How to test a circuit

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How to test a circuit

A circuit tester is a device which is used to test a circuit to determine whether or not power is reaching the circuit. Circuit testers are very inexpensive tools which can be obtained at hardware and home suppliers, and they are critical tools for anyone who works with electricity to have. While they do not provide the detailed information available with a multimeter, they are useful for quick checking of electrical circuits, and they take no time at all to learn to use.

Typically, a circuit tester has a light which illuminates when a circuit is getting power. Some may generate buzzing sounds, and the volume of the sound or the intensity of the light may increase with the amount of power available to the circuit. This can be useful for differentiating between circuits which are being supplied with different amounts of power, or for identifying circuits which are getting too much or too little power, which can be a sign of an electrical short or a similar problem.

To use a circuit tester, one of the probes is placed on the hot or positive pole of the circuit, and the other is attached to either the neutral or the ground. With something like a power outlet, this can be done by sticking the probes into the two primary holes of the outlet. Circuit testers can also be used to test exposed circuits, such as that found under the hood of a car, and they can be used to see whether or not power is reaching junction boxes, wiring for chandeliers, and a variety of other electrical situations.

While using a circuit tester, it is important to only handle the insulated portions of the device. Handling the probes directly could expose someone to a shock or create a short circuit. For people who want more information about the circuit, it will be necessary to use a multimeter to gather data. Multimeters are also very useful tools to have for people who work with electrical circuits and electrical components, although they tend to be more expensive and finicky than basic circuit testers. It’s also possible to build one’s own circuit tester, using two insulated wires which attach to probes on one end and a lightbulb on the other.

By getting into the habit of using a circuit tester before working on any circuit, people can greatly reduce the risk of injuries or property damage caused by unknowingly working with an energized circuit. Even professional electricians utilize circuit testers, because electricity can be very dangerous, and people can very easily make mistakes which can be identified and addressed with a circuit tester.

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

How to test a circuitMary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Using Circuit Testers and Meters

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How to test a circuit

The first step in almost any electrical project is to check for power to make sure the circuit or device is safe to work on. You can do this with a variety of inexpensive testers or even a multimeter.

Testers and How They Work

Standard probe-type circuit testers, such as neon circuit testers, voltmeters, and multimeters, have two wire leads with probes for checking circuit wiring or electrical devices. When you insert the leads into an outlet or touch them to a switch’s screw terminals, a light or readout will indicate whether or not the device has voltage. An even simpler (and decidedly safer) type of tester is a non-contact voltage tester, which doesn’t even need to be inserted into an outlet or to touch bare wire connections; merely bringing the sensor close to a power-carrying wire or device will cause the tool to light up or make an audible noise to indicate the presence of power.

There are also plug-in outlet testers with three small different-colored neon lights. These testers simply plug into an outlet and can check for an open neutral, lack of a ground, wires on the wrong terminals, or no power. A specific pattern of lights indicates each condition, and a chart on the top of the tester tells you how to interpret the light pattern.

While simple voltage testers can test only for the presence of voltage, multimeters have multiple testing functions and can measure voltage, ohms (for resistance), and amperage (electrical current), indicating quantities on a digital readout or analog dial. Testing to see if the power is on is only one function of a multimeter.


Never touch the bare-metal probe ends of a tester during a test because they may have electricity running through them and can give you a dangerous shock. Also, never let the probes touch each other during a test.

Making Sure Your Tester Works

Always check to see if the tester is working properly before using it to check for voltage. The easiest way is to go to an outlet on a circuit that you know is live (has power). Insert the tester leads or sensor into the outlet slots. If the tester lights up, it’s working fine. If it fails to light up, the tester is bad or needs new batteries.

How to Test Outlets for Power

A typical outlet receptacle has three holes in its face. The shorter straight slot is the “hot” lead and connects to the active hot wire in the outlet box. The longer straight slot is the “neutral” lead and connects to the neutral circuit wire in the electrical box. The slot that looks like a small D-shaped hole is the ground slot, and it is connected to the circuit ground wire.

To test an outlet for power, turn off the power to the circuit at the circuit breaker. Insert the two probes of the tester into the two straight vertical slots on the receptacle. If the power is on, the tester will light. Because there is a possibility that the outlet is “split-wired”—with the top and bottom halves of the outlet fed by different circuits—always check both halves for power before removing the receptacle to work on it.

