High-Voltage Flyback Measurements
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A common task performed in the automotive industry is the acquisition of a flyback voltage on an ignition coil. An ignition coil creates the high voltages used to generate sparks in the engine and is made up of a primary coil and a secondary coil. The secondary coil generally has many more turns of wire than the primary coil. This is important because the secondary voltage is simply the turns ratio times the voltage applied to the primary coil. The current flow through the primary coil creates a strong magnetic field between the two coils. When the current is suddenly shutoff, the collapse of the magnetic field induces a large voltage (+20,000 V) onto the secondary coil. This voltage is then routed to the spark plugs. Because the voltages are so high on the secondary coil, all of the tests are made on the primary coil. The flyback waveform is usually on the order of 10 microseconds and has a peak voltage of 40 to 400 V, depending on the ignition coil. The common measurements are the peak firing voltage, dwell time, and burn time.
Peak voltage = the maximum voltage generated by the ignition coil
Dwell time = the amount of time the current is flowing through the primary coil
Burn time = the amount of time it takes the flyback voltage waveform to return to steady state
To accurately analyze a flyback voltage waveform, you need the following capabilities:
Traditionally, this waveform was measured using a specialized and expensive digitizer that has high-voltage capabilities. Alternatively, you could use a general purpose 8-bit oscilloscope with an attenuation probe, but this method reduces your accuracy dramatically.
Figure 1: FlexDMM acquiring a Flyback voltage
Using National Instruments PXI-4070 FlexDMM digitizer capability and the NI LabVIEW analysis functions you can build a flyback voltage measurement system, as shown in Figure 1. In digitizer mode, the FlexDMM has the ability to acquire up to 425 V peak AC waveforms at a maximum sample rate of 1.8 MS/s. The FlexDMM has 10 bits (noise-free) at the 1.8 MS/s sample rate, but with the "Flex" architecture of the FlexDMM you can decrease the sample rate to 10 S/s and 23 bits of resolution. Once you have acquired the flyback voltage waveform with the FlexDMM, you then use the following LabVIEW analysis VI's: Waveform Peak detection.vi and Transition measurements.vi.
Related Links:
Use The PXI-4070 FlexDMM's 1.8 MS/s Digitizer capability to Make Power Measurements
Reader Comments | Submit a comment »
Was this waveform actually sampled faster?
Am I missing something? The waveform
that appears in the graph appears to have
been sampled at >20MS/s. The details
that are shown could not come out with
the 555nS samples taken by the FlexDMM
at 1.8MS/s. Please set me straight.
- Thomas Schaefer, Wells Manufacturing Corp.. tschaefer@wellsmfgcorp.com - Sep 4, 2003
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