# GTB Capacitor network



## Rocksalt (Mar 15, 2006)

I had the bright idea of checking how much my diffs were slipping by holding the car down on the ground and applying some throttle. When I did this the 5600mf cap, blew a lead going to the little board. I couldn't just leave well enough alone and tried to solder the lead back on. For some reason my solder would not stick the 2 leads together. I figured, 'fawk it' I'll run it like this and see what happens.... Noticed a definate power loss off line, no big deal. Now, let's just run this puppy down the road and check out the top end. As I drove it back to me to check it out one of the transient votlage suppressors went POOF and caught the shrink wrap on fire. Took it into work this morning and repaired the 5600 cap, soldered back on one of the capacitors, removed the rest of the blown TVS and am wondering:

#1 if I can run it with one less TVS until I get a new one.

#2 How much current runs though this capacitor network? It appears that the lead size on the cap is to small to handle large amounts of current. The failure may not have been due to current. The way the cap is soldered to the board leaves something to be desired. It was not glued to the board, but only shrink wrapped to it. Maybe the jarring of the cap during running caused stress on the leads, making them weak. Then when the cap discharged at a high rate it blew open the lead. If this were to happen to someone else it could be more detrimental. Once the main cap blows the current passes though the 2 TVS's and the 3 other small caps. Then... what happens if one of the caps short out?!? Then it would short out the supply voltage until either the board frys or the batterys blow up.

#3 If holding the car down so that the wheels don't turn, then applying power, will this always cause a problem? What if you are racing and the driveline becomes locked up. You naturally apply more power to make it move and POOF. Shouldn't this capacitor network be fused to avoid such disasters?

Does anyone have any part numbers for this CB?

Would increasing the cap capacity help in more torque down low. I can see what it's doing to the brushless system to increase 'punch'. More capacitance should make the punch last longer, no?


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## Fred B (Sep 26, 2001)

I don't know what the part number is for the cap on the GTB. You could shoot an email to Charlie at Novak and find out.

The reason that the cap gives you more punch is because it makes the speed control more efficient. The high frequency cap allows the fets to switch on completely faster. Without the cap the fet's switch on more slowly giving less power and becoming less efficient. Fet's are only efficient when they're on (100%) or off. Not in between.

FB


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## Rocksalt (Mar 15, 2006)

The cap network is connected to the input of the motor. This tells me(newbie) the caps are used for high capacitive discharge to the motor for startup current. How does this affect the on/off switching to the field effect transistors? 

Trying to learn that's all.


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## Rocksalt (Mar 15, 2006)

Ok, I did a little research. The capacitor does not help the FET's switch on and off faster. At least my understanding. When at partial throttle the FET's are having to switch on and off at a high rate. As you turn on and off anything you create voltage spikes. The capacitor is used to store these spikes or rather buffer them. When the voltage drops lower than the capacitor's stored voltage it will try and maintain that lower voltage longer, until it cannot keep up anymore. This reduces the voltage drop considerably unless the load stays high and the capacitance is used up. This buffer area will reduce the high current spikes and in effect reduce current going through the FET's.

I am still not sure why the cap leak arc'd off under the load I gave it. Perhaps the lead was not up to snuff, I'm not going to try it again anytime soon to experiment.


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## Fred B (Sep 26, 2001)

If you look at the leads, for the power cap on any of the current speed controls, it's going to the battery side. It's not running to the motor side of the speedo if it's hooked up properly.

The plain and simple fact is that the cap is there to give the fet's the high frequency power that they need to switch on quickly. This is how it works. There have been many people that have tried to argue the theory but the circuit is what it is and nothing more. The cap has nothing to do with how often the fet's are switched but how fast they turn on.

For your research try emailing Charlie at Novak.


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## Rocksalt (Mar 15, 2006)

heck, I looked at the diagram and it looked like the cap network connected to the output to the motor. Still don't understand how it makes the FET's turn on faster. For this one I will believe you instead of opening my ill educated mouth!

Thanks for straightening that out!


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