K40 & Cohesion3D in Scotland - Power always 10mA

(Ian Johnston) #1

Machine: K40 Nano
Board: Cohesion3D
Firmware: (original as shipped)

Problem/ Question: Always getting full power. I ran the command line power test and the lines were the same depth.

I changed the PWM setting to 400 as directed and it was no different.

I then went to the LCD and did a power test there, LOW, 1/4 and FULL etc and the mA meter always goes to 10mA every time………so looks like I have no control over Power.

Attached are photos from my install.

NB. With the original Nano board the power setting via the pot works, as it does now for setting the max power.


(Nicholas Zastrow) #2

I’m having a similar issue over on this thread, Can't get full power from LPSU

There may be some solutions on it for you, but mine still aren’t resolved. I can control power, but it wont get any higher than 10-11ma.

(Ian Johnston) #3


Well, I have done some digging…just a little, but I think I see whats going on (brand new to Cohesion3D, but I’m an electronics guy)…but guess work at this time till I look into it properly.

The ‘pot’ on the K40 provides the 0-5vdc signal to control the laser power.
The Cohesion3D board doesn’t interface with this signal at all, all it does is enable or disable the laser power via the ‘L’ enable input on the K40 power supply. I’m assuming that the Cohesion board simply uses PWM to switch this input on/off and thus give control over laser power.

So, this relies on the K40 power supply being able to accept a PWM switching input whereas it seems to normally be used for a simple on/off control of the laser.

I am hoping the Cohesion guys get this sorted for me, I have spent a lot of $ on the Cohesion3D board only to see no control over power!.
Meanwhile I will scope the PWM signal tomorrow in order to see if the PWM signal is present and/or/if the K40 power supply is able to react to it. Maybe the logic levels are all over the place.

Note:From what I read, ‘L’ input is
Greater than 3Vdc = laser off
Less than 0.3Vdc = laser on

One thing I would like is a full I/O diagram for the Cohesion3D board…so far I can’t find one…only the Mini.


(Jamie Richards) #4

Does the test button on the laser give you more than 10mA?

(Ian Johnston) #5

No, gives me spot on 10mA. It’s how I set the current.


(Ian Johnston) #8

It’s late so I will try disconnect tomorrow…but since the k40 test switch and the pot are both interfaced to the power supply direct then the test switch will fire the laser and the knob will set the current.
Although, will be interesting to see what happens when hit test with ‘L’ disconnected (open circuit).

(Chris Leitch) #10

Before you go disconnecting things and all can I clarify the issue. It seems to be the opposite of @dustylavender
You can get full power however you cannot control the power to be lower through the LaserBoard (you didn’t specify which cohesion board you have but that’s what’s pictured). Am I correct in assuming that?
As an aside where in Scotland are you from?

(Ian Johnston) #11

Hi all,

I am in NE Scotland, just south of Aberdeen.

Just to clarify I have the Cohesion3D Laserboard, purchased days ago.
I get full power, but when I set up layers in Lightburn to vary the power the mA meter always shows 10mA, even if I set at power level of 3%.
I confirm the issue using the Cohesion3D LCD test fire utility, where no matter what I set (full, half, 1/4, low etc…) I always get 10mA output on the mA meter, and my cuts are always the same depth.
The K40 test button and pot are working, I can vary the max power on the pot and the depth of cut varies.

Some testing with the scope.
It’s a triangular shaped PWM being applied to ‘L’ (probably started life as square wave PWM though with the rising edge charging something).
Here are some photos below using the Cohesion3D LCD TEST FIRE utility, in order from FULL ON through to OFF. You can see the PWM in action (mark space ratio varying), and it can be confirmed when a DC voltmeter is put on it, the rms voltage does vary with power.

What is NOT good from an electronics standpoint is that the voltage of the waveform is varying also with the mark space ratio. There is a PWM signal present and therefore it should be doing something.
As the cut-off is <=0.3V (on) and >=3V (off) the waveform is in that area sometimes…but look closer it needs 3vdc or greater to switch off but the overall waveform is mostly well below 3vdc …so no much wonder I get 10mA all the time. Saying that, from my understanding the input on the K40 PSU is an opto diode, so it’s about current, not voltage…so all may be well…I’m not convinced by the 0.3/3.0 spec.
While I wait on Cohesion, it’s time to play by building an interface circuit to clean things up…

EDIT: Confirmed. When I disconnect the L input and put a 1k up to 5vdc externally the PWM output from the Cohesion3D board is much cleaner and the voltages higher (3vdc attained). Looks like the Cohesion3D board is not interfacing properly to the L input on the psu.

So, I wired it all back up and simply fitted a 1k resistor across L and the 5v to the pot (pull up) and I can now vary the power, just need to play with the resistor value to get varyance at all power levels.
What the resistor is doing is bringing the PWM waveform back crossing 3vdc as per the spec.

Other info, 24vdc supply out on the same 4-way header = 20vdc.
DC to pot = 5vdc

Pics before mod, full ‘on’ through to full ‘off’:-

(Chris Leitch) #13

It seems to be doing what it should hardware wise and it appears to be wired correctly.

I’d recommend reflashing the SD card and setting up the device in LightBurn again.

If the issues continue after that we’d need to have a look at your configuartion file in Smoothie and settings in LightBurn.

(Ian Johnston) #14

I disagree (based on my limited knowledge of K40)…See above, there is a hardware interface issue between the Cohesion3D board and my K40 psu, hence the addition of a 1k resistor in order to make it work.

I’d love to see the schematic for the Cohesion3D board specifically the PWM output…and also the K40 ‘L’ input…If I’ve ever got the K40 in bits again I’ll reverse engineer both.



Hi Ian,

This is correct.

Our default setup works for over 1,000 of our customers, without the addition of any other hardware, such as the resistor you mention.

In most cases it can, but we are able to set up other ways of controlling the laser from the C3D if your LPSU is unique and this is needed.

To make absolutely sure I have understood your progress so far, please confirm these details:

  1. You have attempted test firing by sending the G1 lines as demonstrated in our PWM Tuning Guide?
    2.You say that testing the board power control via these means (again please confirm that you tested exactly in the way stated in #1), no power change is noticed via mA meter on the machine?
  2. You are able to successfully change laser power by turning the pot?

Nice Rigol by the way.

(Ian Johnston) #16


  1. Test firing G1 lines with no mods to my setup results in full power cuts on all 10 tests.
  2. I am able to change laser power by turning the pot.

Per photo below, I have now removed the resistors and have added a small circuit to clean up the PWM signal by way of a 4050 six way hex non-inverting buffer. The L signal from the LaserBoard comes into the first buffer and from there is paralleled up to the other 5 inputs. The 5 outputs are paralleled up and drive the K40 L input on the PSU. There is also a 1k pull up resistor on the Cohesion3D output, and another 1k pull up resistor on the PSU L input.
This results in a cleaner more squared off PWM signal driving the K40 PSU.

One thing I notice is that with the Cohesion3D output isolated and with a simple 1k pull-up to +5vdc the signal is not that clean, i.e. there seems to be a capacitive load on the output? I would have expected a real, really clean PWM signal from your hardware when not driving anything other than a pull-up resistor. I would look into this if I were you.

Anyways, I have now achieved a good G1 test across the full range and am happy to leave it at that.


(system) closed #17

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