You can also test to see if the ground system is properly connected to the receptacle. To test the ground, make sure the power to the circuit it on. Insert one tester probe into the hot (short, straight) slot and the other in the ground (D-shaped) slot. If the circuit is working and you have a good ground connection, the tester will light.

Testing Wall Switches

To test a switch for power, turn off the power to the circuit at the circuit breaker. Remove the switch’s cover plate and flip the switch’s toggle so the switch is on. Carefully touch one probe of the tester to one of the screws on the side of the switch. Touch the other probe to the bare copper ground wire or the ground screw on the switch (you can also touch this probe to the electrical box if it is metal, but this test works only if the metal box is properly grounded; plastic boxes are not grounded). Next, touch one probe to the other screw terminal on the switch and touch the other probe to the ground wire or screw. Flip the switch’s toggle to off and repeat the same tests. If the tester does not light for either test, the switch is not getting power.

Testing Light Fixtures for Power

When checking light fixture wiring for power, turn off the power to the circuit at the circuit breaker, then loosen the mounting straps holding the fixture to the ceiling box and pull the light fixture slightly away from the ceiling box for testing. Always test twice—with the fixture’s wall switch on and with it off—because the fixture may get power in either position.

To test for power with a non-contact voltage tester, touch the sensor tip of the tester to each of the circuit wires. If the tester lights up when touching any of the wires, the circuit still has power.

To test a fixture for power using a probe-type tester, you need access to the fixture’s screw terminals or, if the fixture has wire leads, to the ends of the wire leads. Touch one tester probe to the hot (black or red wire) screw terminal, and touch the other probe to the neutral (white wire) terminal. If the tester lights up, the fixture still has power.

If the fixture has wire leads connected to the circuit wiring with wire connectors (wire nuts), stick one probe into the connector for the black (or red) wires and the other probe into the white-wire connector. If the tester does not light up, confirm the test by carefully untwisting each wire connector—without touching the bare-metal wire ends or letting different-colored wires to touch—then touching each probe directly to the group of black (or red) and white wires.

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A check for short circuits is one of the most basic tests you can perform with a multimeter. On the simplest meters, you use the resistance setting; sophisticated models have a continuity setting that flashes a light or beeps a tone to let you know a connection is a short circuit.

Turn Off Power

Turn off all power to the circuit or device under test. Unplug the equipment from the AC outlet.


  • Probing an electrical circuit with a multimeter may pose a dangerous shock hazard if the circuit’s power is on.

Set Multimeter to Resistance or Continuity

Switch the multimeter on and turn its selector knob to the resistance setting. Use the continuity setting if your meter has that function.

  • Some multimeters may have several resistance settings; choose the lowest resistance scale on the meter.

Touch Probe Tips Together

Touch the test probes together and observe that the resistance reading goes to nearly zero. For continuity, the light flashes or a tone sounds.

Locate Circuit Component

Locate the component or portion of the circuit you want to check for a short. The tested part should not normally have zero electrical resistance; for example, the input of an audio amplifier should have a resistance of at least several hundred ohms.

Touch Probe Tips to Circuit

Touch the metal tip of the black probe to the circuit’s chassis or electrical ground, and touch the tip of the red probe to the parts of the circuit you suspect may have a short. The tips of the probes must touch metal parts of the circuit, such as a component lead, circuit board foil or wire.

Observe Meter Display

Observe what the meter does when you touch the probes to the circuit. A high resistance signifies an open circuit. Very low resistance — about 2 ohms or less — indicates a short circuit. A meter with a continuity setting flashes or beeps only if it detects a short circuit.

Diagnosing of Defective Printed Circuit Boards (PCB)

Before getting into details of printed circuit board (PCB), there are some basics you need to know about circuits.

Electricity: It is the power provided to every instrument ranging from small lights to heavy machinery. Electricity is just a flow of electron from one level to other (upper level to lower mostly).How to test a circuitSo in an electrical circuit, there is always a voltage or current source, components of circuit and electricity always go from a positive voltage level to negative voltage level.

Voltage, current, resistors, capacitors and inductors are considered as primary elements of any electrical scenario called circuit. Electrical current can be in two forms either a sinusoid AC (alternating) current or simply a straight line called direct or DC current.

