Unexpected 0v on a simple circuit - Page 1
Forum section: electronics / beginners

author: MattY19
author_rank: Newbie
timestamp: December 25, 2022, 11:51:35 am
content: Hi All, nice to meet you alli'm really new to electronics but I have a breadboard a few potentiometers and some RGB leds, and was building a simple circuit this morning and got something I didn't expect.I've watched lots of youtube videos, but i'd not noticed this effect before.I'm using 5v (USB) to the breadboard and wanted to try and create a voltage between 2 and 2.5v using the potentiometer to allow me to power something later on.When I measured the voltage between the middle pin of the potentiometer and ground on the USB header I got 0v, when I was expecting it to vary between 0 and 2.5v depending on the wiper position.I tried to simulate this using an online tool and I discovered that there is no voltage unless I add a resistor between the middle pin and ground. I'd assumed as long as the voltage was higher at the potentiometer than ground I would see some sort of voltage.Is there some sort of "pull" effect that adding the resistor after the potentiometer has ? I did try googling, but wasn't sure what to look for.An image of my simulation of the original circuit isIMG_0561.PNGAn image with the resistor and working as expected isIMG_0562.PNGWould anyone be able to point me in the direction of what to look for to understand this effect better ?I'm feeling pretty daft that I've not gotten my head around these basics yet!ThanksMatt

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2022, 12:02:24 pm
content: Are you sure the potentiometer is good? Try measuring the resistance between the wiper and either side of the potentiometer.

author: BeBuLamar
author_rank: Super Contributor
timestamp: December 25, 2022, 12:14:33 pm
content: Did you connect the center of the potentiometer to the 0V? If so it should be 0 V. You should connect one terminal of the pot to +5 the other to 0V and the center connect to nothing . Then make the voltage measurement between the center and the 0V terminal.

author: xrunner
author_rank: Super Contributor
timestamp: December 25, 2022, 12:15:31 pm
content: Measure each component out of the circuit before you connect them all together. Check the pot. to see if it acts as it should on it's own (resistance tests).

author: MattY19
author_rank: Newbie
timestamp: December 25, 2022, 12:18:10 pm
content: Thank you. I’ve just measured the resistance across left and right pins and it’s 500 ohms. When I connect the wiper to the positive I can see a change in resistance.I suspect the issue is my lack of understanding.Thanks for replying:)

author: Ian.M
author_rank: Super Contributor
timestamp: December 25, 2022, 12:43:29 pm
content: If you want to use it as a potentiometer, wire it as one! i.e. apply supply voltage and ground to the two track ends and take your adjustable voltage from the wiper, which must *NOT* be shorted to either track end.Currently you have it wired as a rheostat, which is the configuration to use for variable resistance.https://learn.sparkfun.com/tutorials/voltage-dividers/allN.B. a rule of thumb for potentiometers is that the wiper current should never exceed 10% of the standing current through the track. This gives a max. 5% error in the output voltage under load with the wiper in the middle, decreasing non-linearly to very near 0% with the wiper at either track end.

author: BeBuLamar
author_rank: Super Contributor
timestamp: December 25, 2022, 01:00:00 pm
content: Quote from: Ian.M on December 25, 2022, 12:43:29 pmAnd so the potentiometer is a very poor way to make a variable power supply. It wastes 90% power in the potentiometer.

author: Ian.M
author_rank: Super Contributor
timestamp: December 25, 2022, 02:06:59 pm
content: Quote from: BeBuLamar on December 25, 2022, 01:00:00 pmWorse than that, as the load current increases the I2R losses in the upper half of the track increase more than the reduction in I2R losses in the lower half of the track. If you follow the rule of thumb, with the wiper centered, it wastes approx 95.5% of the input power!If you need your variable voltage to supply more than a couple of mA, a potentiometer is invariably the *WRONG* solution.

author: WattsThat
author_rank: Frequent Contributor
timestamp: December 25, 2022, 03:20:21 pm
content: Back to your original question:Ref 561: The voltage on the wiper is always zero because the wiper is connected to ground. The voltage will always be zero with respect to ground, the two point are the same potential.Connect the wiper to the opposite end of the pot (the 500 ohm fixed resistor end) and the voltage on the wiper will vary between zero and 2.5 volts as the wiper position is changed.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 25, 2022, 03:50:04 pm
content: You need to connect the potentiometer between +5V and 0V. Leave the cursor of the potentiometer unconnected (middle pin). There, on the floating cursor is where you will get the variable voltage. Like in this link first schematic:https://www.allaboutcircuits.com/textbook/experiments/chpt-3/potentiometer-voltage-divider/DO NOT CONNECT THE CURSOR TO 0V.If you connect the middle pin (the cursor) to ground, you'll short circuit the voltage and will only measure 0V, so it won't work as expected (unless you insert yet another resistor like you did in the second simulation).

