How to Build a Current Flow Sensor

Sensing current flow involves using Ohm's Law: Voltage = Current x Resistance. This article will show how to make a current flow sensor which will trip an alarm when current at or above a threshold limit is reached.
This is a simple way to provide over-current checks to a system. For example, if current flow in a particular part of the system goes above 1 Amp, this could indicate an unsafe condition and trigger a safety alert.

Things You'll Need

• Precision sensing resistor (value discussed in Step 2)
• Electrical leads
• Audible alarm (3 VDC)
• Solder
• Soldering iron
• Insulated wire (AWG 16 - 22)

Instructions

1. Determine the point in the circuit you wish to monitor. It must be between components (not inside an integrated circuit).

2. Determine the current amount you wish to use as your threshold current. This would be the current at which you wish the audible alarm to sound.
   The sense resistor will be chosen using Ohm's Law and the 3 VDC operating level of the audible alarm as follows: Sense Resistor Value = 3 volts divided by the threshold current.
   Example: Resistor = 3 VDC divided by 1 Amp threshold current = 3.00 Ohms.
   Resistor Wattage rating should be at least twice the maximum wattage the sense resistor should see: If maximum current at that point would be 5 Amps, Watts = Volts X Amps, so maximum expected wattage would be 3 VDC X 5 Amps for 15 Watts. The safe sense resistor would be 3.00 Ohms and 30 Watts.

3. Turn power off in the circuit. Using a soldering iron, solder and wire, insert the sense resistor into the circuit at the point determined in Step 1.

4. Attach one lead between the audible alarm (+) and the side of the sense resistor you expect to see the higher relative voltage; this will be determined by where you have chosen to place the resistor in the circuit.
   Attach the other lead between the audible alarm (-) and the other side of the sense resistor.

5. Return power to the circuit. The audible alarm should sound when current through the sense resistor brings the voltage drop across the resistor to or above 3 VDC.

Tips & Warnings

• If you have a voltmeter available, check the voltage across the sense resistor to ensure the voltage drop is near what you calculated. If it is not, you should return to Step 1 and reevaluate.

• Soldering irons must be hot when used. Exercise caution.