K40 Upgrade from C3D Mini to Laserboard- What to Do with PWM cable?

Machine: Originally a K40 “No Ribbon and White JST Connector Power Supply” with C3D Mini

Board: Laserboard

Firmware: Smoothie

Problem/ Question: Due to clumsiness, I need to upgrade my K40 from a C3D Mini to a Laserboard. I bought the machine and the controller secondhand, and the original owner had completely removed the original control panel, and installed a custom one when he installed the Mini. The laser PWM wiring was plugged directly from the PSU to the C3D Mini. I’m at the tail end of the process of upgrading the entire system to a larger chassis, and managed to rip the microSD socket off of the Mini by dropping the back case panel while test fitting it. I was able to get it to work again briefly by resoldering the socket, but ultimately it failed. I purchased a Laserboard as a replacement, and 99% of it was drop-in.

However, I’m left with the 3-conductor PWM plug. What should I do with it? Is this what the ‘Alternate PWM Out’ pins are for? I understand this was originally connected to either a potentiometer or a digital panel- would I need to locate a compatible pot? I’ve seen several different ratings (anywhere from 1k to 10k) as the ‘recommended replacements’, but I’m not sure which would actually be correct.

Hi Octothorpe, sorry to hear you damaged the micro SD socket on the Mini, but hopefully we can help get the LaserBoard up and running for you. Have you already been through the Getting Started Guide? https://cohesion3d.com/start/ If not, that’s a great place to start.

It would be very helpful to see photos of your setup. The LPSU in the machine, the LaserBoard, and any wiring running between the two, as well as things connected to the custom control panel. Once we’ve got an idea of what you are working with we can offer some suggestions.

Power supply is pictured below - the wire I have questions about is disconnected and held in place right next to its socket.

This is the original location of the other end of the cable

Laserboard with everything else plugged in, and the cable with no spot to plug in

Custom control panel- the empty spot is for a temp probe I don’t currently have installed

aha, you need a potentiometer to replace that. 5V and GND to either side of the pot and IN to the wiper. This will be your hardware maximum power limit (hit the test button and rotate it until the ammeter reads 15mA).

The pot you need is here: https://cohesion3d.com/shop/peripherals/replacement-multi-turn-potentiometer/ (out of stock at the moment - if you can’t wait any 10K linear taper pot will work. 10K, 1K, 20K - whatever - the value of the pot is not super critical since all it’s doing is acting as a resistive voltage divider).

Good luck.

To add a bit of historical insight here, the “2 wire” control mechanism your machine originally had was how we did things with the first 100 C3D Mini’s we shipped back in January 2017.

We quickly found that having the potentiometer remain in place helps with having better power control as explained in the link below. It also alleviated concerns of “my test fire button doesn’t work!” and “my panel is now useless”.

So that “PWM Cable” stopped being a thing 3 years and over 1,000 board ago :slight_smile:

The potentiometer will be the best way to proceed. As an interim measure you can bridge IN to 5v on the LPSU but then you have to understand that 100% power in LightBurn will really be 100% power to the laser and this can quickly degrade the tube.

We are working on some LPSU docs, here is a relevant excerpt:

We also suggest you wire a potentiometer to G IN 5V to set the relative max power the laser will use when firing.

In the image above, the potentiometer has the wiper (yellow wire for IN) at the center. Note that not all potentiometers will have the wiper in the center, so check the specifications on the one you have to determine which is the wiper pin. Also, if turning the knob clockwise lowers instead of raises the current, you’ll need to switch the G and 5V wires.

When connected properly, the potentiometer will control the maximum laser power.

You can simply set it to a safe value like 80% of the tube’s maximum power (This could be 15-17mA for a 40w machine, but you must absolutely do your own testing to determine what the maximum power of your laser, tube, and power supply combo is, and then set a safe maximum value from there.

If you find that engraving is still too powerful when setting the power in software, you can use the potentiometer to lower the laser power for finer detail.

More detail about the potentiometer and power control is here: Cohesion3D PWM Control and Potentiometer vs ‘Digital Panel’ - Cohesion3D


Ah, that’d make sense. Fortunately, it looks like I had a spare 10k pot in a radio shack bits box, so I can wire that in and get by til I can get one of the multi turn ones. I think that’ll also solve some problems I had with the machine before I upgraded it- I couldn’t etch 2-color laser acrylic at all, for example (cutting barely worked, but etching melted and mixed the colors), and even cutting 3mm MDF ended up with edges a bit darker than they should have been. Will be a lot easier to dial in the power when it’s a dial instead of buried in machine settings in my software :stuck_out_tongue: Thanks for the assistance.


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