Question! My power adjustment is very sensitive is this common? I only have use of about 1/8 of the rotation of the potentiometer before it gets into dangerous levels is this common?
There could either be a defect in your potentiometer, or it’s of the wrong value. But more likely the on board trimpot in the LPSU wasn’t properly adjusted from the factory. I found there are typically 2 trimpots in the power supply, one for maximum mA and one for tweaking the gain on the level POT. I do not recommend experimenting unless you are confident in your abilities to work with high voltage devices.
Can I add a resistor to the center tap to increase my range?
I replaced mine with a 10 turn linear pot. Gives the ability to change the power level by very small accurate amounts.
Yeah, you could try adding a 5k resistor to the wiper.
That would give you a bit more adjustment…
@Alex_Krause I’ve been having similar issue. Only about 1/4 of the pot’s rotation is usable & it’s very touchy.
That is why I went to a linear 10 turn POT. Incredibly fine adjustments can be made.
I will have to open up the panel tomorrow and have a look, can’t remember off the top of my head.
Like Anthony, replaced mine with a 10 turn pot, it’s so much better I wish I did it earlier!
5K Ohm
Yup, mine is a 5K linear taper as well. Make sure that it is linear taper or you will not get even adjustments through the range.
Sorry about resurrecting an old thread. I just got a K40 and am experiencing this same issue, i.e.only about 1/8th of a turn of adjustment on the pot.
Wondering if there was an adjustment to the power supply that could fix this or if it’s easier to just punt and get a 10 turn 5k linear pot?
@Alex_Krause There are a few questions above so here goes:
The only adjustment sensitivity problems I have seen are bad pots. The pots they ship are junk.
The replacement pot is a 5K linear. A 10 turn pot is ideal. I also add a small DVM from wiper to gnd so that I can have a digital value of its position.
There are no adjustments in the supply that will improve its resolution that is exclusively done by the pot.
Info on replacement is on my blog.
@Brent_Dowell please see above answer from Don. He has some amazing information about the k40 on his website
Interesting. The original pot That came with it was jumpy all over the board. I checked it with my multimeter and the resistance would bounce from 0 to a value as it was twisted. I did replace it with a new one that when I tested appeared to work better. I’ve got a 10 turn on order and when that gets here will go ahead and replace it it with that.
I had 3 of the pots in inventory and so tried them all on the machine tonight with one of those little led volt meters attached. All of the pots had different response rates. One wouldnt even go below 2 volts, so I guess they are all pretty much trash.
The nice thing is taht the lasers response in milliamps matched the volt meter pretty well.
Looking foward to getting my 10 turn pot on Wednesday, but at least I know whats going on now. Thanks again.
@Brent_Dowell you can also test the pots stand alone using the ohms scale on the DVM. Connect from one side to the wiper and turn the pot while watching the meter.
Strange that all three pots are bad ?
Well the 10 turn got here today and I installed it and It works much better.
I did test the other pots and it just seems like while they work, they also just kind of jump around depending on when you are touching the knob. Maybe they’d be fine in a less critical use.
The 10 turn I find I only use about 3 turns max, but at least it’s usable over that entire range, and it does go from 0 - 5 volts at 10 turns. I’m really only turning my k40 up to about 1.5 volts which gets translated to about 12-14 milliamps.
But this is very usable now. Thanks so much for your help!