I own a bare 6WD chassis "Wild Thumper" by Dagu, designed by Oddbot. And I have (2) "racing packs". Those are basically 6 NiCd cells in series, providing 7.2 Volts nominally. The motors are rated for 6 V. Stall current is listed as 5.5 A.
Even the most basic speed control will need some sort of amplifier for the PWM signal. Let's start very simple. Just speed control. Just one motor. One direction. What kind of transistor shall I buy? And what kind of circuit shall I use?
I know about BJT transistors and I do not really worry about the 0.7 V drop across Collector-Emitter. Or about heat sinks. What would be the (minimum) transistor to buy? What characteristics am I looking at?
Some would say FET transistors are a better choice. Tell me why. Remember, I am a FET newbie.
I bought a few of everything, except relays. Wow, those are pretty expensive, compared to solid state transistors. Even when buying four power transistsors for a full H-bridge, you’d still be cheaper off than buying one DPDT relay for the same rated current.
I’ve used the complementary IRF540 N-channel and the IRF9540 P-channel FETs for building the motor driver in Tiny Tim and they do an excellent job in my opinion plus they’re fairly cheap.
If you’re going for a cheaper solution, I’d look at BJTs like the complementary TIP-120 and TIP-125 which are rated for 5A continuous operation and 8A pulse operation. They come really cheap especially when bought in larger quantities.
FETs have the nice property that they don’t consume power in the on state like BJTs do. They only consume power while switching. This will not make much difference for a motor driver with a small amount of transistors though - it’s more of a nice-to-know thing.
