How many times have you heard that breadboards are unreliable or unusable at hight frequency due to stray capacitance? Perhaps too many times with little to support the assertion. This article suggests a breadboard capacitance between 2 and 5 pF between the breadboard’s adjacent 5-terminal sections. This limits the breadboard to a maximum frequency between 100 kHz and 1 MHz. These observations are supported with both a direct measurement and an indirect method that uses the breadboard capacitance as a component of the oscillator as shown in Figure 1. This oscillator demonstrates that the breadboard capacitance isn’t large. In fact, building an oscillator using breadboard capacitance is surprisingly difficult.
Figure 1: Picture showing capacitors C1, C2, and C3 formed as seven-parallel plates using adjacent breadboard sections.
How much capacitance is associated with a breadboard?
The short empirical answer is that there is about 2 pF capacitance between each of the breadboard’s adjacent 5-terminal connection. This can be measured directly using an instrument such as the Digilent Analog Discovery and associated Impedance module such as pictured in Figure 2.
Measurement procedure
For this experiment a parallel plate capacitor was formed by connecting adjacent breadboard sections. The resulting seven-parallel-plate capacitor is shown in Figure 2. The small, expanded image in Figure 2 highlights the capacitor sections using alternating yellow and red colors. Also visible in Figure 2 is a 1 MΩ resistor connected in parallel with the capacitor. This resistor provides a fixed resistance to which the capacitive reactance may be compared.
Figure 2: The capacitance of the seven-parallel-plate capacitor is measured using the Digilent Analog Discovery via the impedance analyzer module.
Tech Tip: How do you measure the thickness of a piece of paper? The general solution is to measure not one, but many sheets. For example, it is relatively easy to measure 100 pages and then perform a division operation to determine the average page thickness.
This many, instead of one, measurement concept is applied in this article to measure the capacitance of adjacent breadboard sections. Instead of measuring a small section, we measure many such sections operating in parallel and then perform a division. We trust that this action will improve the result by transposing the measurement into a position more favorable to our measurement device.
The measurement results are shown in Figure 3. For this frequency sweep, the resistor appears as a constant for all audio frequencies up to 20 kHz. Observe that there is some deviation around 1 MHz. This deviation in the highest frequencies may be an artifact of the measurement device as it becomes increasingly difficult to measure resistance when the impedance is dominated by the capacitor.
With respect to Figure 3, observe that the capacitive reactance decreases linearly as expected by this familiar equation:
X_C = \dfrac{1}{2 \pi fC }
The reactance of the seven-parallel-plate capacitor is in the GΩ region at the start of the frequency plot dropping to the low kΩ at the end. As a reference point, the resistance and reactance are equal to about 7 kHz.
1 \ M \Omega = \dfrac{1}{2 * \pi * 7 k * C}
It follows that C_{7-plate} \approx 23 \ pF
At this point we can convert the seven-plate capacitor measurement to the capacitance of a single parallel section of breadboard. From Figure 2, we see that our seven-plate capacitor is formed by 12 adjacent breadboard sections. This process suggests that the per-section breadboard capacitance is approximately 2 pF.
Do take this 2 pF measurement as general (ballpark) suggestion as it does not account for the items immediately surrounding the breadboard. For example, the measurement changes in the presence of hand capacitance.
Tech Tip: Hand capacitance is the unintended impact the presence the human hand has on RF measurements. In many situations, the circuit may be detuned as your hand approaches the circuit. Metal shielding of the circuit can mitigate this problem. Also, for those circuits that require tuning, a long handled tuning tool make the process easier.
Figure 3: Analyzer view of the parallel resistance and impedance form the seven-plate capacitor. The resistive and capacitive reactance values are both equal to 1 MΩ at 7 kHz.
Constructing an oscillator using breadboard-based capacitors
The prototype phase shift oscillator is shown in Figure 1 with the schematic included as Figure 4 and the oscillator’s output shown as Figure 5. Recall that there are two general conditions that must be met for a circuit to oscillate.
- There must be positive feedback
- The gain must be greater than unity
The positive feedback for this oscillator is derived from the phase shift associated with the three RC section where the third RC is associated with the virtual ground of the op amp. Each stage provides 60 degrees of shift. The gain is set by the R1 negative feedback resistor.
Figure 4: Schematic of the phase shift oscillator. Capacitors C1, C2, and C3 are constructed as seven-parallel-plate capacitors using adjacent breadboard traces.
Figure 5: Oscilloscope screen capture showing the performance of the oscillator.
Tech Tip: The squeal of a microphone placed too close to a public address speaker is a form of oscillation. The two conditions for oscillation are met when energy from the speaker is coupled back to the speaker. Note that the oscillation occurs at one frequency determined by the unique acoustic features of the electronics and room. Like the singing of a wine glass, the system resonates at this one special frequency.
The classic equations associated with this type of phase shift oscillator are:
f =\dfrac{1}{2 \pi R C \sqrt(6)}
and
\dfrac{R_1}{R_4} = 29
Breadboard capacitance verified using an oscillator technique
Knowing the resistors and the output frequency derived from Figure 5, we can calculate the capacitance.
23 \ kHz =\dfrac{1}{2 * \pi * 47k * C * \sqrt(6) }
If follows that calculated capacitance is about 60 pF.
Recall that the bulk capacitance is formed using 12 adjacent sections of the breadboard’s 5-terminal sections. This experiment suggests that each adjacent breadboard section has a capacitance of 5 pF. Once again, take these measurements with a grain of salt.
Oscillator limitations
Before departing we should mention that this is not a good oscillator. It has high distortion and is very finicky to get started as suggested by the distorted waveform in Figure 5. Understand that the design is dominated by the miniscule breadboard-based capacitors. The resulting frequency is uncomfortably high as are the chosen resistor values. It would be preferable to increase the capacitor value by an order of magnitude, but there is not enough breadboard real estate to do so. Despite these problems, the circuit has value. It shows that breadboard capacitance is very low at audio frequencies. We also had to go to great length to make this tiny capacitance perform useful work.
Difference in capacitor measurements between the two methods
With regards to the differences between the two capacitance measurements, the oscillator is considered the less reliable method. As we mentioned, it’s not a very good oscillator and it appears to deviate from the ideal equations such as the desired R1 value. The circuit may suffer from the inductance of the long wire runs. It may also suffer from non-ideal op amp input resistance especially when we consider the op amp input resistance to the excessive 1.8 MΩ feedback resistor.
Conclusion
This article presents two ways to measure breadboard capacitance. Both converge on an answer suggesting between 2 and 5 pF between any of the breadboards 5-terminal section. This capacitance is nearly invisible for audio circuit relative to reactance of typical components. However, the capacitance can be problematic at higher frequencies. This suggests a 100 kHz to 1 MHz frequency limit for the breadboard.
Many people claim breadboard capacitance negatively impacts their designs. This may be true. However, in my opinion, there are other breadboard gremlins that make themselves known well before capacitance is a problem. You can learn more by exploring this article that suggests a hard limit on breadboard current and this article that explores power supply decoupling.
Perhaps the biggest challenge with the breadboard is the experimental nature of prototyping itself. Not all ideas lead to success. Many will do nothing. Some will fizzle and smoke on the breadboard. However, these are valuable lessons that guide future designs.
Best Wishes,
APDahlen
About the Author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this. LinkedIn | Aaron Dahlen - Application Engineer - DigiKey
Return to the breadboard guide for additional construction tips.