Understanding the K40 Digital Control Panel???

K40 PowerLED Control Panel

Newer versions of the K40 sometimes ship with a digital control panel. This panel retains the main power switch but the Calibrate (potentiometer), Laser Switch, and Laser test buttons are replaced by push buttons and a controller PCB.

Warning: this vintage K40 control panel does not ship with a laser current meter. Therefore it is not possible to know the actual operating power the laser is running at a particular digital setting. It is important to monitor the actual running power of the tube during operation to maximize the tube's life.
All tubes will eventually fail and are generally considered consumable but their life can be optimized. Tube life is proportional to the running current* so setting the machine to run at the minimum current required for the job and staying under the tube's rating will optimize its life.

Some users have made the argument that although running the laser at higher currents may shorten the life of the tube it also shortens job time, so it's worth it.

Add a Laser Current Meter

I recommend adding a current meter to your machine if you do not have one. The meter will assist you in running your laser at a current that is consistent with optimizing its life consistent with your expectations.

Add an analog meter to your K40

Most manufactures specify their tubes reliability @ 1500-2000 operating hrs running a working current of 20-22 ma. Check your tube's specifications, especially if you have replaced the stock tube.

The longest life I have seen specified is:

• 3000-4500 @ working current of 10-13 ma
• 40W tube with lower current longer life specified.

* There are other factors such as shelf life and operating temperature that cause a laser tube to fail. 

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For other information on the K40-S build use the  K40-S BUILD INDEX with schematics

Thanks to +Chris Hawkins for donating a panel for this exploration

Testing

A donated K40 PowerLED V3.0 was bench tested using a simple tester made from a USB adapter a few resistors, a voltmeter, and connectors. This tester provides 5V through a USB brick and listens to the IN and P pins of the panel.

This tester was connected to the panel and various tests were performed as described below.

Schematic

The schematic of the panel and the test board as it develops is here:

Interconnection with the Laser Power Supply (LPS)

You will note that this control panel connects both the P + & P- of the panel to the K+ signal on the LPS rather than the P signals on the LPS. Wow, now that's not confusing :(.

Also, note that nothing is connected to K- on the LPS and P+ & P- on the control panel are shorted together.

A LPS wired to a K40PowerLED Panel note the 4 white wires (from panel) and the black wires (from water sensor)

Operation:

The PowerLED [as best I can tell so far] utilizes an embedded processor to provide the functions of the panel. The main functions of the panel are to set the power level of the LPS, enable the laser to fire, and test fire the laser. The indicators and controls include:

• Laser power display
• Laser Switch
• Laser Test Switch
• Light Instruction
• 3X +/-Digit Controls: 10, 1,.1

Laser Power Display

Indicates the % of the power the LPS is set to. This display indicates the % of the LPS max power it is set to via the IN signals voltage on the interface. This display is operational as long as the Laser Switch is enabled.
The LPS power is controlled via an analog value of 0-5V on the IN pin of the LPS.

Laser Switch

This alternate action PB enables and disables voltage to the IN signal and the Laser Power Display. When this switch is OFF the display is off (except for decimal points). When the display is off the IN pin to the LPS is reduced to zero volts. When this switch is ON the IN signal will be a voltage proportional to the % displayed on the Laser Power Display.

Laser Test Switch

When this PB is pushed the +P/-P signal is grounded for as long as the PB is held. The Laser Test Switch grounds the +P/-P irrespective of the state of the Laser Switch. When this PB is pushed the "Light Instruction" LED illuminates.

Digit Controls

There are 3 sets of digit controls with a + and - control for each. These pushbuttons (PB's) allows the setting of the respective digit in the Laser Power Display.

Testing Data

The following spreadsheet contains:

• A table of the IN voltage vs the digital % power setting taken during the test
• The calculated error between actual and calculated power values for % settings.
• A math model (equation) for the input-output function of the panel

Model: 

VoltsOnLPSIN = 2.4562 * (%PanelSetting^2) + (7.6189 * %PanelSetting) - .2686

R^2 = 99.69% (good fit!)

Plotted Test Data & Error Calculations

5vdc Power Supply Loading

This panel draws about .2 amps from the stock K40 supply. That's about 20% of the safe 5vdc capacity. Note that some new machines have an LPS with the fan missing. I have to believe that the stock LPS is running on its limit (1 amp w/o fan) with these vintage machines.

