| Electronics > Beginners |
| Voltage Divider Affected By Load |
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| Johnex:
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. |
| IanB:
--- Quote from: Johnex on June 12, 2018, 08:27:07 pm ---The question i have is, how do i fix that? --- End quote --- You 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? |
| JourneymanWizard:
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. |
| Johnex:
Sorry for being clear but the voltage increases cos 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: |
| cellularmitosis:
Can you draw us a schematic? |
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