Which Transistor for
Your Pulse Motor?

The transistor in a pulse motor circuit is the switch that fires the coil. Get it right and you have efficient, reliable switching. Get it wrong and you'll have an expensive paperweight with a heatsink. I've blown my share of transistors over the years — here's what I've learned about choosing the right one for your build.

What the Transistor Does in a Pulse Motor

The transistor acts as an electronically controlled switch. When the trigger signal arrives (from a Hall sensor, reed switch, or feedback coil in a Bedini SSG), it briefly turns ON, allowing current to flow from the battery through the coil. When the trigger stops, it turns OFF, cutting current abruptly and creating the back EMF spike.

Key transistor parameters for this job:

• Ic (collector current) — must be higher than the peak current through your coil
• Vceo (collector-emitter voltage) — must withstand back EMF spikes, which can be 5–10× supply voltage
• hFE (current gain) — determines how much base current is needed to switch the transistor fully on
• ft (transition frequency) — faster switching = cleaner pulses = better BEMF spike
• Package — TO-3 and TO-220 are most common; TO-3 dissipates heat better

The Classic: 2N3055

The 2N3055 is the transistor most closely associated with Bedini motor builds. It's a power NPN in a TO-3 metal can package, rated for 15A collector current and 60V Vceo. It's robust, widely available, cheap, and the Bedini community has decades of experience with it.

Pros: Proven, cheap (~$1–2 each), high current capacity, forgiving of abuse
Cons: Relatively slow (ft ~3MHz), requires good thermal contact in TO-3 socket, older design
Best for: Classic Bedini SSG builds, lower-speed rotors, anyone following traditional Bedini circuit designs

The Budget Workhorse: TIP31C

The TIP31C is a TO-220 packaged NPN transistor rated for 3A and 100V. It's extremely cheap (often under $0.50), easy to mount with a standard TO-220 heatsink, and good for beginner-level pulse motor builds where coil current is modest.

Pros: Very cheap, easy TO-220 mounting, 100V Vceo handles most BEMF spikes
Cons: Only 3A — limits coil size; lower hFE than 2N3055
Best for: Small beginner builds, learning the circuit before investing in better components

The High-Power Option: TIP35C / TIP36C

The TIP35C (NPN) is a serious upgrade — 25A, 100V, TO-218 package. When I'm building a coil with substantial current draw or pushing a larger rotor, this is my go-to. Good switching speed, robust construction, handles significant current spikes without drama.

Pros: High current (25A), 100V Vceo, robust
Cons: More expensive than TIP31, needs a good heatsink
Best for: Medium to large builds with heavier coils

The Modern Alternative: IRFP250N MOSFET

Experienced builders sometimes switch to MOSFETs instead of BJT transistors. The IRFP250N is a popular N-channel MOSFET with extremely fast switching, low on-resistance, and 200V rating. The catch: MOSFETs require a gate driver circuit and the trigger winding of a Bedini SSG doesn't directly drive them without modification.

Pros: Extremely fast switching, low heat, higher efficiency
Cons: More complex drive circuit required; not plug-and-play for classic Bedini SSG
Best for: Advanced builders optimizing for efficiency

Papa Bale's Picks

• Beginner / first build: TIP31C — cheap, easy, low risk if you wire it wrong
• Classic Bedini SSG: 2N3055 — use what John Bedini designed the circuit around
• Serious build / large coil: TIP35C — the upgrade path that doesn't require circuit changes
• Advanced efficiency experiments: IRFP250N with gate driver — when you want maximum performance

Don't Forget the Heatsink

Every transistor in a pulse motor circuit needs a heatsink. Even if the transistor barely warms up during brief tests, continuous operation generates real heat. A transistor running at 80°C will fail eventually. A properly heatsinked transistor running at 40°C will last for years. Use thermal paste between the transistor and heatsink — it makes a noticeable difference.