Adding a second PSU to K40

As suggested by Don, I am making a new post for a new topic: adding a second PSU to my stock(ish) K40. I recently ordered a Mean Well RT-85D.

RT-85 Spec Sheet PDF (86.5 KB)

I plan to attempt what Keen discusses in their K40 enhancements post: K40 enchancements

Don supplied a wiring diagram to me (provided by ttaago): New analog panel on FL-K40D

At this time the only thing that I have added to the electronics of my stock K40 is an interlock on the cutting compartment. I will use this post as a build log and a place to ask relevant questions.


Question for all regarding connectors: since I will be moving wiring around, is it ok to add pin end ferrules to all the wires? If two wires go into a single terminal, can they be put into one pin end ferrules and crimped together?

Also: a fair number of my wires are unlabeled (no awg markings). Is there any way to know for sure what the awg is (aside from guess work)?

Yes putting two wires together into the same ferrule is fine. Don’t worry too much about gauge; just use the smallest ferrule that fits. You can tell gauge by measuring, and there are tools for this, but for the ferrules all the matters is fitting in here, as far as I know.


I have mounted my auxiliary PSU in the electronics compartment. I have not done any wiring on it yet.


Well, I have started to identify what each wire does. Looking at the spaghetti hurts my brain. I am currently trying to figure out what to move to the new PSU and what to leave on the old PSU. I haven’t done this before…

Right now, I am labelling every wire where it connects to the PSU so that I know what I am working on without having to check drawings over and over again. The last time I touched a wiring schematic was when I got my tech class ham license in 2005. Trying to figure it all out again.

This is the beginning of wisdom… :relaxed:

I say this as someone who has, just a few times, gone on autopilot and released the magic smoke that had previously been enabling the electronics to work. But really, going methodically and labeling each end will do wonders.

The wiring diagram you linked to isn’t complete; it doesn’t show (for example) the 24V from the LPS to the controller board to power the stepper motors.

Here’s the theory:

  • The LPS controls the laser, so everything that controls the laser including safety interlocks (case, water) connect directly to it.
  • The LPS measures the voltage on the potentiometer, so use the LPS 5V supply for the high side of the potentiometer
  • This LPS has a 24V power supply; you want to use that to supply the stepper motors, so that is the motor voltage that goes to the controller board Edit: As you pointed out, your new power supply has 24V and you can use that to supply the stepper motors. Note that doing this will make it easy to switch to a different LPS later if you ever need to; not all of them supply 24V so this is more flexible.
  • Use the 5V from the new power supply for everything else that needs 5V.
  • Use the 12V from the new power supply for anything that needs 12V, like LEDs
  • All the ⏚ should connect directly to the single ground stud; you don’t want to daisy-chain from ⏚ to ⏚. This is “FG” on the LPS in the diagram, often called “ground”, but also “PE” short for “Protective Earth”. This is separate from the DC GND connections that are a reference against which the DC voltages are supplied and measured.

What size wire you need depends on current. It’s easy to get by with two sizes:

  • Use larger gauge wire (18AWG should be fine¹) to carry power:
    • N
    • L
    • GND between power supplies and power consumers
    • 5V on PS to board
    • 12V
    • 24V
  • You can use smaller gauge wire (24AWG should be fine) for signals:
    • TH
    • TL
    • WP
    • G reference for interlock / etc switches, potentiometer
    • 5V on LPS (it’s only a potentiometer reference)
    • IN (potentiometer wiper)

None of this touches the high-voltage wires from the LPS to the tube.

If @donkjr says I’m wrong about any of this, believe him not me.

¹18AWG is fine as long as the fuse in the IEC socket is 10A or less, I believe.


@mcdanlj thank you for the great information! I have 12, 18, 22, 24, 26 awg wire. 12 and 22-26 are solid core.

I didn’t realize that running the ground in she’s isn’t allowed. I just assumed it was like electrical sockets.

The description posted by @keen seems to suggest that everything 5V and 24V can be moved to the new PSU.

Dumb question: does LSU stand for Laser Supply Unit? I think that I missed the initialism somewhere along the line.

It’s really loops you want to avoid, and connecting everything to the one ground stud is the easy way to not get that wrong.

LPS? Laser Power Supply that came with the unit.

PSU is Power Supply Unit, and is the new 5/24V supply you bought.

I didn’t see one by @keen in that post; thinking of @ttaago? He has the LPS 5V supply connected to the potentiometer. It really shouldn’t matter in theory if the digital grounds are connected, but the LPS is measuring the difference between its idea of 5V and its idea of ground, so I’d definitely stick with it’s 5V for the potentiometer.


Keen’s post is at the link below. Yes, I mean LPS. I wrote that late, sorry for the wrong word.

Ah, that PSU. I was looking at the description for the one that @ttaago found (in the other post you linked to) that had 12V and 5V; and missed that you got the one with 24V, 12V, and 5V. Agreed you can run everything off that, except that again I would use the 5V reference on the LPS for the potentiometer and nothing else. I don’t think it really matters, but the LPS should be measuring relative to its own idea of 5V.


Still nervous about moving wiring around so I just continued labeling today.

Working off diagrams, trying to get wires labeled. Don’t worry, I am not going to run the ground in series. I did that before your message. I am going to move the new PSU ground to the ground but once I have 18AWG wire. That should be in the next couple days.

