 6A and more. Say, one per stepper driver. Or you can buy metal shielded power supply units designed for placing inside the insulating case of a machine able to do 36V or 48V and up to tens of amps. Meanwell is a fairly well regarded brand for either.

author: Langdon
author_rank: Frequent Contributor
timestamp: April 14, 2025, 01:46:15 pm
content: It has a nice toroidal transformer.It can supply a lot of power.