WorldWide Drilling Resource

Vive la Différence! by Britt Storkson Owner, P2FlowLLC Often in electronics, one does not need to know the exact voltage of a signal is, but it is useful to know if one voltage is greater than the other. For example, in an RV (recreational vehicle), if we have an auxiliary battery for the RV lights and appliances, and a separate battery for the starter, we would like the auxiliary battery to be connected to starter battery (for charging from the vehicle alternator) only when the engine is running. This way, the auxiliary battery does not drain the starter battery when the engine is not running. When the engine is running, the starter battery voltage will be higher (around 13.8 to 14.1 volts) to charge the battery while the unconnected auxiliary battery voltage will stay where it was (about 12.0-12.6 volts). We need a way to measure the voltage at each battery and connect the auxiliary battery to the starter battery only when the starter battery is being charged by the running engine. We don’t care exactly what the voltage is, we just need to know which voltage is greater than the other one. One way to do this reliably and inexpensively is to use a device called a “voltage comparator”. From the drawing, if the “Vin” (voltage in) voltage is greater than the “Vref” (voltage ref- erence) voltage, then the Vout (voltage output) is the +Vcc (power supply to the voltage comparator) value. If the “Vref” voltage is greater than the than the “Vin” value, the Vout voltage is 0 volts. With this voltage comparator, we can drive a relay which will connect or disconnect the auxiliary battery from the starter battery. As with most everything electronic, there are a few “rules” one needs to follow to for it to work correctly. For one, the volt- age at the “+” and “-” pins of the comparator must be about 2 volts less than the Vcc voltage (comparator supply voltage). An easy way to do this is to use two 47,000-ohm to 100,000-ohm resistors in series to form a “voltage divider”. Simply put, if the two resistor values are equal, the voltage at the junction of these two resistors (labeled Vref) is half the value as the incoming voltage - which satisfies this requirement. Another consideration is an automotive electrical system produces a lot of electrical voltage spikes and other noise, and since voltage comparators are very fast devices, the output could chatter off and on as a result. A way to filter this noise out is to place a large value (470-micro- farad or more) capacitor at the junction of the two resistors. Since a capacitor stores electrons, it tends to slow the voltage tran- sitions and lends stability to the circuit. While there are other ways to do this, a voltage comparator is inexpensive (less than $1), reliable, and accurate. It would be considered a “pilot duty” control in that it does not have the capability to drive large current (amperage) loads - large current loads in this case being defined as current loads greater than about 10 milliamps. To drive large current loads, we use a transistor or relay at the voltage comparator output to do the job. The voltage comparator is a simple solution to a variety of control challenges - and simpler is always better. Britt Britt Storkson may be contacted via e-mail to michele@ worldwidedrillingresource.com 41 WorldWide Drilling Resource ® NOVEMBER 2017