![]() ![]() Am I missing something here? This circuit is currently on a bread-board by itself, I just wanted to figure out if my idea worked. My LED just stays on all the time until the voltage is too low to actually light it up. I wanted to voltage-drop the 9v through resistors to be close to 5v, so that when the battery voltage started to drop, the voltage that the chip would see would be lower than the reference voltage, causing the output to change. The circuit does not seem to work as I intended. A lead acid 12V battery while charging should go upto 14.4V and after completing. The output can only sink current, so I have an LED wired to 5V, through a 470-ohm resistor then to the output. Low voltage means that either battery is under charged or worn out. I use a 5k potentiometer to simulate voltage changes. ![]() Then, the 9v is hooked to a 2.2K resistor, then to ground through a 4.7K resistor, with the "input 1 plus" pin connected in between them. (voltage drop of like 0.1v on the 10 ohm resistor). What I was going to attempt was to have a 5v voltage regulator connected to "input 1 minus", using a voltage divider circuit with a 10-ohm resistor and a 4.7 k, to make the voltage go down by a very small amount. But there are probably types that can give decent effects with a bias current of 0.5mA.I was going to try and make a low battery voltage indicator using an LM339, and after I figured out the circuit itself I would connect it to an Arduino input, and make my program to give a visible indication of a low battery. Once the battery is at the trip level, the zener will start to cut off, so its loss will slowly minimize, but then, of course your LED will be draining 2mA.įor a reliable operation you want Iz not to be too far from its optimum, i.e. The rest of the "waste" will be determined by your zener. The rest is determined by your choice of Zener and Resistor to determine the initial trip voltage, in this case your transistor set-up will be using about 95uA to 150uA of current when turned off. If we want the transistor to turn on at the first sign of a 0.7V difference, we need to assume a differential voltage of 15 to 30mV over the resistor, making: Both values are below 100uA and we want to put at least a couple of uA into the base of the first transistor, so we'll use 1uA as a minimum. We use that to do the remaining calculations: Now, the first transistor is only switched on when the battery is between 9V and 7.2V to keep the LED transistor from turning on. R(pull-up) = 100kOhm and R(base) = 33kOhm (makes 133kOhm together). Let's choose 133kOhm to stay well on the safe side: The current into the base of the LED's transistor then becomes 25uA at 7.2V and 20uA at 6V (divided by hfe, which is assumed 100). The 2 resistors are used as a potential divider. This way the intensity of the indicator LED does not change with change in the battery voltage you want to check. Lowest V+ = 6V, I(led) = 0.002A, V(led) = 1.9V, means: R(led) = (6V - 1.9V) / 0.002A = 2.1k Ohm. One is a constant 5V to power the two LEDs and the other input is the battery whose voltage you want to check. This again drives an assumption, as no specifications are made, I will assume the LED current is 2mA and that the red LED adheres to commonalities, needing about 1.9V at that current. a BC547C type may go up to 500 in the right circumstances.įirst off, you mention a low-power LED. Further, for ease of the example I am going to assume a hfe of the BC547 of 100. I am going to make the assumption you want it to work in the range of 6V to 7.2V, so we'll use those two as the extremes of our calculation. You want the LED to turn on at about 7.2V. I would propose a little math to find the best solution to your exact situation with lowest losses: This means some experimentation or graph-hunting may be required to find your value. Reverse biased zeners have their specified knee voltage at a specied current, your schematic allows for very small "leakage current", but this also influences the biasing of the zener. Your calculations based on the Vz of the zener are a little bit dependant on the zener current. ![]()
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