Event Detector Trigger Board Help!!!!

[Jonesy_135]

Hi All,

I was hoping to get some help with a project im working on

Im trying to build a trigger board to test an event detector

The Event detector is used to test for solder joint failures
so when a joint fails (thus becoming open circuit) the event detectors flags it up
the detecttion threshold can be set from 200 to 5000Ω

i need to build a trigger board that will simulate an open circuit event...
so the track needs to go to GND then switch to a resistance (using a pot to allow me to adjust for the detection threshold)

however the problem im having is is has to be 250-300ηs open circuit

there for Relays and switches are out (as far as i can make out!)



so far i have a 266ηs pulse from a 74LS123 which can be configured for either 0.2v or 3.4v output

ive tried looking at transistor switches and diode switches

i guess im just not experience enough to be able to put the pieces together





So can anyone offer any suggestions to help?

is there a relay out there that that will work?
or use a transistor to switch from GND to the resistor.



Any help would be much appreciated.

i dont know how clear i have been with what i need, so ask all the questions you want


Cheers guys!!

 

[Tunelabguy]

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.

 

[Jonesy_135]

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.

This is great, thanks!! ill get hunting for a FET ASAP and give this ago!!

thank you! ill let you know how i get on!