Home  Articles Advanced Gearbox Tech II: Solid State Switching with MOSFETs
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Advanced Gearbox Tech II: Solid State Switching with MOSFETs |
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Written by Jay
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Tuesday, 10 May 2005 |
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Page 7 of 10 Wiring Schematic The MOSFET must be installed such that when the trigger and selector plate contacts are closed, a large positive voltage is applied to the Gate pin relative to the Source pin. Additionally, the MOSFET drain and source pins must be placed in series with the conduction path of the motor, thus allowing or disallowing current to flow from battery to motor.
The nature of the MOSFET Gate is capacitative, while the Source and Drain are inductive. These traits lead to a few additional concerns that must be addressed for the MOSFET to function properly as an AEG switch.
Problem 1: Gate Capacitance
The Gate pin of the MOSFET behaves similarly to a very small capacitor, storing charge and retaining its charged state if it is not discharged. This is an undesirable condition, as it means that once we turn on the MOSFET, it will remain on unless we explicitly discharge the Gate. Simply opening the trigger and selector contacts will not accomplish this; a discharge path to the negative battery terminal must be provided for the Gate pin. Additionally, the path must be properly loaded with a resistor so as to prevent excess current flow throught the discharge path, in essence creating a short circuit around the MOSFET.
Problem 2: L-C "Ringing"
Inductor-Capacitor circuits can be induced to oscillate under certain condtions, and this is also the case with MOSFETs. Oscillations, or "ringing" can be prevented by introducing a dampener into the system, such as a small value resistor added in series to the Gate. The resistor slows down the charge and discharge of the MOSFET Gate, reducing the likelihood of oscillation. The resistor value must be kept low however, or risk reducing MOSFET switching efficiency and generating excess heat.
Following all the constraints above, a schematic can be drawn representing the proper wiring of a MOSFET:
In the schematic, a new wire (green) has been added to the stock wiring schematic. The original red wire going to the trigger contact has been removed and resoldered to the "lower" selector plate contact red wire. The new green wire, is now soldered to the original trigger contact position, providing a path to the MOSFET gate pin. When the trigger and selector contacts close, positive charge flows from the battery to the red wire, through the contacts, then to the green wire, which terminates at the Gate. This allows a positive charge to build at the Gate and turn the MOSFET "on". Since the motor black wire is now interrupted by the MOSFET, current will only flow through the motor if the MOSFET is "on", which can only occur if the trigger and selector plate contacts are closed. A discharge path with a 30k ohm resistor has been added between the Gate and Source pins, and a small 100 ohm resistor has been added in series with the Gate pin. When the trigger or selector contact switch is broken, the Gate pin discharges through the 30k ohm resistor back to the battery negative terminal, effectively turning the MOSFET "off". Although larger resistances for the discharge resistor and smaller resistances for the Gate resistor can be used to increase the efficiency of the MOSFET, we chose to stick with known working solutions referenced from the Filipino airsoft community* for the purposes of this article.
*readers RiotSC of California and johnyew of Hong Kong have notified me that the stated resistor values did not originate from the Filipino airsoft board but instead came from this article at a Japanese airsoft site called "Kiss in the Dark".
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