Voltage Divider Affected By Load - Page 1
Forum section: electronics / beginners

author: Johnex
author_rank: Contributor
timestamp: June 12, 2018, 08:27:07 pm
content: Hi!So i have a simple voltage divider for measuring voltage. It is connected to an ADC.The input voltage range is 0 to 30V, and the output is 0 to 4.091V (the max of the ADC input).It is wired like so:The voltage divider is in parallel to larger wires that then have any load, the voltage divider acting like a voltmeter.That works great, until i add a load. Adding a 20A load for example is altering the voltage reading after doing the proper scaling at the ADC by almost 1.5V(an increase in voltage, not a voltage drop).That means the load is acting like a resistor, and is altering the top or the bottom portion of the divider from what i can understand.I want to be able to apply any size load to the wires the voltage divider is sensing on, without it getting affected by the load resistance.The question i have is, how do i fix that?Edited to clarify its a voltage increase at the divider output, not a voltage drop as expected.

author: IanB
author_rank: Super Contributor
timestamp: June 12, 2018, 08:34:02 pm
content: Quote from: Johnex on June 12, 2018, 08:27:07 pmYou don't. There is nothing to fix.You are trying to measure a voltage, and if the voltage you are measuring happens to change, then you will (correctly) measure the change in voltage. Or do you want the voltage measuring circuit to lie and give incorrect readings?

author: JourneymanWizard
author_rank: Newbie
timestamp: June 12, 2018, 08:38:27 pm
content: First, check whether the 1.5V drop really is or isn't there: put your multimeter across the 30V to GND nodes of your divider and measure.If you are designing this as something you can plug into a high current and moderate voltage area (e.g. across battery terminals of a lead-acid battery with solar charger), make sure that your module is a) not interrupting the high-current path [if you are only measuring voltage] or/and b) capable of passing high current through all its wires and interconnects. Dropping 1.5V at 20A of load will happen across 75mR of resistance - small resistances add up quick at high current.Finally, realize that if you are pulling 20A from something like a lead-acid battery array across a room, it may be that a 5% voltage drop is expected operation.

author: Johnex
author_rank: Contributor
timestamp: June 12, 2018, 09:39:49 pm
content: Sorry for being clear but the voltageincreasescos of the resistance change at the divider, by the 1.5V, not a voltage drop like you would expect.This is obviously not correct, and the voltage meter across the same load shows as you would expect, a slight voltage drop.The voltmeter is showing the correct voltage at the same spot, while the voltage divider is not.The load is adding its own resistance into the divider, causing the change in the reading the wrong way. This is what i want to fix.There was a vague mention somewhere about using a diode instead of one of the resistors in the divider, or using an opamp, but none of the examples were clear how it worked.Here is a video showing the issue. As the load increases so does the voltage, not a voltage drop as you would expect, so the load is clearly affecting the voltage divider:

author: cellularmitosis
author_rank: Unknown
timestamp: June 12, 2018, 09:44:49 pm
content: Can you draw us a schematic?

author: Johnex
author_rank: Contributor
timestamp: June 12, 2018, 09:50:19 pm
content: I posted it in the first post, its not more complicated than that. The ADC has 10Mohm input equivalent.I have 2 large wires coming from a power supply, they are feeding into a drone. The drone is the load. I have 2 "sense" wires connected to the large ones going to the voltage divider, the output of the divider feeding into an ADC connected to an arduino. With the drone at idle, less than 1A, the voltage divider works as expected, if i change the voltage on the power supply, the voltage changes correctly.Under load, the divider increases it's output voltage, while a multimeter at the same "sense" spot shows a slight voltage drop, since the power supply is not perfectly holding the voltage.

author: IanB
author_rank: Super Contributor
timestamp: June 12, 2018, 09:55:09 pm
content: It could help to draw a picture of the whole circuit, not just the divider. Always remember that a voltage is a potential difference between two points. The voltage in the potential divider doesn't exist in isolation.In your first post you show a ground symbol, but it is important to know what is behind that symbol. How is the connection between that point and the ADC reference ground? If the ground potential floats then measurements will be affected.

author: Johnex
author_rank: Contributor
timestamp: June 12, 2018, 10:11:43 pm
content: Crude but you should be able to see all

author: JourneymanWizard
author_rank: Newbie
timestamp: June 12, 2018, 10:26:41 pm
content: Hmmm.First guess would be back EMF from the rotors? Can you 'scope your power lines? Try filtering your ADC node to ground with some ceramics? 0.1uF, 1.0uF, something like thatSecond guess would be voltage drop between reference points (ADC ground reference different from divider ground reference). Try a star connection for all your power to Arduino and voltage divider, and ground of your ADC and divider and Arduino.

author: Johnex
author_rank: Contributor
timestamp: June 12, 2018, 10:33:24 pm
content: I will try adding some caps, they should be ceramic and not electrolytic? I don't have a scope so i can't really test thatEverything is connected to the ground of the power supply, wouldn't that be enough for the ground levels?Everything is also getting power from the power supply, the drone is not running of a battery or anything, just from the power supply too.If that is not what you mean, do you have an example of how a star connection would work?

author: free_electron
author_rank: Super Contributor
timestamp: June 12, 2018, 10:45:47 pm
content: i'm betting the ground wire is longer than the power wire ... with those currents, the drop in that wire 'lifts' the reference point of the voltage divider. hence the larger reading under load..with these kinds of current it is very important to sense at the right spot so you don;t see the voltage drop in the cables....

author: Johnex
author_rank: Contributor
timestamp: June 12, 2018, 10:48:18 pm
content: It's possible yes, but would just say 1cm or so make that drastic difference?Why would the voltmeter show the correct voltage at the exact same point too?

author: Brumby
author_rank: Unknown
timestamp: June 13, 2018, 01:39:52 am
content: My first suggestion is also to establish 'star' connection points, which looks like this...All the connections are made at a single point with the conductors radiating out from that point - just like a star with rays of light shining out.

author: ArthurDent
author_rank: Super Contributor
timestamp: June 13, 2018, 03:52:05 am
content: I think you have the answer and it is that where you sense the voltage makes a big difference. On one project I was working on we were generating a 20,000A pulse and had to measure it. The connecting 'wires' were 6 foot long heavy copper (1/2" x 3") buss bars but at those currents we could measure 1.5 volts drop or zero along the length of the bar if we changed the ground point for our instrumentation.Here is a link with the explanation and a helpful hint that a capacitor might be needed to keep the system stable and prevent oscillations. In most cases like yours the sensing should be right at the load because that is the voltage you are interested in.http://www.tmatlantic.com/encyclopedia/index.php?ELEMENT_ID=22008

author: Brumby
author_rank: Unknown
timestamp: June 13, 2018, 05:16:53 am
content: Quote from: ArthurDent on June 13, 2018, 03:52:05 amOops, yes. Move the star points from my diagram to the right, next to the load.

author: Johnex
author_rank: Contributor
timestamp: June 13, 2018, 09:40:11 am
content: Quote from: JourneymanWizard on June 12, 2018, 10:26:41 pmI think you might be on to something with the different ground levels. I will try making a new ground link and see if it changes anything.

author: Johnex
author_rank: Contributor
timestamp: June 14, 2018, 08:33:57 pm
content: @JourneymanWizardYou were totally correct. I made a new link from the large wires to the ADC ground and that fixed the issue, now i get a small voltage drop as expected!Thanks to everyone for the ideas and help, really appreciated!