![]() ![]() To read a battery voltage, don’t you need an analog-to-digital converter? Yes, you do, sometimes. Raspberry Pi Battery Measurement (without Analog) So you can tell when they are low, but voltage alone cannot tell you how long they will operate. (For those that are going to do it anyway.)Their voltage will drop below 3.7 volts when they are about to die. But I’m asked about it often, so I wanted to mention it here. Note: Using voltage divider circuits to measure the voltage of Lithium-Polymer batteries isn’t a reliable method to determine how much energy they have left. And you have full control over how long the divider is activated. With this configruation, no current can sneak through the analog pin’s protection diodes. A digital I/O pin controls a NPN, which turns the PNP on and off. Add a high-side PNP transistor to the simple voltage divider. What if you could just turn on the voltage divider when you make the measurement? Ah-ha! Here’s a solution. Most applications do not need continuous battery voltage monitoring. This path can damage the microprocessor and means your battery-powered circuit stays powered! So you need a way to avoid back powering your circuit.Īdd transistors to voltage divider circuits This circuit will cause the voltage divider to “power” the microcontroller through the I/O pin. Now, this switch turns off VCC but not the battery voltage. At the microcontroller, you add a switch to turn it off. Here, let’s say you take the simple divider from above and leave it connected to the analog pin. If your board stays on all of the time the battery is connected, then “back feed” or “back powering” is an issue for you. When powered by batteries, however, that leakage means less battery life. ![]() With USB there is little concern about that much leakage. Sounds small, but that means 325 µWatts of wasted power. No matter what else is going on in the circuit, 44 ♚ will flow through the divider. Simple enough, right?įirst, connecting these two resistors in series to the battery will create a leakage current. ![]() With R1 at 100kOhm and R2 at 68kOhm, the divider outputs about 3.0 volts. (At least the ones you want to last.) When splitting the voltage in half does not work, I turn to this voltage divider calculator. While easy, that is still too high for a most 3.3 volt systems. Two 100kohm resistors divide the voltage from 7.4 to 3.7 volts. Problem using Simple Voltage Divider Circuits Together these batteries create a supply of 7.4 volts. Your board is being powered by two LiPos in series. Let’s say you are using a 3v3 microprocessor (Raspberry Pi) or ESP8266 (Microcontroller). Instead, you might need to step down the battery voltage to measure it. In this case, this is NOT a substitute for a buck regulator. There are a couple of different cases where you might need to “step down” your battery voltage. For me, the voltage level shifter is the most common. ![]() If you need a voltage regulated, get a voltage regulator! At some point or another, I’ve built all five of these voltage divider circuits. To be upfront, one of these uses is NOT as a voltage regulator. This elementary circuit has a few inventive uses. Here are some ideas of what you can do with the humble voltage divider. Hey Newsletter Readers: Looking for Best Resistor Sorting Method? My Bad! ![]()
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