@T Chap: That drift seems a bit high. Have you hooked a multi-meter directly to the precision resistor while cooking it? I strongly suspect your intuition that the rest of the circuit is drifting is accurate. I've used a similar foil resistor in a precision thermistor measuring circuit. (5k 5ppm temp coefficient) While I didn't get the circuit smoking when I heat-gunned it, the temperature of the best channel changed ~4 miliKelvin peak to peak with the worst channel shifting ~12 milikelvin. (I started waving the heat gun around half-way through the test.)
I was reading the voltage directly off the output of the AD628 in the sense circuit. I need to test systems as close to a real fire as I can get without really destroying the board to have a rough idea of when and where things will break down. I have now killed 3 of these resistors($19.50ea). The rating for 8watts is with a rather larger heat sink which I cannot afford the real estate for. In a case where I stall the motor, basically representing a shorted output, the driver fuse ( 2amp slo) takes the punishment. In this case, the 1ohm resistors all opened up to different values(ie 8ohm, 151ohm). My standard 4watt 1ohm non inductive wirewound resistors never fail even when turned black due to a short( ok well usually they survive until the fuse catches it). Like I stated, I was heating a large chunk of the board, including the sense IC and all passives, so this was not a valid test for the vishay. However, in the real world, clients do the craziest things and the resistor must survive a dead short or stalled motor. Back to the tried and true!
Comments
Lawson
Perhaps a new thread.
Graham