NEW! Go figure, the IN output is a PWM signal

After being prompted to dynamically look at the IN pin of the panel by +Lukas Bachschwell who showed us a scope trace of the signal I was surprised to see that these panels acutely drive the IN pin of the LPS with a PWM signal!!! Thanks, +Lukas Bachschwell

PWM signal on IN

As you can see in my test setup the power was set to 50% and the IN voltage read 2.9. The readings here are average since the signal is actually a PWM square wave. My cheap DVM read 2.9 (which we now know is an average) 

"IN" Scope readings

Vavg: 2.98VDC. 

Duty: 58.8 [*seems like an 8.8% error from panel settings to actual?]

Freq: 24.55kHZ

*it would be useful to test and plot PWM linearity, i.e. panel settings vs actual PWM DF.

Why is the "IN" pin driven with a PWM? 

Keeping in mind that the digital panel wants 

• Digital control of the power by switch settings
• Power value display

Since most embedded controllers have PWM capable outputs it's rational to use PWM rather than a D/A approach. The controller can manage the display, switches, and the power this way.

Can we connect our controllers PWM output to "IN" on the LPS

Yes. An open-drain could drive the IN pin on the LPS but in my assessment, it does not gain you anything that I can fathom and has downsides.

Should we use the "IN" pin vs the "L" pin for power control?

No. Although this is an ok way to control power locally between the panel and the LPS, connecting a controller to the IN pin is a bad idea because:

• We need manual control of the power for testing without a controller connected. 
• We need a manual offset adjustment. This allows us to avoid having to make job setting* changes to account for laser depletion. The laser wears over time and uses so the power setting for any given job will change over time.
• The IN pin is not optically isolated from the HV supply but the L pin is. Using the L pin is close to the same as driving the IN pin but it's optically isolated.

*the software that is sending GCODE

Using both the "IN" and "L" pins give us good local power control as well as isolated power control from an external controller. 

 Assessment:

The display and controls worked as expected. See the graph for data on accuracy and linearity errors.
I felt the .1 digit was a bit overkill for a Laser Engraver as I think there are many variables in the process that are much larger than a .1% laser power change. I did not plot the tenths.

The % settings vs actual output voltage were pretty linear. The error increased to its largest value (10.15%) exactly at 50%  and then decreased until it reached 100%. I suspect this is related to the type of D/A that the controller employs (actually it's likely because the IN signal is actually a PWM).
From a practical perspective, the linearity error doesn't really matter. What is important is whether the panel can provide a full range of adjustable power from 0-100%. In normal use the operator adjusts the power to suit a particular job noting the best setting and as the laser "wears" that setting will change anyway.

During testing, I noticed that sometimes the % value would decrease when the + key was pressed and sometimes it did not recognize a keypress. More of a nuisance when setting the power but a safety issue if the Laser Switch behaves that way.

Safety

My biggest concern is how the laser is enabled. The embedded controller apparently reads the Laser Switch (the fact it is a momentary PB yet operates as an alternate action switch is the giveaway) and sends the set voltage to the LPS.
If the Laser Switch is alternated to the OFF position it puts 0V on the IN pin and this is the only way the laser is "turned off". Essentially the laser is enabled, just at 0 power. I guess that is not theoretically different than disabling the power with one of the other enable functions (K or P) but it just feels wrong to me. It's reducing the power NOT disabling the LPS.

Let's consider how this safety mechanism can fail:

1. A firmware bug does not turn off the IN voltage when the Laser Switch PB is alternated. Example: a bug turns off the display but leaves a voltage on "IN".
2. The firmware does not recognize an alternate push of the Laser Switch and does not turn off the IN voltage. The operator does not notice the display is ON and thinks the Laser Switch was pushed. I have noticed during testing that sometimes the +/- keys and the Laser Test Switch does not work. The switch seems to be intermittent or the firmware is missing the press. 
3. There is a failure on the control panel electronics that keeps the IN pin at a voltage.
4. Somehow a voltage gets on the "IN" pin from somewhere else in the machine's circuitry there is no secondary means of inhibiting a beam.
5. The display being ON or OFF is a confusing way to tell the operator the state of the "laser enabled" function. In most machines, the display turns on when the power turns on and stays on during operation. A specific indication that the laser is enabled is more appropriate. 
6. Example dangerous scenario: the laser is enabled and the operator pushes the Laser Enable button, the display turns off but the IN voltage is left at whatever value it was before. The operator thinks the panel being off means the machine is powered off. During troubleshooting inside the covers, the operator starts a job ...... the laser is now active.

I doubt that this kind of circuit strategy for making a laser safe would be considered "fail-safe" according to OSHA regulations.

1. Under the requirements of the ANSI Z 136 Standard, for embedded Class IIIB and Class IV lasers only, the interlocks are to be "fail-safe." This usually means that dual, redundant, electrical series-connected interlocks are associated with each removable panel.