In hardware development of electrical circuit, to get all the components of circuit on one single place or board is called PCB designing.

Printed Circuit Board (PCB) is the common name that is used for these electrical boards. In history, PCBs were been developed by going through a complicated procedure of point-to-point wiring and these circuits were highly exposed to get failure or damage. After those more accurate design techniques were developed which were more secure.

These days composition of printed circuit board consists of four major components.

  1. Silkscreen
  2. Solder Mask
  3. Copper
  4. Substrate that is fine fiberglass

Older PCBs were single layered but these days multi layer PCBs are present and used in the market .PCBs are multi layered because these days complexity of electrical circuit is also increased.

Newly developed PCBs have high pitch parts in which most of the parts are unidentified, on testable and more they involve complex troubleshooting and repairing techniques. Older circuit boards were able to be repaired by using automatic test equipment but these days it is not possible. Techniques used for troubleshooting wereHow to test a circuit

PCB Troubleshooting Techniques

  1. Checking of solder joints
  2. Tracking of problems
  3. Troubleshooting of discrete elements
  4. Checking of integrated circuits (ICs)
  5. By using software help
  6. Visual Inspection
  7. Functionality test

Most of these techniques become non functional when they need to cope with modern circuit boards.

A newly developed VI signature analysis technique which is best for troubleshooting of completed circuit elements

Analogue Signature Analysis to Test the Unpowered Printed Circuit Boards (PCBs)

One of the best essential device that is used for detailed analysis of faulty components in a circuit. It is one of the best choices to test PCB when component signatures or documentation has been lost. This test require no power supply so it is best when we want to check faulty or dead boards as it is not safe to power them up.

A sine wave is provided to particular component under test using two probes. Resulting currents, voltages and shifts in phase are displayed on LCD. Current is on y axis while voltage is on x-axis and resultant trace is displayed as signature on screen. To use this device you need to have solid theoretical knowledge of signatures of different components and its complete understanding is required.How to test a circuit

Strategy to diagnose a faulty PCB

There are three different stages of this technique.

  1. Detection of fault using VI instrument. Unidentified high pin count is tested by alternating voltage.
  2. Second stage is to detect location of the faults. It deals with minute analysis to pinpoint the faulty component. But not like functionality test it performs tests only on input and output stages.
  3. In third stage new functional components are placed in circuit after removing the faulty components


In an electrical circuit all the components are either in series, parallel or a mixed combination (Series-Parallel) both so it become impossible to identify their signatures so the only suitable solution in this scenario is to take a new PCB and compare the signatures of defective one with the signatures of functional one.

To compare the signatures, first thing is to take all signatures of defective PCB new functional PCB if available other you must have saved signatures of components. But if you have a fully functional PCB take all its signatures by using multimeter .Voltage resistance current and inductance of every component is computed and then is compared with all the signatures of defective PCB (Printed Circuit Board).

  1. First of all refresh all the points (remove dry or damaged solder if any) and compare signature if signature matches fault has been removed otherwise go to next step.
  2. In this step tracking is performed as there are many tracks in a PCB so there are a lot of chances for tracks to get damaged, if any track is damaged jumper wire can be used to repair tracks.
  3. This is the last step in which functionality test is performed on every pin of linear integrated circuits, input and output on every pin of IC is checked if it matched with the original data sheet it is fine otherwise you have to remove that IC.

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How to test a circuit

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How to test a circuit

How to test a circuit

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Ohm’s Law states the voltage across a resistor, R (or impedance, Z ) is directly proportional to the current passing through it (the resistance/impedance is the proportionality constant)

Kirchhoff’s Voltage Law (KVL): the algebraic sum of the voltages around any loop of N elements is zero (like pressure drops through a closed pipe loop)

Kirchhoff’s Current Law (KCL): the algebraic sum of the currents entering any node is zero, i.e. , sum of currents entering equals sum of currents leaving (like mass flow at a junction in a pipe)

Nodal analysis is generally best in the case of several voltage sources. In nodal analysis, the variables (unknowns) are the “node voltages.”

