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Old 2015.05.05, 01:55 PM   #9
TheSteve
VE7FM
 
Join Date: Oct 2010
Location: Langley, BC
Posts: 629
Jshwaa - Just as a bit of a bit of info. I don't come here to wage technical war with others or brag about my technical knowledge. I come to this forum because I enjoy Mini-Z's.

Now to address your reply...

I didn't go into further detail because your reply was off topic. This thread is about why a stock motor is faster in reverse then forward. Plain and simple it is related to the timing of the motor(the angular relationship of the brushes to the permanent magnets).

As you would like further info though related to my reply here it is.

Your statement "It is due to the path of current being lower for reverse than forward under the stock-like FET configurations." is still 100% not true. The motor drive of a Mini-Z is a standard H-bridge as your own diagram indicates. The total resistance of forward and reverse drive is the same as the current flows through both a P and an N junction no matter the direction of the motor.

As to your second point regarding lowering the gate pull up resistance on the P channel fet your reply was phrased such that it really seemed as though you were thinking in the bipolar realm. So assuming you understand that a mosfet type device is voltage driven and not current driven, then yes, turning a fet on faster will reduce the time it spends in the linear region. But really we should dig a little deeper shouldn't we? If you're going to lower that resistance will it really turn the fets on quicker? Well that depends on the transistor driving that resistance. It may respond slower if it doesn't have enough gain or has a high internal resistance on its own. Further if your specific board does perform better with a lowered resistance will all hardware variations of the Mini-Z controller perform better? Keep in mind the older hardware generally used cheaper/worse components as the technology/hardware used was spec'd 10 years ago.

Then there is the side effect of switching mosfets quicker - noise. Even the highest end electronic speed controls in the world purposely use higher value resistors to slow switching time to reduce the interference the speed control emits. This is pretty critical when it is just inches from a receiver - sometimes a very sensitive 27 MHz AM.

So even if your "mod" happens to work for you it is poor judgement to suggest others do it without understanding the potential risks. They could end up ruining the race for someone else because the car is now a random noise generator.

btw, the PTC (or polyfuse) that Kyosho adds has the same resistance no matter the direction of current flow so it doesn't come into play here.

And lastly let me add again that I don't come here to try to match technical wits with others or brag about my education or experience. I come here for the enjoyment of the hobby. I totally respect your sense of adventure and desire to modify, customize, improve etc. I also appreciate that you want to share your knowledge with others. Just keep in mind what seems like an improvement to you might not be for others or in the best interests of others.
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