Open Circuit Measurements – a BAD Idea


Open circuit measurements are never a good idea! This type of measurement is done with the circuit under no load and unwanted resistance of several million ohms may not show leading to a possible wrong diagnosis. In most cases opening the circuit will not affect your measurement. But, there will be the times that it will and you may end up chasing your tail for a whole shift without solving the problem.




A = Amperage or current

E = Voltage

I = Current or amperage

V = Voltage

R = Resistance

Rt = Resistance Total

It = Current Total

Vd = Voltage Drop



Ohm’s Law:

Before being able to understand why open circuit measurements are not reliable; let’s review the concept of ohm’s law. It explains the relationship between voltage, current, and resistance. Simply stated “it takes 1 volt to push 1 amp through 1 ohm of resistance”.

In a electrical circuit, the amount of current flow is what performs work. The higher the current flow is the more work that is performed, this may not necessarily be a problem unless the circuit fuse is blowing. On the other hand lower current flow will perform less work, this is a problem because the load/s in the circuit will not perform the require work. Any deviation in the voltage or resistance requirements will affect the current flow; thus, affecting the amount of work performed.


To perform circuit calculations the following formulas are used:

(Volts) V = R x A

(Resistance) R = V/A

(Amps) A or I = V/R

Note: Current has different names and symbols. It can be referred to as amperes or amps. The symbols in Ohm’s law can be I or A.


The Ohm’s Law pie or triangle chart is often used to simply the understanding of the formulas.

ohms law, V=Axr



Series Circuits:

A series circuit has only one path for current to flow, the following rules apply to series circuits:


1. Total resistance is the sum of each individual resistance.
2. Current flow is the same in the entire circuit.
3. The sum of voltage drops equal source voltage.
4. Voltage drop is proportional to the resistance of each component.


Let’s  use the circuit below to perform series circuit calculations. Refer to abbreviations section for formula nomenclature. 

Series Circuit Calculations

The formula for series circuits is Rt = R1 + R2 + R3 + Rn……

Rt = R1 + R2 = 5 + 5 = 10Ω

It = 12/10 = 1.2 A

Voltage Drop of the 1st light bulb is:

Vd1 = R x A = 5 x 1.2 = 6 volts ( this light bulb uses 6 of the 12 volts supplied)

Voltage Drop of the 2nd light bulb is:

Vd2 = R x A = 5 x 1.2 = 6 volts ( this light bulb uses the remaining voltage)

Total Circuit Voltage Drop is:
6 + 6 = 12v (same as source voltage)

Now let’s insert some unwanted resistance into the circuit.

series circuit, unwanted resistance, ohm's law

Rt = R1 + R2 + R3 + R4 = 1 + 5 + 5 + 2 = 13Ω

It = V/A = 12/13 = .923A (923 mA)

Voltage Drop through the bad connection:

Vd1 = R x A = 1 x .923 = .923V ( the bad connection uses almost 1 volt of the supplied voltage; this will affect the performance of the light bulbs (loads) in the circuit).

Voltage Drop of the 1st light bulb is:

Vd2 = R x A = 5 x .923 = 4.62 volts 

Voltage Drop of the 2nd light bulb is:

Vd3 = R x A = 5 x .923 = 4.62 volts

Voltage Drop of the corroded ground:

Vd4 = R x A = 2 x .923 = 1.85 volts ( this connection is using almost 2 volts; this will affect the performance of the light bulbs (loads) in the circuit).

Total Voltage Drop of this circuit is:

Vd1 + Vd2 + Vd3 + Vd4 = .923 + 4.62 + 4.62 + 1.85 = 12 volts



Parallel Circuits:

A parallel circuit has more than one path for current flow, the following rules apply to parallel circuits:


1. Amperage flow through each path depends on its resistance.
2. Total current is the sum of the current flow in each path.
3. If one path of circuit is broken, current will continue to flow in the other path.
4. Total circuit resistance always less than resistance of smallest resistor.


The formulas for parallel circuits are:

It = I1 + I2 + I3 + 1n……….


So, let’s use the circuit below to practice these formulas. Refer to abbreviations section for formula nomenclature.

parallel circuit, ohm's law

The total resistance of this circuit using both formulas is 2.5Ω:


The total current flow for this circuit is:

It = V/R = 12/2.5 = 4.8 A


Sample Circuit Calculations:

So, why are open circuit measurements a bad idea? When the circuit is open to measure voltage; the meter becomes a load in the circuit effectively creating a series circuit. Like in any other series circuit the voltage is divided between the loads proportional to the resistance. 

The total resistance of this circuit is:
Rt = 6 + 6 =12
The current flow through this circuit is:
A= V/R
A= 12/12
A = 1amp
So, the voltage drop of the bad connection will be:
V= R x A = 1 x 6 = 6 volts
The voltage available to the light bulb will be 6 volts which will make the bulb dim or inoperative.

ohms law, calculations, bad connection



DMM Connection:

The proper method of connecting the DMM to measure voltage is to connect the meter in parallel to the circuit; such as shown in the drawing below. The meter will take whatever voltage is available at the positive lead and drop that voltage thru its internal resistance. Using the numbers from the previous drawing; the available voltage after the first resistance (bad connection) is 6 volts. Since only 6 volts are available at that point that is what the DMM will drop and display as the measured value.

measuring voltage, dmm voltage

DMM Connection: Circuit Open = WRONG!

So, with the circuit disconnected and the DMM connected, you are now working with a series circuit. in the example given the meter is the second load. 


Going back to using Ohm’s Law, resistance in a series circuit is the sum of all. So in the circuit we are working with, Rt=  6 + 10,000,000 = 10,000,006
Our current flow for this circuit will be: A= V/R, A= 12/10,000,006 = .0000012 amps
That means our voltage drop across our loads are: V= A x R
Bad Connection:
V= 6 x .0000012 = .0000072 V
V = 10,000,000 x .0000012 = 12 V
The DMM will display 12V, which may lead you to conclude that the component (light bulb) is defective. When in reality the connection is defective, but, you cannot see it since you are measuring the wrong way! Remember….NEVER open the circuit when measuring voltage!

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