Event Detector Trigger Board Help!!!!

Tunelabguy said:

A transistor switch is a good idea, but not a bipolar transistor. Those devices have a minimum saturation voltage that could interfere with your resistance measurement. Instead use an enhancement mode FET switch. These devices have no minimum voltage drop from the drain to the source and appear very much like a very low resistance resistor. Plus they can be controlled with a positive voltage, just like an NPN bipolar transistor. There are many enhancement mode FETs available with various drain current ratings and various gate turn-on voltages. Pick one that can be fully turned on by your fairly low voltage pulse, and pick one that is rated for a bit more drain current than you ever anticipate needing to flow through that switch. That way your switching time will be nice and fast. 250-300 nsec. is not too hard. But you will need a low-impedance gate drive. Even though the gate of an enhancement mode needs no DC current, it does have some capacitance. To charge and discharge that gate capacitance quickly, you need a low-impedance drive. That is another reason for picking as small a FET as you can get away with. The larger FETs have larger gate capacitance. A small FET will be easier to drive. Then connect your variable resistor to the drain of the FET and ground the source.

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