Ammeter in my K40

Hey everyone,

I recently bought a K40 (G3020) and want to add an Ammeter. I bought this one from amazon :

Before I dive in and tackle installing it - has anyone else installed simular? Can anyone confirm it will work ?

Many thanks


Welcome Julien!

Sorry to be the bearer of bad news, but you really want an analog ammeter, not a digital one. Fortunately, not a very expensive problem…

An analog ammeter, for units with only digital display. The digital display is not accurate. An analog ammeter may not be essential before your first test, but is a strongly recommended add-on. @donkjr describes how to add an analog ammeter.

The K40: Getting Started category has lots of useful information in it, if you haven’t seen it yet! Highly recommended!

Ah ok - wont work ? I actually ordered an analogue one that’s taking forever to come - but would have preferred digital for the aesthetics - never mind - thanks for answering.

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Well, it at least won’t be accurate enough to be useful. So “won’t work” as in “won’t do the job” not “won’t have any apparent function” :slight_smile:

No thanks to COVID-19 for all the disruption to everyday life. Slow shipping for our hobbies is probably the most minor of the inconveniences we’re encountering, I guess. :frowning:


Just out of curiosity, why wouldn’t a digital ammeter be accurate?

Two separate issues:

  • That particular digital ammeter’s max precision is 10ma and thus on its own insufficient for this task, and usually precision in cheap electronics exceeds accuracy.
  • In general, digital ammeters use the voltage drop across a shunt to calculate current. This voltage drop is small, so they have to amplify it which both amplifies Johnson-Nyquist noise in the shunt and introduces additional noise from the amplifier stages. Then you have contend with both the sample period and sample rate across a PWM signal. It is easy to have bad aliasing in different ways. This rabbit hole goes down to the Shannon limit and square waves being approximated by an infinite series of transcendentals and could be a long article all on its own.

Gotcha, thanks for the explanation. :slightly_smiling_face:


IMO the biggest problem is that it is hard to see and interpret short analog signals on a digital meter.

For example you will not be able to easily see the peak noise in the cathode current when the LPS arcs.


I installed a small digital voltmeter across the power pot (center pin, wiper, to ground). It gives a reading of the power setting before the laser is enabled. That, along with the analog ammeter makes the power setting easy and accurate.

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Yes, been using that approach for some time :slight_smile:

An analog 30mA is fine for K40.
Percentage power indicator is nothing more that an indicator. mA is the way to judge how the laser is used.