Definition of a Fail-safe Interlock

An interlock where the failure of a single mechanical or electrical component of the interlock will cause the system to go into, or remain in, a safe mode.

Editorial Comments

Safety

I am not under any illusion that the K40 is SAFE as shipped although I do wonder what the FDA symbol on the K40PowerLED panel infers???? That said, I endeavor to follow OSHA and FDA recommendations whenever possible while doing conversions. This is to enhance my own safety and the safety of those mimicking my builds. To that end my machine has interlocks. These interlocks are not in series with any firmware and consist of nothing but wire, connectors, and mechanical switches. They all fail in the disabled mode...

For those using this panel; ensure that you install interlocks on all accessible covers using the P+ loop on the LPS. Put front and rear cover interlocks in series with the water-flow switch. 

Operation Improvement

The K40PowerLED panel would at first glance present itself as a high-tech implementation of K40 control. In my assessment, it does not provide much if any, advancement in the operation of the K40. In fact, the missing laser current meter makes an important operational measurement invisible to the user. Running overcurrent is the fasted way to shorten the life of a tube. Without a substantial improvement in functionality I am not willing to take the safety risk I think this panel may present.

The Good!

• You can see a digital display of the % of max power the laser power is set to.

The BAD!

• The buttons do not always respond or respond correctly
• The panel draws an additional 20% of the already loaded 5V supply
• No laser current meter leaves the operator blind to laser tube stress
• Less linear than a linear pot 

The UGLY

• Potential safety hazard in that the laser enables indication is confusing and the circuitry is not fail-safe.

Next steps

I do not think that further exploration into this panel will reveal more than the actual design that was used.

I am noodling if a safer, and more comprehensive control panel is wanted, needed, and cost-effective to improve the operability and safety of K40 conversions. Let me know your opinion in the comments..

Enjoy,

Comments and corrections expected;

Don

Will a 40 Watt LPS Drive a 60W Laser?

Under-powering a Laser Tube

Caution: these are preliminary musings .....

Recently +Chuck Comito presented me with a question regarding how his 60W laser would perform with his stock 40W Laser Power Supply (LPS).

I hadn't spent much time looking at LPS specs matched against Laser Tube specs. I just assumed that if you got a bigger tube you had to get a bigger supply. 

Although to get to full power you will need a LPS that will output enough current to match the tubes specification, you can under power your laser ... theoretically. 

That said, +Chuck Comito is running a 60W K40 with a 40W LPS somewhat successfully, still testing. 

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Why do we care?

Like most users when my tube goes bad I plan to upgrade to a higher power tube. We all know that our K40 really is a K30-35 in that these tubes don't actually output 40 watts.

30 watts is marginal for cutting thick materials and experience has shown us 50-60 watts is a better range for us makers.

The cost of upgrade would logically include the cost of a new tube and a higher wattage supply. This post suggests that you can upgrade your tube using your current K40 40 watt supply. That't a $100-150 cost savings and it allows us to use the stock supply to end of life.

The short theoretical analysis

1.) You can run your 60 watt laser with your 40 watt supply if you keep the current <=16MA
2.) Although at 16ma you will not reach 60 watts you will achieve a marked increase over what you had before at the same current.
3.) You can reach 60 watts if you run the supply at full capacity (22ma) but this will likely reduce its life.

My learning: you can upgrade your machine to 60 watts without getting a new supply unless you need run at the full 60 watts.

The long theoretical analysis...............

We know what the voltage vs current curve looks like, see the ref. at the bottom of this post.
I do not know what laser output vs current curve looks like when the tube is in the Normal Glow region [update: I added an attempt to model current vs optical power in the model below].
I assume that as long as the tube ionizes and the voltage is held at the operating voltage it will output light proportional to its current.

...We know that the needed trigger voltage relates to the length of the tube
...We know that the supply will current limit based on its pot setting up to close to 100% DF where is will be at max current.

Can you use a 40W supply to power a 60W laser?

This question of LPS wattage vs laser tube wattage is one I never did any research on. a I never compared the specs of supplies and laser tubes. So in the model below I captured some specs for comparison. I used similar supplies and tubes just different wattage's.
If you look at the specs for tubes and supplies in the model you will notice that the HV specs for a 60W tube can be met with a 40W supply! This means the supply will provide enough trigger to ionize and hold that ionization level.
There is a small mismatch between the current specs when using a 40w supply with a 60w tube. Note that the 40W supply must run at MAX current to provide enough current for the 60W tube, whereas the 60W supply provides enough current running below its max output i.e. its operating curent.