Nodal Analysis Procedure :

  1. Label the N node voltages. The node voltages are defined positive with respect to a common point ( i.e. , the reference node) in the circuit generally designated as the ground ( V = 0).
  2. Apply KCL at each node in terms of node voltages.
    1. Use KCL to write a current balance at N -1 of the N nodes of the circuit using assumed current directions, as necessary. This will create N -1 linearly independent equations.
    2. Take advantage of supernodes , which create constraint equations. For circuits containing independent voltage sources, a supernode is generally used when two nodes of interest are separated by a voltage source instead of a resistor or current source. Since the current ( i ) is unknown through the voltage source, this extra constraint equation is needed.
    3. Compute the currents based on voltage differences between nodes. Each resistive element in the circuit is connected between two nodes; the current in this branch is obtained via Ohm’s Law where V m is the positive side and current flows from node m to n (that is, I is m –> n ).
  3. Determine the unknown node voltages; that is, solve the N -1 simultaneous equations for the unknowns, for example using Gaussian elimination or matrix solution methods.

Nodal Analysis Example

  1. Label the nodal voltages.
  2. Apply KCL.
    1. KCL at top node gives I S = I L + I C
    2. Supernode constraint eq. of V L = V S
    3. Solve for V T for instance.

Mesh (loop) analysis is generally best in the case of several current sources. In loop analysis, the unknowns are the loop currents. Mesh analysis means that we choose loops that have no loops inside them.

Loop Analysis Procedure :

  1. Label each of the loop/mesh currents.
  2. Apply KVL to loops/meshes to form equations with current variables.
    1. For N independent loops, we may write N total equations using KVL around each loop. Loop currents are those currents flowing in a loop; they are used to define branch currents .
    2. Current sources provide constraint equations.
  3. Solve the equations to determine the user defined loop currents.

Mesh Analysis Example :

  1. Label mesh currents.
  2. Apply KVL.
    1. Left loop KVL: V S = R 1 I 1 + R 2 (I 1 -I 2 )
    2. Constraint equation I 2 = -I S .
  3. Solve for I 1 and I 2 . Note: Branch current from mesh currents: I M = I 1 – I 2

In any linear circuit containing multiple independent sources, the current or voltage at any point in the network may be calculated as the algebraic sum of the individual contributions of each source acting alone.

  1. For each independent voltage and current source (repeat the following):
    1. Replace the other independent voltage sources with a short circuit ( i.e. , v = 0).
    2. Replace the other independent current sources with an open circuit ( i.e. , i = 0). Note: Dependent sources are not changed!
    3. Calculate the contribution of this particular voltage or current source to the desired output parameter.
  2. Algebraically sum the individual contributions (current and/or voltage) from each independent source.

An ac voltage source V in series with an impedance Z can be replaced with an ac current source of value I = V / Z in parallel with the impedance Z .
An ac current source I in parallel with an impedance Z can be replaced with an ac voltage source of value V = IZ in series with the impedance Z .

Likewise, a dc voltage source V in series with a resistor R can be replaced with a dc current source of value i = v / R in parallel with the resistor R ; and vice versa.

Thévenin’s Theorem states that we can replace entire network, exclusive of the load, by an equivalent circuit that contains only an independent voltage source in series with an impedance (resistance) such that the current-voltage relationship at the load is unchanged.

Norton’s Thereom is identical to Thévenin’s Theorem except that the equivalent circuit is an independent current source in parallel with an impedance (resistor). Hence, the Norton equivalent circuit is a source transformation of the Thévenin equivalent circuit.

Thévenin Equivalent Circuit Norton Equivalent Circuit
  1. Pick a good breaking point in the circuit (cannot split a dependent source and its control variable).
  2. Thevenin : Compute the open circuit voltage, V OC .
    Norton : Compute the short circuit current, I SC .
  3. Compute the Thevenin equivalent resistance, R Th (or impedance, Z Th ).
    1. If there are only independent sources, then short circuit all the voltage sources and open circuit the current sources (just like superposition).
    2. If there are only dependent sources, then must use a test voltage or current source in order to calculate R Th = v Test / i Test (or Z Th = V Test / I Test ).
    3. If there are both independent and dependent sources, then compute R Th (or Z Th ) from R Th = v OC / i SC (or Z Th = V OC / I SC ).
  4. Replace circuit with Thevenin/Norton equivalent.
    Thevenin : V OC in series with R Th (or Z Th ).
    Norton : I SC in parallel with R Th (or Z Th ). Note: for 3(b) the equivalent network is merely R Th (or Z Th ), that is, no current or voltage sources.

