Connecting 2 milliampmeters to k40 laser

I want to put 2 MA meters on my k 40 1 that is a 0-30 range and 1-10

I want to see with greater resolution how many MA i am using especially in the low power range since I cut mostly thin paper with my k40

Will the MA meters read correctly if there are 2 of them?

I have 2 really old American made in usa gauges, I think they have a more classy look and I like to help support my fellow local eBay-er.

You can’t hook them up in parallel like that.

You could hook them up in series if the higher current won’t break the lower current ammeter.

However, you definitely don’t want either ammeter to break, because then you’ll get high voltage across the broken ammeter, and using two instead of one roughly doubles the chances of failure…


Alternate Suggestions:

  1. Use a switch to select one or the other meter.
  2. The tube ionizes at around 4ma and is pretty unstable at that point. You may find that measuring low currents 0 - 6 ma is not very repeatable.
  3. Use a DVM to measure current as they autorange. I don’t like using DVM’s as they do not show fast changes in current like the analog meter.
  4. Measure the tube current indirectly. The tube current is controlled by the “IN” signal on the LPS which is 0-5V. Put a cheap DVM on that signal. With that setup you can get an accurate voltage value that somewhat corresponds to the current setting (its just in volts not ma). Adjust the current for results that you want and note the voltage on IN. Then you can return to that voltage by resetting the pot to that same voltage.

Note: its a good idea to characterize your tube. Create a table that shows what current the tube runs at for a given voltage on IN. You can use this table as a reference as your tube wears and its characteristics change.

Some detail on panel upgrade here:


Thanks for taking such time to make a response. I have been doing my best to search the forum and expand my knowledge on my own but there are ah-ha moments that you can only learn by being in it for so long and eventually saying stuff like I’ve been doing this for 30 year and I still learn something new here and there.

Use a switch to select one or the other meter.
I love it, So in the future, I don’t have to ask other ppl, how do I go about teaching myself what switch amperage to choose on this application?

Mike, that danger is still present with 1 correct? What does that mean, you could be running a project and open your electronics side and zap yourself with pure laser hi voltage signal by touching the leads on the meter on accident while lazing or hitting test fire?

To determine what current rating a switch for a given application you just need to find out what the current in the circuit is and then select a switch that handles that current or larger. Specs for switches are provided by the manuf and most times stamped on the switch.

The typical current you are going to see at the cathode of the tube is 0-30 ma way less than 1 amp.
Most any switch can handle that level of current.

I would use a spdt switch so one or the other meter is always connected.

Keep in mind that “pegging” [exceeding max reading] on a meter for an extended length of time could [depends on meter] burn out the movement.

If you use a switch, you’ll want a “make before break” switch. Otherwise, if you switch it while the laser is energized, you will momentarily have up to 20,000V across the switch.

You really don’t want that.

I strongly suggest that this is complexity that is unlikely to reward you with value…

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The cathode should not have 20,000 volts on it.

It is one end of a gas discharge tube.

When not ionized it is an OPEN and there is no electrical connection to the anode.
When it ionizes the current flows in the serially placed meter circuit leaving a small drop on the meter and the rest across the tube.

I have never measured the voltage on the meter but I know it is small as I have looked at it with a scope.

That said, I agree that two meters will not likely provide much benefit.

Honestly the massive display on the 30ma reader probably is accurate enough for notes anyway.

Feeling dumb but back to eBay it goes.

I was really getting excited about getting a ratheon on off on and doing the project.

I honestly had really repeatable results with my snowflake cutting out with the digital potentimeter already present in my mini. I will be more than happy with just the 30ma DC.

kind of unrelated,
My panel has the membrane from around the switches cut out.

previous owner stated, during test firing the laser the rebound of the plastic membrane would occasionally keep the laser on.

I never hear anything about that from the community at all.

You can use a digital 50 mA ammeter instead. They only update at about 3 FPS, but they are more accurate than analog ones.

If you go with a digital one, I highly recommend to use a 4-digit one which uses a fixed 2.2 format (0.00 - 99.99).

I currently got a 5-digit one which always uses all the digits which is super annoying since anything two places after the decimal point is just noise. E.g. it displays 1.23XX (two noise digits) instead of just 1.23 which is what it can fairly accurately measure.

Fun fact: The generic digital ammeter modules are the same size as the generic thermometer modules used in K40s. So, if you want to use something else to measure the temperature (e.g. if your chiller does that), you can put an ammeter into that cutout.

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