K40 power supply information

Here is some great information on the basics of the K40 power supply.
This information came from a great friend of the G+ days Scott Marshall who unfortunately is no longer with us.

There’s currently 2 kinds of power supplies for the K40 around. Inside they’re the exact same supply, but just to see if we’re paying attention, the K40 makers seem to like to change the connectors they solder into the PSU board.

To simplify things, I’ve taken to calling them “White” and Green” power supplies. This must make sense, because I’ve even seen sellers using that terminology.

The “Green” power supply is the most common, used in about 60% of the K40s currently in the wild.
It’s name comes from the Low Voltage output connector, the rightmost connector looking at the terminal side of the supply.
Many of these supplies have mounting provisions and the connector for a Fan, but are not so equipped. Adding one is a worthwhile and easy upgrade. The fan required is (annoyingly) a 24V 70mm model with 60mm bolt pattern. It’s mounted using standard self threading computer fan screws. A 5V fan could be used by simply connecting it to pins 2 (-) and 3 (+) of the output connector. ALL-TEK SYSTEMS has a Plug and play fan kit on our website.

The Green connector is a 4 pin type of connector invented by Phoenix Contact back in the 1980s if I recall correctly. They are now made my many companies, but are generically known as “Phoenix connectors” – along with at least 5 other types I know of, somewhat obscuring the clarity of the definition. Phoenix makes well over a thousand style connectors.

This type is actually a variation of a Phoenix terminal block, and offers several advantages over it’s white counterpart. It’s screw terminated, so anyone with a screwdriver can swap wires in and out, and it unplugs, allowing a power supply replacement to be ALMOST a plug in affair (but for the high voltage connection to the Laser Tube).

MOST times the switches and control pot connect to the Green supply using a 6 pin variant of the 4 pin Low Voltage Out connector. Likewise Line voltage comes in thru a 4 pin model identical to the Low Voltage out one (Yes they are interchangeable, be careful to make sure what you plug in where)

The 2nd most common supply based on my survey has a white rectangular Low Voltage power out connector. The connector on the PSU (the far right one) looks like 4 square pins sticking straight up with a white plastic locking tab. The tab curiously doesn’t engage anything, but seems more to discourage plugging the connector in wrong way, as the pins are off center.
These (“WHITE”) supplies are usually equipped with “XH” style connectors for control inputs. These are arranged as 2 2 pin connections (one for each switch) and 1 3 pin connector (for the level control).

The White Power Supply uses the exact same internals as the Green version. If you look under the PC Board… you will see the hole to line up with the Phoenix Style Green connectors. They can be easily interchanged by someone with the proper skills.

Th complicate things furthur than done already, the K40 folks have seen fit to use 7 pins here instead of the 6 pins found on the Green variation. They also saw fit to use totally different and meaningless nomenclature for the pin designations. The 1st connector is a 2 pin white, pin 1 is ground, and Pin 2 is Laser enable (The switch is usually labeled “Laser Switch”) Pin 1 is marked “P+” and pin 2 is marked “G” (at least THAT makes sense). Connector #2 pin 1 is Marked “K-” and it’s really Ground. Pin #2 is marked K+ and is actually the TEST input. These 2 pins connect to the Laser Test switch (Marked “Test Switch” on most K40s).
Connector #3 is the level control, and is Marked “G, IN, 5V” – this marking makes sense and it does what it says. “G” goes the pot CCW, “IN” goes to the pot wiper, and 5″V” goes to +5V.

The White control connectors are what’s commonly known as XH connectors (same story on the naming, but this time it’s Molex who invented them). They can be sourced thru ALL-TEK, Ebay and many hobby shops. This style connector is commonly used for charging balance connectors on RC aircraft Lithium Polymer batteries.

Like the Large white Low Voltage Out connector, XH terminals require special crimping tools and a supply of appropriate pins.
This makes things a little tougher on your average laser owner.
ALL-TEK carries custom built adapters so plugging one supply into a K40 originally equipped can be a plug and play operation.

Here’s the pinout of each. Notice that the White Supply typically uses 3 ground pins where the Green model shares 2.
You can interconnect them or daisy chain them in any order and it will all work fine. Under the board, they are all tied to the same ground point.

While on the Ground subject, there’s another odd thing done in the K40’s power supply connection arrangement. You’ll notice the AC Mains connection (the far left connector, ALWAYS Green (so far)) has 4 connections. The right 2 are AC power (220 or 120v), and pin 2 is earth ground, all as expected. It’s pin 1 you have to watch out for. It’s the DC return for 15000v tube power. It MUST be connected (properly!) or you will expose anything near the wire to 15kv at lethal current levels. The K40s tube power + goes straight to the tube, but the ground is connected from the tube to the Ma meter, then the Ma meter runs to Pin 1 of the mains connectior. It is indeed tied to earth (and system) ground. ALWAYS make sure it’s connected to the correct pin, and tight.