Assuming your tube and LPS have specs like those below, I would theorize that you can operate your 60W tube with a 40W LPS operating in the range <=16 ma and not >22.

The 40W LPS current will not likely get you to a full 60W. If things were linear, which they never are, you could estimate that with a safe LPS current of 16ma you will run 16/20 = 73% of rated power or 43.6W.

This may seem like a hugh loss of power but consider that your 40W was probably putting out 30W (as some high current). You now have a 13.6 watt (45%) increase over your previous setup. This means you have more power at a lower current. Not bad since you saved $100-200 in your 60W upgrade.

A Model

Using a purely specification based comparison of LPS's and laser tubes we can build a model to better understand how matching/mismatching LPS and laser tubes might perform.  Here is the analysis that led me to the conclusions I summarized above:

Enjoy;

Comments and corrections expected

Don

K40 LPS Silk Screen Error

K+/K- Reversed On Newer Machines

Reversed Silk Screen [K40 with digital panel]

Correct silkscreen [non digital panel]

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K+ & K- Silkscreen

On some new machines it seems that K+ and K- are reversed. This does not create a problem except when you are rewiring or testing. The silkscreen is printed wrong not the signal.

LPS Connection Function & Test

Here is a table that shows the voltage at these terminals when functions are asserted. 

Donate:

Please consider donating (button to the right of this post).
Your donations help fund additional research, tools and parts that I will return to the community as information.

For other information on the K40-S build use the  K40-S BUILD INDEX with schematics

Enjoy,
Comments and corrections expected...

Don

Enhanced K40 Temperature Monitoring

Updated K40 Temperature Sensing

I have one point of temperature monitoring  in my K40, water. I recently moved my K40 to the garage shop and decided to get rid of all the hacks and partially complete conversions. Temperature monitoring was one of those on the agenda to be completed. Mostly the temperature controller needed to be remounted but while I was at it I decided to add some more gauges .....

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Please consider donating (button to the right of this post).
Your donations help fund additional research, tools and parts that I will return to the community as information.

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More Temperature Monitoring

I already had a sensor in the water bucket connected to a controller that I had mounted lazily on the frame of the K40's kart.

This controller will alarm at an over temperature and since its relay is in the K40's interlock circuit it will shut off the laser.

This is the controller.  If its no longer available check for other similar ones. It needs to have an alarm function that opens the interlock when the temp setpoint it exceeded.

Gauge Mounting

The current controller/gauges were mounted on the upper part of the K40 control panel. It would have been nice if all these gauges were the same type. Then again their difference in appearance makes their varying purpose more obvious. In any case I went with function and cost over aesthetics.

One of my next projects will be to engrave acrylic labels for these. In the mean time their functions are:

• Upper left: Cabinet temp
• Upper right: Coolant temp
• Lower left: Laser tube temp
• Lower right: Laser power level

Gauge Installation

After removing the current control panel and then the hinged cover I used my nibbler to cut the holes for the new gauges. That nibbler is a must have "maker" tool!

Cabinet Temperature

This sensor is more of an experiment than anything. The sensor is located on the gantry near the air assist nozzle and is wired into the K40 interlock circuit. Its purpose it to sense a fire in the cabinet and turn the laser off. I have no way of testing or proving if this will work but I figured considering the damage a fire can do it was worth the effort and money! The idea is that a fire will melt the sensor and open the circuit. The controller will alarm if the sensor is disconnected and open the interlock circuit..... I hope.

I can imagine many cases where this may not work:

• The head is not near the fire
• The sensor shorts from the fire
• The cabinet self destructs before the sensor sees it

I used this 2 stage controller which is overkill but can be used if I ever build in a water cooler. 

I plan to set this alarm on this sensor to the lowest practical operating temperature I can.

Laser Tube Temperature

A sensor was tie wrapped to the laser tube's glass and routed to the gauge. This sensor monitors only and is not wired into the K40 interlock circuit. I hope to learn more about the temperature in the lasers cabinet and eventually plan to add air to that cavity. This gauge required a spacer since the panel sheet metal was to thin for the mounting tabs to firmly hold the meter in place.

As an FYI you can see the new water sensor and air assist connections at the bottom of the photo.

Laser Power Meter

The last meter in this set is the digital voltmeter that is connected to the LPS's control pot. I had to add a frame and filter to the naked small digital voltmeter so that it had a better mounting, is more readable and somewhat nicer looking.

I added gel filter material between the display and the face plate to get the display to be more readable.

Old Power Setting Meter ... dangling

Mounting and Wiring

Meter mounting. Note the adapters on the upper meters.

Enjoy, comments and suggestions expected :)

Don