Gophert CPS-3205C and a 1 ohm 1% 4 lead  - Page 1
Forum section: electronics / beginners

author: Purduephotog
author_rank: Regular Contributor
timestamp: April 23, 2017, 05:50:59 am
content: I've read this and a few other calibration pieces. I do have access to a full up NIST lab.. but before I go borrow/bother Gary I'd like to get it done myself.I just attached my new gophert to a 7W 1 ohm 1% resistor.1.000V and 1.400A current limiting.The reported current from the power supply was 0.859.At 2.000V it reported 1.712A: current limit wasNot quite what I expected. But it's late and I decided I'd head to bed... And another video suggested picking up a used 175 for 50$ to help recalibrate and a free voltage supplies from voltagestandard.comhttps://www.eevblog.com/forum/testgear/checking-test-equipment-calibration-on-the-cheap/

author: jeroen79
author_rank: Frequent Contributor
timestamp: April 23, 2017, 09:08:25 am
content: I have no idea what exactly you were doing.Can you post the circuit you made?And where did you read what?Anyway: 1Ohm does not equal 0.859A at 1V.But was the load really 1Ohm? Did you account for the leads?The current doubling when the voltage doubled dies make sense though.

author: Purduephotog
author_rank: Regular Contributor
timestamp: April 23, 2017, 12:40:46 pm
content: The circuit was just clipping the 1 ohm 10W 1%power resistor to the power output leads of the supply.I had dialed in 1.000V and set the current limit to 1.400A.I was trying to figure out how to go about establishing what is in calibration and not without schlepping down to the calibration lab and begging some gear.Lot of assumptions, but how should I go about figuring what is in cal (grossly at this point?)

author: tggzzz
author_rank: Super Contributor
timestamp: April 23, 2017, 01:09:07 pm
content: Quote from: Purduephotog on April 23, 2017, 05:50:59 amResearch and understand "Kelvin connections", 2-wire, and 4-wire resistance measurements.Learn what all the equipment tolerances mean, and calculate the effect they will have on the measurements.

author: Purduephotog
author_rank: Regular Contributor
timestamp: April 23, 2017, 01:11:37 pm
content: Quote from: jeroen79 on April 23, 2017, 09:08:25 amI was apparently not thinking.After reading your post I removed the probes and cranked the lugs directly down to the resistor.1.000V 1.001A.So yeah, apparently this set of probes suck. Time to order from Franky's store again.Thank you for helping reattach my head. Now I've just got to figure out what that means for all of my previous measurements... And whether or not to redo them.

author: Purduephotog
author_rank: Regular Contributor
timestamp: April 23, 2017, 01:14:21 pm
content: Quote from: tggzzz on April 23, 2017, 01:09:07 pmYep. I knew these in college and have been working thru the actual measurements on my own. So I was stymied when I hit something so simple.I just wasn't thinking. I can't believe I hadn't tested the leads. More caffeine.

author: tggzzz
author_rank: Super Contributor
timestamp: April 23, 2017, 01:16:23 pm
content: Quote from: Purduephotog on April 23, 2017, 01:14:21 pmCollege knowledge is very practical"Let's makenewmistakes"!

author: electr_peter
author_rank: Unknown
timestamp: April 23, 2017, 01:38:19 pm
content: Be aware that Gopher PSU does not limit current at the set limit exactly. For example, if you set 1.4A current limit, it can limit at 1.3 or 1.5A.However, Gopher PSU shows actual current pretty accurately. Check current with DMM if in doubt.It is bad practise to "trust" set current limit for precision on cheaper supplies. Always look at actual live current reading.