Variable power supply modification - Page 1
Forum section: electronics / projects

author: bwilson13087
author_rank: Newbie
timestamp: February 16, 2018, 03:59:17 pm
content: Hey guys and gals,I'm working with an Eventek KPS305D 0-30V 0-5A DC power supply. So far, it does what I need it to do. My only problem is that the course voltage adjustment is too sensitive. The adjustment knob is just a simple 10k ohm single turn rotary potentiometer. At first I was thinking I could replace it with something like a 10 turn pot, but I don't think it will fit without a lot of other modifications. Now I'm thinking I could replace the 10k pot with a 5k. I'm guessing this would just change it to a 0-15V supply with half the sensitivity, which I'm fine with.Are my assumptions correct? If it really needs the full 10k I figured I could also add a 5k resistor (or something close) in series with the pot. Any input or other ideas would be greatly appreciated.Thanks.EDIT:Just to be more clear, I use this supply as a convenient way to change my input voltage for various things. But having the voltage jump a couple volts from barely touching the knob is a problem. Also there is a fine adjust with a 1K pot, but it is too fine for my liking. Overall I either need the course adj to be less sensitive or the fine adj to be more. As it stands now, the full range of the course adjust is 0-30V in just 3/4 of a turn (small movement = too big of change) and the fine adjust is around 0-0.3V in 3/4 of a turn (big movement = too small of change). I hope this makes sense.

author: Paul Price
author_rank: Super Contributor
timestamp: February 16, 2018, 06:17:07 pm
content: You could add a fine resolution adj. by connecting a second pot in series with the hi-end of the existing pot. You could try using a 1K pot connected as a rheostat for much greater than X 10 resolution. The idea here in use is to center the added fine-adj. pot and use the 10k pot to get near to the desired voltage then trim the output voltage to what you want using the fine-adj. pot.A schematic of the power supply showing how the existing pot is connected would be helpful for me to offer other fine-adj. modifications.The only thing you must consider is finding a knob you like and a place to drill a hole to mount the new fine-adj. pot.Alternatively, there are 3-turn 10K adj. pots you can buy that might just fit as an exact mechan. replacement. You probably will need a linear-taper pot.There are so many more other ways to accomplish what you want as well, including digital ways using a rotary encoder and a MCU and a few resistors and much more complexity.Changing the 10K to a 5K is the worst solution to this problem.

author: bwilson13087
author_rank: Newbie
timestamp: February 16, 2018, 06:36:02 pm
content: Quote from: Paul Price on February 16, 2018, 06:17:07 pmThanks for the reply. The power supply actually already has a 1k pot for fine adjustment. I kind of also have an issue with it being too fine as well. The fine adjustments full range only changes voltage by about 0.3 volts. I can't find any schematics to share but I'll try to add a pic of the front board. I have also looked at 3 turn pot but I'm pretty sure they are also too big. The board screws directly to the front panel so the standard rotary pot thickness is all the more room I have.

author: Paul Price
author_rank: Super Contributor
timestamp: February 17, 2018, 12:46:24 am
content: You could determine how the course and fine adj. pots are wired together by eye-balling the PCB traces and using a DVM.If they are connected together, you might substitute a 2k linear pot for the 1k fine adj. pot and find you get twice the fine voltage adj. range at full output.

author: bwilson13087
author_rank: Newbie
timestamp: February 17, 2018, 06:05:46 am
content: Quote from: Paul Price on February 17, 2018, 12:46:24 amI had considered this. Since I'm not 100% sure how it all works, I wasn't sure if the added resistance would cause problems if both adjusts were as high as they could go.

author: Nusa
author_rank: Super Contributor
timestamp: February 17, 2018, 06:25:17 am
content: Quote from: bwilson13087 on February 17, 2018, 06:05:46 amA full range for the two pots in series (I'm assuming they are in series!) of 0-12K instead of 0-11K is much less likely to have issues than your first plan of replacing 0-11K with 0-6K. And even if it does, 12K is only going to happen if you do it on purpose.

author: bwilson13087
author_rank: Newbie
timestamp: February 17, 2018, 08:06:33 pm
content: Quote from: Nusa on February 17, 2018, 06:25:17 amAs far as I can tell the course and fine adjustments are in series. To be clear my original plan was to maybe use a 5k pot in series with an additional 5k (probably 4k7) resistor. I was assuming this would reduce the max voltage to around 15V but halve the sensitivity.