Here’s a “cheat sheet” on the connections for each PSU.

Green Supply

Connector #1
Pin #1 HIGH VOLTAGE Return (from ma meter)
Pin #2 AC Ground – goes to Chassis Ground and Mains Ground
Pin #3 AC IN #1
Pin #4 AC IN #2 (In US, either can be Neutral)

Connector #2
Pin #1 (G) “Laser Switch”, “Test Switch”
Pin #2 (P) “Laser Switch”
Pin #3 (L) “Test Switch”
Pin #4 (G) – Pot CCW Pin (turning knob ccw contacts this)
Pin #5 (IN) – Pot Center Pin (0-5V DC output level control)
Pin #6 (5V) – Pot CW Pin (turning knob cw contacts this)

Connector #3
Pin #1 24Vdc out – runs to motor controllers
Pin #2 Ground
Pin #3 5Vdc – Control power
Pin #4 “L” or “LO” Laser control. This turns ON the 15KV power when pulled to ZERO volts (Negative logic)

White Supply

Connector #1
Pin #1 HIGH VOLTAGE Return (from ma meter)
Pin #2 AC Ground – goes to Chassis Ground and Mains Ground
Pin #3 AC IN #1
Pin #4 AC IN #2 (In US, either can be Neutral)

Small 2 pin ‘XH’
Connector #2

Pin #1 (P+) – “Laser Switch”
Pin #2 (G) – “Laser Switch”

Small 2 pin ‘XH’
Connector #3

Pin #1 (K-){really Gnd} – “Test Switch”
Pin #2 (K+) – “Test Switch”

Small 3 pin ‘XH’
Connector #4

Pin #1 (G) – Pot CCW Pin (turning knob ccw contacts this)
Pin #2 (IN) – 0-5V level control to High Voltage Power Supply
Pin #3 (5V) – Pot CW Pin (turning knob cw contacts this)


Great stuff.
I had wondered about the difference between the cathode earth connection and the other earth connections, this clears that up.
I’ve put those illustrations into the documentation I’m preparing for our Maker Space.


… in addition and FYI all of this is on my blog and more :slight_smile: Including schematics…

It is important to connect the cathode to the ground on the leftmost connector for proper operation however I do not think that wire is at 15,000 volts nor high current.

I have looked at this point many times with my scope.

The laser tube is a gas discharge device and the current is actually small [look at what the meter in the cathode circuit reads]. The worst I would expect from that point is a low current electrostatic discharge.

The reason the other end is lethal is because it is 20,000V @ 30ma.


@donkjr Thanks for the clarification. The information is verbatim from Scott Marshall. I would think that getting between the tube and ground at any point would not be a good idea! :astonished:

The original post of looking for PS info does have a link back to your blog.

If that point was really at 15,000V it would arc to the pin next to it!

Cheap tesla coil!

I am not the big power electrician, so I ask here if the standard blue connector power supply has a pot to limit the actual output power?

I am not sure I know what …


The LPS I have worked on have green and/or white connectors?


Oh my god, now I have also become color blind … :astonished:
No and sorry for the confusion, the contacts are of course green.

In the stock setup, there are two ways power is usually controlled:

  1. A pot that controls the LPS output power by providing 0-5V to the IN pin.
  2. A digital panel that provides a PWM signal on the IN pin

The actual power control schema may differ depending on how a non-stock controller is connected to the LPS.
Looks like you have a grbl controller but I cannot tell how it is connected to the LPS.

If you need more details/help let me know

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Many thanks Don, mainly I asked because I am a little jealous of the power supplies with the small hole in the side (and a pot on the inside), which can be used very easily to adjust the output power.

Hi Bernd,

I am setting up my laser with a Grbl board and was wondering if you are using PWM for raster engraving or are you just using “on/off”. Would you be able to detail how you have connected you board to the LSP?

Thanks in advanced,


Hi Ed, I use the “default settings” from Awesome Tech and have never touched it because it works fine as it is. On the AwesomeTech page there is some nice information which also deals with the PWM settings.
The pure physical connection is plug and play, Nano out Mini-Gerbil in.
The original potentiometer from K40 is “disconnected” when you change the board, i.e. you can only adjust your powersettings via LightBurn. Be aware that your max. power does not exceed the approx. 60% in LightBurn, unless you have the option to throttle your power supply. (it was not possible with mine)
And of course, an analog ammeter is needed if you do not have it in advance.

by the way, $28=5 (1,5KHz),- Super Gebil.com says $ 28 = 4 (1KHz) I have no idea why there is this difference.

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