Reading Oscilloscope Output on a Pulse Motor:
What to Look For

A multimeter tells you average voltage and current. An oscilloscope shows you what's actually happening in your pulse motor circuit in real time — the shape, timing, and amplitude of every pulse. Once you've seen a back-EMF spike on a scope, you understand the circuit in a fundamentally different way. Here's how to read oscilloscope output on your pulse motor and what the key features mean.

Do You Need an Oscilloscope?

Strictly speaking: no. Many builders get excellent results with just a multimeter. But an oscilloscope reveals phenomena that a multimeter completely misses:

• The actual peak voltage of the back-EMF spike (which can be 5–10× supply voltage)
• Pulse width — how long the coil is energized per pulse
• Pulse timing relative to rotor position
• Ringing oscillations after the transistor switches off
• Whether the transistor is fully saturating (switching cleanly) or partially conducting

A cheap entry-level scope — even the DSO138 kit (~$30) or a Hantek 2 channel ($60–80) — is enough to see all of these. A scope transforms debugging from guesswork into diagnosis.

Where to Probe: Key Measurement Points

Three main probe positions for a basic SSG-style pulse motor circuit:

1. Collector of the Transistor

This is the most revealing probe point. Connect your scope probe to the transistor collector, ground clip to circuit negative. What you'll see:

• Low voltage plateau — transistor is ON, current flowing through coil, collector near ground
• High voltage spike — transistor switches OFF, back-EMF spike appears. This is often shown as a negative-going spike if the collector goes high and the scope trigger isn't adjusted, or a positive spike above supply voltage depending on your circuit configuration.
• Supply rail voltage — resting state between pulses when transistor is off

2. Base of the Transistor

Probe the transistor base to see your trigger signal. You should see clean switching between low (transistor off) and approximately 0.7V above emitter (transistor on, base-emitter junction conducting). If the trigger signal is noisy, rounded, or slow-edged, your transistor may be slow to switch, causing inefficiency and heat.

3. Output Battery or Capacitor Terminals

If you have a secondary charging battery or cap bank, probe across it to watch voltage rise over time. On a fast time scale, you'll see individual charge pulses arriving. On a slow time scale, you'll see the cumulative voltage climb.

Reading the Back-EMF Spike

The back-EMF spike is the most important waveform feature in a pulse motor circuit. When the transistor cuts off, the coil's collapsing magnetic field generates a voltage spike that can reach 50V, 100V, or higher from a 12V supply.

On your oscilloscope:

• Spike amplitude — higher spikes indicate more inductive energy stored. Larger coils with more turns produce bigger spikes. This is what charges your secondary battery.
• Spike duration — sharper (shorter) spikes indicate a fast transistor switch. Slow transistors create slow-edged spikes where the energy dissipates as heat in the transistor rather than transferring to the output.
• Ringing after the spike — oscillations following the spike are parasitic resonance between coil inductance and stray capacitance. Some ringing is normal; excessive ringing can interfere with the trigger circuit.

Pulse Width and Duty Cycle

Zoom in on a single pulse on your scope. The ratio of "on time" (coil energized) to "off time" (coil de-energized) is the duty cycle. For most pulse motor circuits, this is naturally determined by the physical geometry — how long a rotor magnet faces the trigger sensor as it sweeps past.

A typical well-tuned pulse motor might show 10–20% duty cycle — meaning the coil is on for only 10–20% of the rotation period. This is a key advantage of pulse motors: they draw current only briefly each cycle, which is why average current draw can be remarkably low even when peak currents during the pulse are high.

What Good Looks Like

A healthy, well-tuned pulse motor waveform at the collector shows:

• Clean, fast-rising pulse edges (transistor saturates quickly)
• Low collector voltage during the ON state (transistor fully saturated, not partially conducting)
• Sharp, high-amplitude back-EMF spike at switch-off
• Stable pulse repetition with consistent spacing (stable motor speed)
• Minimal ringing after the spike (good circuit layout, appropriate diode)

What Bad Looks Like

• Slow edges — transistor taking too long to switch. Causes heat, inefficiency. May need a faster transistor or lower base resistor value.
• High collector voltage during ON state — transistor not fully saturated. Often a base resistor that's too high, starving the base current.
• Erratic pulse spacing — unstable triggering, possibly a loose coil or sensor, or a magnet with incorrect polarity.
• Missing spikes — the flyback diode may be shorted, or the coil isn't building up enough current before the transistor cuts off.