Yahoo Answers is shutting down on May 4th, 2021 (Eastern Time) and beginning April 20th, 2021 (Eastern Time) the Yahoo Answers website will be in read-only mode. There will be no changes to other Yahoo properties or services, or your Yahoo account. You can find more information about the Yahoo Answers shutdown and how to download your data on this help page.
Trending News
Voltage Divider Calculator is wrong ?
ohmslawcalculator dot com says i need so and so resistors to get desired voltage, but when i actually solder everything in place, results are close not not close enough. I would be wrong by at least a volt.
I was doing 15V drop to 12 and i ended up getting it with just random resistors VS what the formula suggested.
I tested each resistor before i solder them, all were new and in good value, yet resulting voltage was wrong.
Any ideas why that is ?
From a source of 15V i used 560k for positive and 2.2m for negative. Output is projected to be 11.957 but it was around 11 or 13, i cant remember now.
3 Answers
- derframLv 72 years agoFavorite Answer
To drop 15 volts to 12 volts, R2 will need to be 4x R1: Such as R1 = 3K and R2 = 12K. This will only hold true if there is no load on the output. Any load will act like a resistor in parallel with R2 and will require the appropriate adjustment of R1.
Just saw your addition. With resistors that high, *any* load will make a big difference - even the 10 meg-ohm input impedance of your voltmeter will produce a voltage lower than you expect. Drop those resistor values by 2 orders of magnitude. And remember, the resistors are not exact, and have some variation from marked values.
- ?Lv 62 years ago
Lesson i learned was:
Using resistors to divide voltage creates 2 problems:
#1 Cannot connect any device because output amps becomes just few micro amps. Even a cellphone wont work.
#2 Lowering resistance value will increase amps (not by much) but will cause resistors to catch on fire.
- ?Lv 72 years ago
PERHAPS YOU OVERLOAD THE VOLTAGE DIVIDER .
VOLTAGE DIVIDER COULD NOT SUPPORT HEAVY CURRENT. HIGH RESISTANCE VOLTAGE DIVIDER SUPPORT SMALL CURRENT LIKE A FEW mA. WITH A SMALL RESISTANCE LOAD ATTACHES TO THE VOLTAGE DIVIDER OUTPUT, THIS LOAD DISTURB THE VOLTAGE DIVIDER RESISTANCE AND NO MORE ABLE TO SUPPORT 12V WHILE IT IS UNDER CLOSE TO NO LOAD CONDITION.
TO DROP 15V DOWN INTO 12V, USE LM7812 , ABLE TO SUPPORT UP TO 1A WITH PROPER HEAT SINK ATTACH TO IT.