It looks like the AC input comes through the main and then to the outlets at the back. This is the bit that actually scares me. Want to ensure I don’t light up my K40 with flames or arcing instead of LEDs.

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It’s almost certainly OK. I don’t see how you would create a loop there. @donkjr would know better than me. I’m a programmer who dabbles with electrons; he’s an expert.

That’s right. Typical for accessory outlets. They, along with everything else in the K40, should be protected by a fuse in the main C14 plug that connects your K40 to the outlet. That fuse being 10A or less is why I said 18AWG — most C14 sockets aren’t rated for more than 10A anyway; many are rated for less.


My electronics work is limited to radios (basically modding them) for ham use. I only recently wired an outlet myself after my father in law couldn’t figure out the issue. I am enjoying this new stuff but an taking my time rather than rushing into the work before I truly understand it.


My advice:

Before rewiring-

Draw-up the wiring as it is. Include wire colors and endpoint designations.
You can use a schematic tool or just pencil and paper. I often use graph paper while I am wire tracing and then move that to a schematic capture tool at completion.
Most K40 owners skip this step but I have found it to be a good investment over the long haul. Especially when you make a mod and then forget what you did. In actuality, if you are tracing and marking the wiring the extra time to write it down isn’t significant.


Then mark on that same drawing what you are going to change in some visible way.
Make the wiring changes one subsystem at a time and test before moving on to the next.


If users that come to this forum had documented the wiring in their machine before and after mods most problems we encounter could be solved in 1/2 the time just by sharing the schematic.


If and when you get these drawings done then I will be happy to review them before you start the wiring.


I often use Digikey schematic capture for simple stuff, stuff that does not need electrical or PCB modeling. This tool is crude and limited but free and simpler to learn than most EE tools. It’s also easy to embed diagrams in blog posts. It’s better to use a tool like Eagle but that takes more of an investment in time and $$. I am sure there are other open-source and free tools.

My original machine with simple mods:

This is my modified machine (probably out of date);


Looking back at your post.
What are you trying to accomplish with the new triple output power supply?

@ttaago added a supply because his replacement LPS did not have enough 5V and no 24V to run the controller. Your situation is different?

In any case here is a drawing to help you decide how to add the DC supply to power the nano.

I separated the wiring into Sections A, B, C.
The dotted lines represent what I think is current wiring
The solid lines represent wiring changes
The X on a wire represent were a wire is removed
I did this from what I could glean from the pictures of your machine and the PS specs.
Your machine may vary from this so adjust accordingly.

Please check everything carefully. Let me know if anything looks strange or wrong.


  • The LPS-L to Nano-LO wire is left alone
  • The LPS ends of the nano 5V & 24 wire is disconnected at the LPS and connected as shown to the new supply. If the wires are not long enough I would solder-splice/shrinkwrap longer wires onto them.
  • The Gnd that goes between the LPS and Nano is left alone. A wire is added from the LPS-G screw terminal to the DC supplies Com connection.


  • The AC, AC, & Gnd wires from the LPS ac connector [left connector] are daisy chained over to the DC supplies AC connections as shown.
  • Check to make sure that the GND on the LPS is connected to the frame.


  • Connect the DCPS-FG to a screw in the frame. I would put a stud in the floor between your DC supply and the LPS so you have easy access from both.
  • The 24v, 5V are already connected from previous wiring
  • I do not know where you want the 12V connected
  • Check with an ohm meter that the DC supplies FG and Com terminals are connected together internally

Wiring and testing:

  • I would wire up B first and before connecting anything to the DC supply verify the output voltages are on the terminals you expect and the correct value.
    EDIT: The DC supply specs refer to voltages as both V1-3 and also Channel 1-3. I assumed channels and V are the same so you should check on the actual supply.
  • Then wire A and C.
  • With the DC power plug to the nano unplugged from the nano verify the correct voltages on the correct pins.
  • At this point there should be nothing connected to:
    • LPS-24V
    • LPS-5V
  • Then plug in the nano and power up and test everything


@donkjr Thank you for the incredible information and work that you provided for me. I truly appreciate it. I worked on my drawing of the unit as it arrived to me today. The only change that I have made is adding the mA meter.

As to what I am attempting to achieve: not a lot yet. I want to take some of the load off of the LPS and move it onto the new PSU. From my reading, this will help since LPS is at its limit with stock wiring. I want to be able to add lighting, gauges, whatever other electronics I need to, in the future.

I attempted to figure out Scheme-It but that is going to take me a bit to figure out. I drew (by hand) a schematic of the system. I apologize that the colors of the wires are so close. I have not changed wire colors at all since I received the machine. The wires from the bottom of the E-Stop are Red and run to AC N/L. But, they are also spliced into outlet 1 (right most outlet near Main) with Blue wires.


Looking good.
One way to make the drawing more legible is to make two drawings one that just shows AC and one that shows all non AC stuff.

What is the box on the left labeled Pot.?

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Good idea! That is the digital potentiometer (if that is what it is called). I am working in scheme-it now trying to get it figured out. I am not entirely sure how to make the main connector there (the C14/15).

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Share the schem-it drawing and I will help if you want.

Not sure what you mean by “digital” potentiometer? Picture?
Do you have a digital control panel?

Sorry for the delay. Not feeling great today. I wasn’t sure what the control panel is called. It is a digital control panel (not a digital potentiometer). I added the mA meter. I think that scheme-it at least looks right even if my choice of symbols is not perfect.