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The Search Engineer finds solutions to all your questions, problems, and dilemmas. Occasionally, he could be wrong. But he doubts it.
By Design News Staff -- Design News, December 5, 2005
Steel slide . . . Ideal gate resistor value . . . Screen material . . .
Dear Search Engineer: I need to sense the presence of four ball bearings in an assembled steel slide, and the ball bearings aren't visible. Can I use an inductive sensor to sense the changing permeabilty of the cross section as it is fed through a sensor with a servo?—C.P., DN reader
Dear C.P.: One reader says to try an ultrasonic probe. The presence or absence of the return signal ought to be substantially different.
Dear Search Engineer: Can you tell me the formula for calculating the ideal gate resistor value (RG) for a P channel enhanced FET switching circuit driven from a CMOS microprocessor output? I understand that you could drive the FET directly from the CMOS output, but if you use a series resistor for the gate, you can reduce potential long-term damage to the CMOS output. Do you know how to calculate this ideal value? —M.D., DN reader
Dear M.D.: One reader replied that there is no reason why you cannot drive a FET directly from a CMOS uP pin as long as the pin is current limited. However, you will get a sluggish response (the dv/dt will be proportional to current limit/capacitance) which will result in increased heat dissipation in the FET due to switching losses. You can always put in a FET driver such as the MAX5078 as this will provide a high impedance to the uP, and give a good low impedance kick to the FET. To ensure good operation, make sure you have thick tracks going to the FET driver and have local decoupling (the output current will come directly from the rails). A discrete solution will require a bit more design, but will provide adequate results. Resistors are normally put in series with the gate primarily to stop ringing (due to the gate inductance and gate-source capacitance resonating). The current limiting is a secondary issue as you normally want the FET to turn on as fast as possible. 100V is normally good enough.
Dear Search Engineer: I am looking for a window screen material that is highly transparent and nearly invisible. It will be used in a luxury recreational vehicle for the screen door on the entrance. Our current door screen limits the visibility for the passenger, and, at times, due to light conditions, the driver's vision is also impaired. The shape of the door doesn't lend itself to a rollup or removable screen so that is probably not an option. Gore-tex makes a material that might work, but at this time it has an exclusive relationship with Pella windows and isn't available for other manufacturers to use. Any suggestions? —M. W., DN reader
Dear M. W.: One reader says to investigate partial metallization of the window itself. It will not affect driver vision i.e. light entering, but impairs light escaping out. It is also the principle of one-way mirrors. Careful however, as a mirror might cause blinding of other drivers when reflecting sunlight or bright headlights. Slight tilting of the glass will help reflect the incoming strong light downwards, away from other drivers.
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