I bought one of these K40 lasers a few years ago and since then I’ve had to replace the power supply as well as the potentiometer. I think I need to replace the potentiometer, again, because the power output seems to stop at about 8-10mA. Anything above that number, and I get nothing. The laser fires but the power levels seem to be very small.
Question. What numbers should I see when testing the pot? I saw a video there a guy was showing as high as 2.5v. I’m not getting anything higher than 1.5v.
Could this be a bad pot or do you think it’s the power supply? I just replaced the power supply last weekend.
The laser current can draw smaller than expected current for multiple reasons:
Bad tube
Bad LPS
Bad pot.
The pot should be connected this way:
One end of the pot connected to 5V on the LPS
The other end of the pot is connected to the ground on the LPS.
The wiper should be connected to IN on the LPS.
When the machine is powered on the wiper should read 0-5V as the pot is turned up.
If not check that the 5V side of the pot = 5V and the GND side is GND.
You also check your pot by disconnecting it from the circuit and connecting a DVM on ohms from either end and the wiper.
The resistance should change from 0-[Max resistance]. Max resistance in whatever size pot you have, usually 1 or 5K.
Doesn’t the test button on the LPS fire the laser at full power? if it does and the power output is above what you’re seeing via POT adjustment then I would think that would indicate a problem on the POT.
The wire colors for the pot are black, white and light brown / tan. Which one is ground? And, should it be connected to my case grounding wire?
Back story.
I haven’t used the machine in several months. Last weekend I went to use it when I noticed the lack of power on start up. I checked my water level. Checked the grounding line that goes from the outlet on the back of the machine to the case, that was loose, so I tightened it up. Nothing.
The last time this happened, I ended up replacing the POT. Is it normal for these to go back so quickly?
I know it’s hard diagnosing someone’s problem over the internet but I appreciate it none the less.
So annoying that sellers have promoted cheaper “digital” displays over still-very-inexpensive analog displays. It’s a source of endless frustration.
The digital meters only show what power might be going through the tube; they aren’t actually ammeters. Analog meters show what power is going through the tube. A digital meter can’t tell you anything about the health of your tube, but an analog ammeter can. In a K40, a 30ma ammeter is right.
I’ll run down to my local car parts store to see if they have one. I’m pretty sure I’ve seen them on the shelves. Besides, I was wondering if it could be the tube as well. Thanks.
I also went ahead an ordered a new POT. It should be in tomorrow.
Sorry, I didn’t actually measure anything to get the 8-10mA reading. That number is based on what the ammeter is saying. Anything above that number and the laser fires but no beam shows up to burn the wood. I checked the mirrors and everything is aligned correctly.
Okay, used my digital multimeter and got 3.8v for about a second with the POT turned up all the way. That was about the average number. Sometimes a little less sometimes a little more. But only for a second. If I held the fire button down for more than a second, the voltage would drop back down to minimum voltage. All the while, the ammeter is pegged while holding down the test fire button on the control panel.
The voltage on the wiper (IN) should not change (assuming you are not moving the pot) when you push that test fire button.
Do the same but this time measure the voltage on the 5V terminal which should be to the right of were you just measured. It should stay at 5V when you push the testfire…
@donkjr it sounds a bit like what I was seeing on my machine when I was seeing my power drop when I fired the laser but more exaggerated. 3.8V is much higher than the 2.6V I set on my K40 for 18ma max power.
One way to test that POT would be to disconnect the connector at the LPS and measure the resistance across the POT. Across the Orange and Blue wires should be 1K ohm for a 1K POT. Then, move one meter wire to the white terminal and you should be able to measure 0 ohms to 1K ohm as you turn the knob from end to end.
During operation, there must have 5VDC on the LPS(across the POT) when measured across the Orange and Blue wires.
Sorry, the visible burning of the laser seems to stop around 8-10mA according to the ammeter. Anything above that number on the ammeter and the burning stops. When I said the ammeter is pegged, I can get it to peg by turning it up all the way but nothing comes out of the laser. The laser is still firing (tube is lighting up) but nothing burns above 8-10mA. In fact, nothing comes out of the laser itself above those numbers. I put a piece of paper directly in front of the laser tube.
I just got the new POT installed. Nothing changed.
I’m going to post another video. That will probably better explain what’s happening.
Yes! It’s losing power. Or, at least it seems that way.
In the video link you’ll notice at full power, the laser doesn’t burn a hole in the paper directly in front of the laser tube but, at a lower setting it does.
so the symptoms are that you turn up the POT and the output power drops. We got that.
What you should be wanting to know is what is causing this. ie is the power supply failing, is the tube failing or is the power control to the LPS failing. Once you know what is causing these symptoms you’ll have a better idea knowing what needs replacing.
So you replaced the POT with a new 1K Ohm POT and saw no change. What seems odd is that you say the max voltage on the IN(white wire) pin to the LPS is 3.8V. With the new POT can you make 2 measurements?
with laser enabled but not firing, measure the DC voltage across the gnd and 5V at the LPS and then the same wires at the POT?
with the laser enabled but not firing, measure the DC voltage across the gnd and IN at the LPS and the same wires at the POT moving the POT from 100% CCW and 100% CW?
