Steel slide…Laser welding…Ideal gate resistor value…

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: I am in the process of investigating the use of laser welding to bond polyethersulfone (PES) parts together. I would appreciate any leads for who does this (job shop) and what needs to be considered for the joint design. —K.W., DN reader

Dear K.W.: One reader replied that his company recently went through the same vendor/technology review and search. From what they found, there are no readily available contract manufacturers that have polymer laser welding capabilities in the U.S. at this time. Europe's laser welding industry is further along. Germany and Sweden seem to be the leading countries where they found several equipment vendors, as well as several contract manufacturers offering equipment for welding services. Review the exhibitors from the K-shows in Germany for 2002 through 2005 for all the major players' names. Joint design is fairly simple, but is also dependent on how you plan on introducing the beam through the joining materials. Material thickness, transparency, and flatness of your components are the major factors found to affect the weld joint the most. Consult several of the equipment vendors in the U.S. for more specific details and input. Check out Branson, Leister, as well as a few others.

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. 100Ù is normally good enough.