How to tune the power supply to the tube?

The procedure is : check the manufacturers rating for your tube (my Reci W4 is calibrated at 30mA with a maximum running current of 28mA, but run at 26mA for longevity)
measure your output to the tube using a digital multimeter
Set your desired maximum by adjusting the psu potentiometer while at 100% power

Then, you will have the full range of 0-255 (or 0-100%) available to you without worrying that you might be over-driving your tube.

Here’s an instructable:

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This will work but you would be much better off just installing an analog milliammeter (0-30 or 0-50 mA). With the milliammeter installed you can not only adjust your power supply but you can also monitor the current going through the tube. It can give an early indication of tube failure.


Most people have better access to a digital multimeter than to a 0-30mA analogue ammeter.

Standard word of advice that most new users get is to install a mA meter in their laser if it did not come equipped with one. It is a cheap investment that will make things better for not overdriving tubes, being able to set the current on a K40 more accurately, and as Rich mentioned, helps to diagnose a failing tube or power supply. Well worth the little bit of effort to install one if your laser did not come equipped with one. Just my opinion.


I live in the third world. Where do I go to buy these fabled milliamp meters?

You, and the previous poster, are ‘correcting’ something that wasn’t wrong.

He wanted to know the procedure for checking/setting his laser. I gave him one, one that was correct and would give him the result he required.

Yes, fitting a meter is a good idea - it should be fitted at the factory - but that isn’t the only solution, nor the most accurate when you are dealing with very low numbers.

How accurate are sweep milliamp meters compared to my digital multimeter?

Seems like there’s a lot of willingness to wave willies around here. I think I’ll give this forum a swerve and leave it to you more argumentative types.

I did not think I was being argumentative, all I did was make a statement of something I thought was in the OP’s best interest. I was not correcting anything, just adding more information into the discussion. I even made sure to state that it was just my opinion. I am sorry if you took this to be argumentative, that was not the intention. As to your question on where to buy one, they are available on Aliexpress or Ebay. Most people have access to these online purchasing sites. They can be shipped just about anywhere in the world.

I was under the impression he wanted to solve his problem immediately, not in six weeks.

No need to fight guys, I appreciate all responses. I can sort out what’s applicable for me.

I also live in the third world (Sweden). I ordered one from China for about $4. This is for long term permanent installation.

For the short term, I’ll do tests with my digital multimeter.

LOL @ third world Sweden.

I’ll believe you when you can’t buy a metric bolt or some isopropyl or a digital multimeter. Heck, it would be nice sometimes to be able to buy some ibuprofen or a pie…

It’s rare that I can buy anything locally - I even import my wire, as the only stuff available locally is extension-cord 3-wire flex and house wiring solid-core.

Not a great place to try and run an engineering business.

LOL @ third world Sweden.

Well, first world is the countries that aligned with the west after WW2, second world is the countries that aligned with the east, which leaves neutral Sweden (and Switzerland) among the un-aligned third world.

I had not encountered that definition of “third world” before so I googled it. Learn something new every day.

Third World

your tube should have a max. ma reading to use

Don’t know about you guys, but my new Bodor came with a 80W RECI tube, and AFTER I installed a 30ma meter, I noticed that at the bottom of the controller LCD screen, it actually gives me the HV, tube current, and voltage of the controller signal to the power supply. Didn’t need to cut a hole in my new machine after all. The controller is a BC6442S.

The 6442 can read the info from some power supplies, especially Reci supplies. Doees not work with all LPSU’s

For those who have access to Amazon, the analog mA meters like most of us have installed are available there. Less than $10 in the U.S. Delivery from 1 to 5 days depending on how much you want to pay. Here are a couple of examples from the many listings. Make sure to fully research the specifications for these before you buy because I just did a quick search.

50 mA:

30 mA:


The voltage drop occurs across the tube, so as in a voltage divider, the voltage is very low on the ground side of the tube. That’s why the ground wire is not insulated like the high-voltage wire.