We now need to check the power dissipated in the Zener diode and resistor at different voltages so we can select suitably rated components, and check that the current flowing through the LED when the battery bank is at its maximum voltage is below the manufacturer recommended maximum. Choosing Correctly Power Rated Components Therefore add to the parts list for this project:ġ x 127 Ohm Resisitor. Normally you would select the resistor with the nearest value above the resistance value calculated with Ohm’s Law. These 1.9 Volts must be dropped across a suitable resisitor with a current of no more than 15mA – therefore using Ohm’s Law we find that resistance = 1.9 Volts / 0.015 Amps = 127 Ohms coincidently the exact value of a manufactured resistor. The difference between the battery bank target voltage of 12.6 Volts, and the voltage dropped by the Zener and LED of 10.7 Volts is 1.9 Volts. A resistor is therefore required to prevent too much current getting to the LED and destroying it. The total voltage drop across the Zener diode and LED will be 8.2+2.5=10.7 Volts. In order to make the LED last as long as possible we will not use the maximum current – instead we will aim for around 15mA. Let’s use the following components:ġ x standard green LED (Specfications: maximum current 30mA, voltage drop 2.5 V). We want our LED to light when the voltage of the 12V battery bank is 12.6 Volts or higher to indicate the batteries are being charged by our renewable energy set-up. Therefore, if you put a Zener diode in series with an LED in a circuit, the LED will light if the circuit voltage is greater than the Zener voltage plus the voltage drop across the LED. However, if the voltage in the circuit is less than the Zener voltage then no current flows. the voltage reading across the diode) is equal to this Zener voltage. If the voltage in the circuit is greater than the Zener voltage, then the voltage drop (ie. The status monitor uses a Zener Diode, a Light Emitting Diode (LED), and a Resistor – components which can be bought for pennies each.Įach Zener Diode has a specified Zener voltage. Therefore it would be interesting to have an indication of the status of the battery bank using an LED to show if it is being charged.įor this example we will use the arbitrary figure of 12.6 Volts to indicate battery bank under charge however this could value could be set lower or higher according to your own needs and system configuration. A healthy full 12V battery bank would usually have a voltage of around 12.6 Volts when not under a load and between 13-14 Volts when being charged correctly – i.e. In a typical 12V system the voltage of the battery bank can fluctuate from 10.6 Volts (below this the battery is very dead) when heavily depleted and under load, to as much as 15 Volts when being heavily charged. It can be easily measured with a multimeter or a voltmeter, however a fun and inexpensive project is to make a very simple battery status monitor. This means that the voltage of the battery bank must be known. It is essential that the battery bank in a renewable energy system is well looked after.
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