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Sunday, April 9, 2017

Adding an Analog Milliamp Meter to a K40

Instructions for Adding an Analog Meter to a K40

Some vintages of K40's now have digital meters and pots. Some users find then sufficient and convenient others have found that having more information about the position of the pot and an analog representation of the laser tubes current to be advantageous.  

While you are adding this meter to your K40 you may also consider adding a high resolution pot and/or a pot position indicator: 
Thanks, to +Bob Buechler for testing out these instructions, doing the drawings and reporting on the results in this post.


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

Installing the Analog Meter

Summary of the installation.

The analog meter is going to be installed in series with the wire that routes from the lasers cathode to the -L connection on the Laser Power Supply (LPS). 
[the -L connection is the leftmost pin on the leftmost connector on most LPS's]

Here is a simplified wiring drawing of the installation, courtesy of +Bob Buechler:


You will need the following materials

  • New Meter Note: this meter is larger than used in the stock K40
  • Wire, as needed for your installation
  • Heat shrink tubing, as needed for your installation
  • Ring tongues, as needed for the meter you choose
  • Terminal Pin  and crimper

You will need the following tools:

  • Wire cutters
  • Soldering iron & or crimp-er to match the terminals you are using
  • Pliers or small wrench for tightening the meter nuts

Meter Mechanical Installation

  • Pick an appropriate place to install your meter, cut hole(s) as needed and install

Meter Wiring

  1. 1. Pre-check:
    1.  Verify that with the "Laser Switch" enabled, when you push the test function the laser fires.
    2. Power down and unplug the mains from your machine
  2. Find the wire that connects to the cathode (the end the laser light comes out of) end of the laser tube. Often its a black or green wire. 
  3. Trace that wire to its other end which should be connected to ground (L-) at the LPS. Note: never not ground the cathode or the (-) side of the meter directly to the frame. The lasers current must return to the LPS itself on pin -L. 
  4. Find the lasers ground port on the LPS. Its usually called L-, its the leftmost connection on the leftmost connector. Do not confuse this with the L that is on the rightmost connector with the DC voltages.
    L- on LPS with all green connectors

    L- on LPS with green and white connectors
  5. Remove the existing wire from L- and verify with an ohmmeter that that the LPS pin (L-) is connected to the FG pin on the LPS and that both of those pins (L- & FG) are connected to the frame of the machine. There should be close to 0 resistance to ground (frame) on these pins. Note: this is a good time to test that there is 0 ohms to GND at the frame pin on the mains connector.
  6. We want to reroute the existing wiring (that went from the tubes cathode to the LPS) to the (+) side of the meter. Do such by pulling the L- end of the wire that connects to LPS out of the harness enough to reroute it to the + side of the newly installed meters terminals. Note: Unless absolutely necessary do NOT disconnect the wire from the cathode as that is difficult to replace. When this step is finished the wire that previously was routed from the lasers cathode to the L- is now rerouted to the + side of the meter.
  7. Connect the wire of step #6 to the meters + terminal with an appropriate terminating terminal. Usually the meter has threaded studs with nuts and washers. I recommend using a ring tongue terminal soldered to the wire. See parts list above.
  8. It is common for the meters terminals not to be marked. If not marked start by connecting this wire to the left terminal of the meter (looking from the back), its a guess!
  9. Get a new piece of wire that is long enough to route back to the LPS (L-) pin from the (-) of the meter. 
    1. Use the same size or larger wire (yes it matters) and the same color if possible (colors do not matter but will be easier to trace later). 
    2. Connect this wire from the meters (-) terminal to the L- of the LPS.  
    3. Terminate the meter end with a ring tongue like step #7. For the LPS end use a crimped pin of the correct wire size. If you do not have the ability to crimp a pin at least strip back and tin the wire with solder. Insert the wire into the L- terminal and tighten securely.
  10. After insuring that you have not shorted anything with shards of wire etc prepare to return power to the machine. As a rule I vacuum my machine in the area I have been working with a crevice tool. Be careful not to create a static charge.
  11. Return power to the machine with your hand on the switch in case of smoke. No smoke? Then proceed.
  12. Turn the power adjustment pot (or digital control) to about 1/3 or less of its range. In case the meter is in backward we do not want to stress it. 
  13. To test the meter enable the laser [Laser Switch] and then push the [Laser Switch] while watching for movement in the meters needle. The meter should read the lasers current and you are done.
  14. If no movement is noticeable on the meter these things could be wrong:
    1. An error in the wiring, recheck using the steps above.
    2. The meter is in backward. Swap the wires on the back of the meter and return to step 10.
    3. The laser is not firing, check to see if the tube ionizes?
    4. If you cannot get it to work post a picture of all of the above connections and wiring with my G+ address +Don Kleinschnitz in the Laser Engraving 

Links on G+

"Ideally you want to cut the wire in a way that the meter can physically be placed in series with it leaving the cathode and the LPS end terminations alone. Just putting ring tongues where you cut it to connect the the meter. i.e The meter is placed in series with the current wire.

If the wire is not long enough cut the cathode wire long enough to reach the meter and put a ring tongue on it and connect it to + of the meter. Get another wire that is long enough to reach the LPS and put a ring tongue on it connected to the - side of the meter. Put a pin terminal on the LPS end.

Don't have a pin terminal and crimp-er? Alternatively tin the wire with a liberal amount of solder and insert and screw that into the LPS terminal.

If you want to keep the pin terminal you can splice it and a section of wire to a longer piece just insure you solder properly and cover it with shrink wrap."

Soldering Ring Tongues

I solder these type terminals because I have had problems with crimps corroding and/or vibrating loose. Theory is that if you crimp correctly this will not happen however soldering insures it does not.

Prepare the wire:

Strip the wire back far enough so that the bare end inserts fully into the barrel to its end


Cut and slide over the wire a piece of heat-shrink that will cover the barrel of the terminal after soldering. The plastic cover may or may not come off or loose from heating. Most of the time I remove the plastic before starting.


Put the ring tongue on a heat restive surface or in a clamp. I lay mine flat on a piece of 600 grit sandpaper (the surface is heat resistant). Insert the wire through the barrel. Press with the iron on the ring tongue side of the terminal and heat while applying solder until the barrel of the terminal fills up. Depending on the size it may take a fair amount of heat. Don't put so much solder on it that it flows around the ring as that will impede attaching it to screws.


Clean flux from the terminal and slide the heat shrink up over the barrel and shrink it over the plastic if its still there. Sometimes the plastic falls off or needs to be cut off.

Some say this is overkill because its a pain to do but I have never had one fail over years of use.

Enjoy and comment!


  1. Hey Don,
    You rock. Just converted my K40 from digital to analog and got everything to work, laser firing. My one question, I just hooked up the meter and also installed a digital one, and the digital meter gives me what I believe is a correct number, but when I click test fire, the analog meter just shoots up to the highest reading and drops back down. Is this normal? When I run an actual cut will it go to its correct power and stay there?

  2. Tony,
    The analog meter should not peg.
    Are you sure you have the right meter? (not a lower current range one)
    What do you mean "you installed a digital one". A digital amp meter?
    Connect with me at:

  3. This comment has been removed by a blog administrator.

    Paragraph 3: There is a "never not" statement in the grounding description. I think the "not" should be removed to avoid confusion and error.

  5. Hi, laser is firing when I turn the pot, but not when I press the fire button, any ideas? Have checked wires go from switch to L & G

  6. Hi Don thanks for doing this a great help. Can you tell me what happens to the on /off switch next to the light switch? I have installed a laser on off switch and every thing is working apart from my steppers they will not move and I have a red light flashing on the Mini Gerbil board, many thanks