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Circuits designed by David Johnson, P.E.
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DC CURRENT INDICATOR #4
December 15, 2008
The circuit below uses some common components to turn on an LED whenever DC current above
a certain level is detected. The circuit uses a very popular LM393 dual voltage
comparator from National Semiconductor and a common 1N4148 signal diode. The diode
acts as a crude 0.7v voltage reference. Only one of the comparators inside the 8 pin
package is used. A pair of resistors across the diode forms a voltage divider, which
produce a reference voltage of about 0.015v. The circuit compares this reference voltage
to a voltage produced across a current shunt resistor. A section of wire already in
place can be used as the current shunt. The circuit can be used to indicate current
drawn from a battery and routed to a load or current from a battery charger to the
battery. Both circuits are shown on the drawing. The circuit shows components for a 12v
system but it will work well using any DC supply from 3v to 30v. |
| The circuit is sensitive enough
that it can indicate fairly small currents through a section of thick wire. As
an example, an 18ga wire only 24 inches long will have a resistance of about 0.013 ohms.
The needed 0.015 volts will be generated with a current flow of about 1.2 amps. If more
sensitivity is needed, a longer section of wire can be used as a shunt resistor.
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| Use Ohm’s Law to calculate the
resistance needed. Ohm’s Law equation is: E = RI, where R is the resistance in ohms,
E is the voltage developed across the resistance in volts and I is the current flowing
through the resistor in amps. If we set E at 0.015 volts and R at 0.0012 ohms, then
I = 1.2 amps. |
| Here is one example how this
circuit might be used. Suppose you have a large 12v battery, which supplies current
to various loads inside a recreational vehicle (RV). You noticed that a 10ga wire 10
feet long connects the battery negative terminal to a junction box. You look up the
resistance of the 10ga wire from a chart and find out that the typical resistance is about
0.001 ohms per foot. Therefore, if you used the battery negative terminal as one
point of reference and the connection in the junction box as another point, then the
resistance should be about 0.01 ohms. If the current indicator circuit were
connected between those two points, then the circuit should switch on the LED whenever
current exceeding 1.5 amps is drawn from the battery. |
| In another example, suppose you
have a battery changer connected to a 12v lead acid battery. The charger uses a pair
of 18ga wires 12 feet long. The resistance of 18ga wire is about 0.0064 ohms per
foot, so the resistance of one of the 12 foot wires should be about 0.077 ohms.
Using the current indicator circuit and using the two ends of the cable as reference
points, the LED should turn on whenever the charging current exceeds 0.20 amps. The
indicator light should turn off when the battery is fully charged. |
| Here is a link to a
Table for Copper
Wire |
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Click on Drawing Below
to view PDF version of Schematic |
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