author: tooki
author_rank: Super Contributor
timestamp: December 26, 2022, 05:49:50 pm
content: Don’t invent terminology, RoGeorge. The movable contact of a potentiometer is called the wiper, not the cursor.The cursor is the blinky bar on a computer screen that indicates where typed text will appear (aka the insertion point). (And many people call the mouse pointer the cursor, too, but I prefer to avoid it altogether since it’s ambiguous.)

author: rstofer
author_rank: Super Contributor
timestamp: December 26, 2022, 07:14:27 pm
content: I certainly hope this experiment is being done on a USB wall wart or powered hub. It could be disastrous to a PC or laptop.

author: Zero999
author_rank: Super Contributor
timestamp: December 26, 2022, 07:42:37 pm
content: Quote from: tooki on December 26, 2022, 05:49:50 pmEnglish is clearly not his first language. It was probably a mistranslation. I don't see anything wrong with calling a mouse pointer a cursor. Technically it's a sprite, if we're going to be pedantic.

author: tooki
author_rank: Super Contributor
timestamp: December 26, 2022, 09:05:09 pm
content: Quote from: Zero999 on December 26, 2022, 07:42:37 pmYeah but he could have looked at the very article he linked to, which uses the right word.Quote from: Zero999 on December 26, 2022, 07:42:37 pmI said right there why I prefer to avoid the word “cursor” altogether, both for the mouse pointer and insertion point: because people use it forboth, it can be ambiguous. I didn’t say it was “wrong”.Quote from: Zero999 on December 26, 2022, 07:42:37 pmThat’s an orthogonal thing, in that sprites are one way (not the only way!) toimplementa mouse pointer. On NES Super Mario Bros, a programmer knows that Mario is a sprite, but functionally, he’s the player. Remember, my discussion is about what the thing is called, not how it’s implemented.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 26, 2022, 09:07:18 pm
content: Quote from: tooki on December 26, 2022, 05:49:50 pmYou really had to explain all that? Could have just wrote "proper term iswipernotcursor".Don't invent terminology,tooki. That's a "text cursor", not a cursor, not a blinky, and not a bar.Mine is not blinking, and it's a filled rectangle, not a bar (has no volume). It's not always an insertion point, can be in overwrite mode sometimes, and it's rather a character than a point.Sorry for misusing the Romanian word "cursor" while writing in English. In Ro it's "cursor potențiometru".https://ro.wikipedia.org/wiki/Poten%C8%9Biometru_(instrument_de_m%C4%83sur%C4%83)

author: tooki
author_rank: Super Contributor
timestamp: December 26, 2022, 09:17:35 pm
content: Quote from: RoGeorge on December 26, 2022, 09:07:18 pmDon’t eventhinkof trying to “school” me on tech terminology in English, dude. It’s my native language, and I worked for years as a technical writer.https://www.intel.com/content/www/us/en/programmable/quartushelp/17.0/reference/glossary/def_insertion_point.htmBesides, I wasn’t giving an exhaustive definition of all possible appearances of an insertion point. They are almost always blinky.As for a “bar” having no volume… dunno where you got the idea that a bar must be three-dimensional. One dictionary contains this definition (among several): “a straight stripe, band, or line much longer than it is wide”.Quote from: RoGeorge on December 26, 2022, 09:07:18 pmThat is interesting, though, and I wonder how it came to be!

author: Zero999
author_rank: Super Contributor
timestamp: December 26, 2022, 09:56:44 pm
content: Quote from: tooki on December 26, 2022, 09:05:09 pmThat's not how the human brain works. When one reads something, they typically memorise the meaning, not the actual words used. It's understandable for a non-native speaker to mentally translate the article into their native language, then back into English and it come out slightly mangled. I'm dyslexic and got tired and frustrated with teachers chastising me for misspelling a word, which was spelled correctly on the black board or in a textbook, right in front of me. I also have a problem with my handwriting, so it's possible for me to misspell a word, even though I know how to spell it. I tell my hand to write one letter, but it does a completely different one. It'll often do the same again, even if I cross it out and write it again, it still comes out wrong. It used to make me swear and curse under my breath, which also got me in trouble. I hated school.

author: Jwillis
author_rank: Super Contributor
timestamp: December 26, 2022, 11:04:46 pm
content: Holy smokes! Look at your load. It's a direct connection to ground. Your Voltage is referenced to ground . Of course your measurement will be zero Volts . Measure across your rheostat or cut the ground after your rheostat and take you measurement.