Let's Make Robots!

Motor Control for 6WD Wild Thumper

Open call for suggestions:

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.

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Update: I now own te "official" Wild Thumper motor driver, or should I say Robot driver? Call it a complete Thumperuino!

Look forward to a review.

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.


The bad news is that the factory which was supposed to produce a speed controller for us has failed miserably. The good news is that I am now designing one myself (if you want something done right...)

The bad news is it will still be weeks before I have a final product ready to sell. The good news is that Pololu now sell the 6WD chassis and have a few suitable drivers available. Robot shop will also sell them but are a bit slower setting up the webpage.

If you do wait for one of mine here is a list of "planned" features: (No promises).

1. Controlled by a built in Arduino Nano for hackability.
2. Can charge up to 4x 7.2V NiMh / NiCd Battery packs.
3. 5.4V @ 6A supply rail for servos.
4. 5V @ 1.5A for logic sensors.
5. USB interface with 3.3V output.
6. 2x 20A FET "H" bridges with brake ability.
7. Able to acept serial, analog and RC inputs.


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.

Am I shopping at the wrong place?

Conrad is a very high priced webshop. My favorite webshop for electronic components in europe is tme. Fair prices, fast delivery, low shipping costs.

One thing to remind: the prices are without tax and in polnish zloty (can be changed to Euro).





A suitable FET transistor when saturated can have a resistance of less than 0.05 ohms allowing plenty of current to flow without the FET getting too hot.

A BJT with a similar current rating will convert a lot more of your precious battery energy into wasted heat while limiting your motors performance. A big heatsink will also waste your space.

Suggest me a modest FET to play with. Should handle the stall current of one motor. Just the easiest scenario, so that I can compare. Preferably one I can buy at Conrad.com.

Forward rotation, speed control by PWM - picaxe. Motor power source 7.2 V. Logic power 5.0 V.

I would suggest the IRLZ34N from International Rectifier, a cheap logic level type N-FET, available at Conrad.

The important thing is to get a logic level type. Only this types can be switched on/off directly from a microcontroller at 5V. The L in the type name indicates a logic level FET. Unfortunatly there doesn't exist a similar logic level P-FET for building a H-Bridge.

Even with a deep link to conrad. That's impressive. My deeplinking always broke on website session management.

Look for 'direct product link' or 'link to this article' to get a popup window with a clean link to the article without session ID.