Using a Scope for Diagnostics
Scopes give you eyes for trouble shooting. A multi-meter is helpful, but only shows numeric values. Scopes display voltage amplitude on a time scale. Newer scope have automatic measurements, showing voltage, frequency and more. There are knobs that adjust amplitude, and time scale, and trigger point on waveform. Once familiar with knobs, not much different than driving an automatic, using throttle, steering wheel and brake.
Scopes are now affordable. Dual channel scopes are available starting at $100 to $300,, for something suitable, for typical automotive work. Automotive work for trouble shooting sensors, ignitions, EFI, and alternators, is less demanding than working with ECU internals. Expensive scopes can cost thousands, but most internal ECU work, can be done with a logic analyzer PC dongle, for $15 to $300.
$150 will buy this 4 channel (2 analog, 2 digital) it measures about 2" x 4" x 1/4".
$300 will buy this color 2 channel, it requires a power cord, so not portable.
View attachment 1715699059
The first scope actually has a few more features than second. It could be used for under hood or test drive, the second is best for bench testing or development work. This post is not about scope review, so enough about that.
Automotive work for observing coil primary voltage requires a high voltage probe. I purchase a 2kV probe for $25 on ebay. Standard probes are often limited to 100 to 300V max. Do not attempt to measure secondary voltage, it will require huge and expensive 100kV probe. The primary voltage times coil turns ratio is a good estimate for secondary voltage.
To use this probe on mini scope, it needs a BNC adapter, shown in clear plastic bag. Probe shown included with mini scope, show for connector example.
Since scopes measure voltage, current is measured with a current probe, or a current to voltage converter. Current probes are expensive, they often have isolation and high frequency capability, gain and other features and are used for development of power conversion electronics. A simple low value power resistor, is a passive current converter, often called a metering shunt. I = V/R. The shunt is placed in series with load. If a 0.1 Ohm resistor has a voltage of 0.1 V one 1A flows.
Below is an example using a scope for measuring an ignition coil primary current, and voltage. The scope used is my favorite, but about 25 years old. The current was measured with a shunt on ground leg of coil drive transistor. Scope grounds are typically common! Beware how they are connected! Do not place grounds across voltage! There are scopes with isolated inputs, but they often add hundreds for high voltage isolation. Isolated scoped are used to develop 3 phase power controls, and other complex power electronics.
The top trace (chan 2) shows coil primary voltage, bottom trace (chan 1) shows coil primary current. Scope grounds connected to power ground. The peak ~210V in top trace (chan 2) is a spark event, it happens when bottom trace chan 1 reaches cycle maximum, then drops coil drive transistor opens. Peak current reached ~1.8A. I was using only 3.7V coil(+) instead of 13.8V, resulting extended coil charge time and reduced peak current and voltage. The COP coil is designed for 8.5A. This experiment is for test only, does not relate to a typical ignition, coil secondary, did have a sparkplug with 0.04" gap. Spark energy increases with square of coil current, an actual ignition would have ~ 22x more peak energy. Even with reduced current, spark was present. The peak coil voltage on channel 2 would limit to about 390V due to avalanche protection in transistor. Opening plug gap or failed connection to plug results in avalanche dissipation. There is much more available on this topic.
Scope Settings:
Scope setting vary by scope manual, so best to always read the manual first. There are typical setting available on nearly all scopes.
Scopes often have a common feature of a calibration signal available on front panel. It is typically a square wave at a specific frequency and amplitude. It is a good starting point to verify probes, and settings. It is a good place top start experimenting with settings.
Above picture shows scope calibration signal on CH1, it measures at 5V, and a frequency of 1kHz. Note that CH1 input setting is 1V, and "M" is 250us, one cycle is 4x250us or 1ms. Frequency is 1/Time, 1/0.001 is 1000Hz or 1kHz. Newer scopes often have "measure" feature shown in next picture, this reduces, counting divisions and calculations.
The picture below shows rounding of square waveform, when probe compensation adjustment was turned for maximum. This is incorrect adjustment, it will result in inaccurate (lower than normal) readings of transient signals and high frequencies.
Picture below shows the calibration set incorrectly, it will result in transients and high frequencies measuring too high.
