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Originally Posted by Joe Ross
This site shows what fets the different IFI and Innovation FIRST speed controllers use. http://www.enigmaindustries.com/links.htm Note that they all use different fets, with different maximum currents, however, all of them are rated for 30v. |
Joe,
Nice find on this site. It not only gives very specific info about the all the controllers available for robot builders, you can follow the links to the exact spec sheets for the FETs used in each.
Now on to answering the other pending questions.
1. 2 12 volt fans in series may or may not work depending on the fan. Brushless DC fans may get very unhappy being in series.
2. As pointed out in the above website, the FETs used on most of the IFI controllers have a breakdown voltage of 30 volts. That would make you guess that 30 volt power supply should allow safe operation but... other things to consider are the interface electronics and internal power regulators may not be able to handle voltage higher than the 6-15 volt range specified by IFI. The FET is not the limiting factor, there is so much more inside those little controllers.
3. Sanddrag, could you elaborate on the time period a little more, I am confused on that one.
4. The overall characteristics of these controllers is a function of the junction temperature (the actual temperature of the solid state material inside the case), load and other things. Having a good fan does not help a device that has a spike in junction temperature, the heat will take too long to conduct to the case and then be removed by the fan. With a fixed load, raising the voltage raises the current supplied to the load and therefore the junction temperature.
5. Safety or design margin is a figure designers calculate to prevent real world circumstances from destroying their products. Knowing the guys at IFI and how some people use their products, I would bet that these devices have a very good margin at 12 volts. At a higher voltage that margin is vastly reduced and failure of components is inevitable. Since each leg of the controller "H" bridge is actually 3 FETs in parallel, the failure of one will bring down the controller as it would provide a short circuit to the other FETs.
6. On the devices used in these controllers, a more important spec is gate to source or gate to drain breakdown which is 16 volts. Assuming the drive electronics run at 5 volts, a power supply of 24 volts would exceed this rating by four volts. When a FET fails it usually creates a short between terminals that is permanent.