So let’s quickly recap the representations for voltage, resistance, and amperage in the Ohm’s Law triangle:
E = Voltage (electromagnetic force)
I = Current (amps)
R = Resistance
For a given resistance, the current is directly proportional to voltage. That said, if you increase the voltage in a circuit with a fixed resistance, the current will increase. If you decrease the voltage in a circuit with a fixed resistance, the current will decrease.
Ohm’s law follows a simple formula:
E = I x R
To quickly recap again; E is voltage, I is current (amps), and R resistance (ohms).
Using Ohm’s Law allows you to find an unknown voltage, current, or resistance as long as you have at least two values of the triangle.
Now, I said you could solve for voltage, current or resistance as long as you have at least two values, yet the formula above only allows for voltage (E) to be solved. Ohm’s Law triangle can be rearranged for different values to be solved.
Solving for unknown voltage: E = I x R
Solving for unknown current: I = E / R
Solving for unknown resistance: R = E / I
When using the triangle, just cover the unknown value with your finger and solve it. Imagine a division symbol under E and a multiplication symbol between I and R.
Example 1: You’re working on a circuit with an unknown voltage. We know the current is 0.5A and the resistance is 10Ω. What’s the voltage?
Keep in mind; we only cover the unknown value we want to solve for. So, we would cover the E in the triangle since we’re trying to determine the voltage. We know we will be using multiplication since we read earlier to imagine a multiplication symbol between I and R.
E = I x R
E = 0.5A x 10Ω
E = 5V
Example 2: You’re working on a circuit with an unknown current. We know the voltage is 5V and the resistance is 10Ω. What’s the current?
Just like earlier, we’re going to cover the symbol we’d like to solve for. So, we will cover the I in the triangle now. We know we will be using division since we read earlier to imagine a division symbol below E
I = E / R
I = 5V / 10Ω
I = 0.5A
Example 3: You’re working on a circuit with unknown resistance. We know the voltage is 5V and the current is 0.5A. What’s the resistance?
Nothing has changed in how we are going to solve this. Just like before, we are going to cover the symbol we’re want to find. So, we would cover the R in the triangle since we need to solve for resistance. We know we will be using division since we read earlier to imagine a division symbol below E
R = E / I
R = 5V / 0.5A
R = 10Ω
What this means on a service call
Ohm’s Law is the check that ties a reading to a decision. If you know two of the three values, voltage, current, and resistance, the third value must agree. Use it before you replace a part. A reading that does not agree with the other two usually points to a test problem, a wrong test point, or another part in the path, not to a failed load by itself.
On a service call, measure the voltage across the load and the current through it, then work out the resistance and compare it with the power-off resistance check. If the numbers do not line up, retest before you condemn the part. Record the test points with each reading.
Numbers and measurements you will see
The numbers below use the examples in this post and the series and parallel checks. All three forms must agree for the same circuit.
- Voltage from current and resistance. E equals I times R. In the example, 0.5A times 10 ohms gives 5V.
- Current from voltage and resistance. I equals E divided by R. In the example, 5V divided by 10 ohms gives 0.5A.
- Resistance from voltage and current. R equals E divided by I. In the example, 5V divided by 0.5A gives 10 ohms.
- Series check. In the series example, 12V divided by 10 ohms gives 1.2A, and 1.2A times 10 ohms gives 12V. The same current, total resistance, and total voltage must agree.
- Parallel check. In the parallel example, 12V divided by 8 ohms gives 1.5A total. The branch currents, 1A and 0.5A, add to 1.5A.
- Direction of change. For a fixed resistance, current follows voltage. For a fixed voltage, current falls when resistance rises.
A common mistake
The most common mistake is to use the wrong two values in the formula. The voltage, current, and resistance must belong to the same part of the circuit, measured at the same test points. Do not use the applied voltage for the whole circuit when you want the drop across one part in series, and do not use the total resistance when you want the current in one parallel branch. Pick the part, pick the test points, then use the values for that part.
A second mistake is to trust one calculated value and stop. If the calculated resistance does not fit what the resistance check shows, or the calculated current does not fit what the meter shows, something is wrong with a test, a connection, or the circuit path. Retest the two known values first. Do not replace a load on one calculation alone.
Check it in the field
- Record two measured values and the test points. Note whether the circuit was live for a voltage or current check, or off for a resistance check.
- Use the correct form for the unknown value. Cover the unknown in the triangle, then write the formula, the numbers, and the answer before you act on it.
- Compare the calculated value with a direct measurement where you can make one. In the example, a calculated 10 ohms should agree with the resistance check, and a calculated 0.5A should agree with the current check.
- Cross-check with the circuit rules. In series, check that the current is the same and the voltage drops add to the applied voltage. In parallel, check that the voltage is the same and the branch currents add to the total.
Make resistance checks with the power off, and prove the circuit dead with a voltage test first. Keep hands clear of bare parts during live voltage and current checks. If a reading does not agree after a retest, stop and check the meter leads, the range, and the test points before you change a part.
Related posts
These posts belong together.
- Voltage, current, and resistance summary: a short recap of voltage, current, and resistance together.
- Voltage: the push that makes current flow, and how to read a voltage test.
- Current (amps): what flow means, and how to check it without guessing.
- Resistance: what opposition to flow means, and why a load must have resistance.
- Series Circuits: one path for current, with the same current through each part.
- Parallel Circuits: more than one path for current, with the same voltage across each branch.