Scopes give you eyes for trouble shooting. A multi-meter is helpful, but only shows numeric values. Scopes display voltage amplitude on a time scale. Newer scope have automatic measurements, showing voltage, frequency and more. There are knobs that adjust amplitude, and time scale, and trigger point on waveform. Once familiar with knobs, not much different than driving an automatic, using throttle, steering wheel and brake.
Scopes are now affordable. Dual channel scopes are available starting at $100 to $300,, for something suitable, for typical automotive work. Automotive work for trouble shooting sensors, ignitions, EFI, and alternators, is less demanding than working with ECU internals. Expensive scopes can cost thousands, but most internal ECU work, can be done with a logic analyzer PC dongle, for $15 to $300.
$150 will buy this 4 channel (2 analog, 2 digital) it measures about 2" x 4" x 1/4".
$300 will buy this color 2 channel, it requires a power cord, so not portable.
View attachment 1715699059
The first scope actually has a few more features than second. It could be used for under hood or test drive, the second is best for bench testing or development work. This post is not about scope review, so enough about that.
Automotive work for observing coil primary voltage requires a high voltage probe. I purchase a 2kV probe for $25 on ebay. Standard probes are often limited to 100 to 300V max. Do not attempt to measure secondary voltage, it will require huge and expensive 100kV probe. The primary voltage times coil turns ratio is a good estimate for secondary voltage.
To use this probe on mini scope, it needs a BNC adapter, shown in clear plastic bag. Probe shown included with mini scope, show for connector example.
Since scopes measure voltage, current is measured with a current probe, or a current to voltage converter. Current probes are expensive, they often have isolation and high frequency capability, gain and other features and are used for development of power conversion electronics. A simple low value power resistor, is a passive current converter, often called a metering shunt. I = V/R. The shunt is placed in series with load. If a 0.1 Ohm resistor has a voltage of 0.1 V one 1A flows.
Below is an example using a scope for measuring an ignition coil primary current, and voltage. The scope used is my favorite, but about 25 years old. The current was measured with a shunt on ground leg of coil drive transistor. Scope grounds are typically common! Beware how they are connected! Do not place grounds across voltage! There are scopes with isolated inputs, but they often add hundreds for high voltage isolation. Isolated scoped are used to develop 3 phase power controls, and other complex power electronics.
The top trace (chan 2) shows coil primary voltage, bottom trace (chan 1) shows coil primary current. Scope grounds connected to power ground. The peak ~210V in top trace (chan 2) is a spark event, it happens when bottom trace chan 1 reaches cycle maximum, then drops coil drive transistor opens. Peak current reached ~1.8A. I was using only 3.7V coil(+) instead of 13.8V, resulting extended coil charge time and reduced peak current and voltage. The COP coil is designed for 8.5A. This experiment is for test only, does not relate to a typical ignition, coil secondary, did have a sparkplug with 0.04" gap. Spark energy increases with square of coil current, an actual ignition would have ~ 22x more peak energy. Even with reduced current, spark was present. The peak coil voltage on channel 2 would limit to about 390V due to avalanche protection in transistor. Opening plug gap or failed connection to plug results in avalanche dissipation. There is much more available on this topic.
Scope Settings:
Scope setting vary by scope manual, so best to always read the manual first. There are typical setting available on nearly all scopes.
Scopes often have a common feature of a calibration signal available on front panel. It is typically a square wave at a specific frequency and amplitude. It is a good starting point to verify probes, and settings. It is a good place top start experimenting with settings.
Above picture shows scope calibration signal on CH1, it measures at 5V, and a frequency of 1kHz. Note that CH1 input setting is 1V, and "M" is 250us, one cycle is 4x250us or 1ms. Frequency is 1/Time, 1/0.001 is 1000Hz or 1kHz. Newer scopes often have "measure" feature shown in next picture, this reduces, counting divisions and calculations.
The picture below shows rounding of square waveform, when probe compensation adjustment was turned for maximum. This is incorrect adjustment, it will result in inaccurate (lower than normal) readings of transient signals and high frequencies.
Picture below shows the calibration set incorrectly, it will result in transients and high frequencies measuring too high.
