By Papa Bale · April 5, 2026 · 2N3055, TIP31, and More
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.
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:
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 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 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
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
While traditional pulse motor designs use Bipolar Junction Transistors (BJTs) like the 2N3055, many advanced builders are switching to MOSFETs. Here's how they compare:
The main barrier to using MOSFETs in simple pulse motor circuits is the gate drive requirement. MOSFETs need a specific voltage at the gate to turn on fully (typically 10-12V for power MOSFETs). The small signal from a trigger coil often isn't enough. Solutions include:
For beginners, stick with BJTs. For advanced builders optimizing for efficiency, MOSFETs are worth exploring.
The back EMF spike from the coil can reach 100V or more — enough to destroy an unprotected transistor. Proper protection is essential:
Every pulse motor circuit needs a flyback diode (also called a freewheeling diode or snubber diode) across the coil. This diode provides a path for the back EMF current when the transistor turns off, preventing voltage spikes. Use a fast-recovery diode like the UF4007 or 1N4007.
In Bedini-style circuits, add a diode (1N4148 or 1N4001) from the transistor base to ground (cathode to base). This protects against negative voltage spikes that can damage the base-emitter junction.
For advanced protection, some builders add an RC (resistor-capacitor) snubber across the coil or transistor. This absorbs high-frequency transients that even fast diodes might miss. A typical snubber uses a 100Ω resistor in series with a 0.1µF capacitor.
The key to reliable operation is ensuring your transistor can handle what your coil demands:
Measure your coil's DC resistance with a multimeter, then calculate:
Peak Current = Supply Voltage ÷ Coil Resistance
For example, with a 12V supply and 3Ω coil: 12 ÷ 3 = 4A peak current. Your transistor must have a collector current rating (Ic) higher than this value — preferably with a 50% safety margin.
The transistor's Vceo rating must exceed your supply voltage plus the back EMF spike. For a 12V supply, use a transistor rated for at least 60V (like the 2N3055) to handle the inductive kick safely.
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.
No — not all NPN transistors are suitable. You need a power transistor capable of handling the current your coil will draw. Small signal transistors like the 2N2222 will burn out immediately. Look for transistors with Ic ratings of at least 3A and Vceo ratings of at least 60V. The 2N3055, TIP31C, and TIP35C are proven choices.
Transistor heating is caused by: (1) excessive current through the coil, (2) insufficient heatsinking, (3) slow switching causing the transistor to spend too much time in the linear region, or (4) operating without a flyback diode (which can cause avalanche breakdown). Check your coil resistance, add a proper heatsink, and ensure your protection diodes are correctly installed.
Yes, Darlington transistors like the TIP120 can work in pulse motor circuits. They offer very high current gain (hFE), meaning they need very little base current to switch. However, they have higher saturation voltage, which means more power dissipation as heat. For high-efficiency builds, standard single transistors are usually preferred.
The 2N3055 is an NPN transistor; the MJ2955 is its PNP complement. They have similar ratings but opposite polarity. For standard pulse motor circuits, you want the NPN version (2N3055). PNP transistors would require a significantly different circuit design with negative supply rails.
Use a multimeter in diode test mode. Between base and emitter, and base and collector, you should see a diode drop (0.5-0.7V) in one direction and open circuit in the other. If you see short circuits (0V) in both directions, or open circuits in both directions, the transistor is likely damaged. A blown transistor often shows visible signs like cracks, burn marks, or a burnt smell.
See transistor selection and circuit assembly in action on YouTube.