Last updated: June 10, 1998
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Simple SCR tester circuit diagram

    Meters & Detectors
  • Updated: February 24, 2021

As usual, we can check a SCR with a plain multimeter. But it is not easy. The Simple SCR tester circuit diagram is very useful. We can know the location pin on the gate lead, anode lead and cathode lead. And can also test diode, LED and Triac.

The SCR devices kind, when take to application with DC voltage. When have trigger current at gate lead SCR will conduction all time. There is one way to stop them is, Remove the power supply voltage that feed it go out it will stop conducts current.

How it works.

As Figure 1 is the SCR tester circuit will see that it is few parts include just three resistors , two LEDs only. The working is very easy. When take SCR to input connector (correctly). When press SW1, LED will still glow.

How to test a circuit

Then if press SW2, LED will go out all time, indicate that SCR already to uses. But if is tested LED glow. By still not press switch. It indicates this SCR “short”.

The resistors-R1 will limit properly gate current. The resistor-R3 is current limiting to LED about 20 mA and R2 will allow to have current flow in range between 110 mA. The SW1-switch is trigger to SCR stop . Then when press this SW2 , LED1 will go out.

Building and Application
This project is easy and has a few part. So can solder all components and wire by without PCB.

Application when measure LED or Diode will use just A and K terminal only. If they is good will makes LED1 glow. If backward but still light show that “short”. But id correct polarity LED1 not glow indicate that “blow“

The measuring the SCR testing also use by connecting correct position then press SW1, LED glow. Then press SW2, LED1 should go out indicate that “good” available.

How to test a circuit
Figure 2 how to test triac.

If insert them but LED1 glow by not press anything to indicate that “short”.

Measuring the Triac can measure them as Figure 2 (A), (B). See in (A) section then press SW1, LED1 will glow. Press SW2, LED1 will go out then measure as Figure (B) again. By press SW1, LED1 will glow. Next press SW2, LED1 will go out as these ways is show that Triac is good.

Since we need to the second measure because the Triac have two ways feature depends on the polarity of voltage between lead gate (G) and lead A1 by A2 is positive. And when lead G is (+) lead A1 will need is (-), when G is (-) lead A1 will is (+) or must not same together.

The component list

Resistors ¼ watts +-5%
R1, R2: 100 ohms
R3: 220 ohms
SW1: Normally open pushbutton switch.
SW2: Normally closed pushbutton switch.
Copper alligator clips, 9 volts battery with 9v battery snap

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I always try to make Electronics Learning Easy .

The Video Course teaches you everything about modern cars.

If there is trouble without an obvious cause in any electrical component, test the circuit to find the cause.

How to test a circuit

A circuit tester is a useful and inexpensive tool for making electrical tests.

Checking a simple circuit is straightforward – the lighting circuits are among the simpler ones – but the electrical wiring in a car contains many interlinking and branching circuits, which bring complications.

All car wiring is colour-coded; unfortunately there are no national or international standards for colours. Colour codes for individual cars can be found in wiring diagrams , in the car handbook or in a service manual.

Study these diagrams so that you can find short cuts which save you having to check an entire circuit.

For example, if you know that the power for a suspect circuit comes from the ignition switch , and if other items fed from that switch are working, there can be no fault between the battery and the ignition . So you can save time by starting at the ignition switch.

How to use a circuit tester

How to test a circuit

Connect the tester clip to the negative terminal of the battery and touch the probe to the positive one.

If the tester lamp does not light, the battery is dead (or the bulb in the tester has blown).

If it lights, try again with the clip earthed to the car body: if the lamp fails to light, the battery negative terminal is not earthed properly.

Earth the clip near the switch of the circuit being tested and touch the probe to the ‘live’ (battery) side of the switch. If the lamp does not light, the wiring between the battery and the switch is faulty, or a fuse has blown.

If the lamp lights, turn the switch on and probe its other

side: if the lamp does not light, the switch is faulty.

If the switch works, leave it on, earth the clip near the component and probe the live side of the component. If the lamp does not light, the wiring from switch to component is faulty, or a fuse has blown.

If all of the checks so far are satisfactory, transfer the clip to the live side of the battery.