Yep. I cut my teeth being a technician at MIT's famous Building 20, building equipment for Professors Woodson and Jackson, and PhD candidate Jim Melcher (later to be EE Dept Head)... MHD research. ...Jim Thompson
I hate that term, "good engineering practice." It invariably means: I want to do it this way, I don't want to do any math, I don't want to think this through, because I have opinions.
More smug bloviation and BS. Damn! I think you're trying to be a PhD candidate >:-} ...Jim Thompson
Drive it from a realistic gate impedance and the spike is even smaller. Include the PCB trace capacitance (making the trace a transmission line) and it's even less.
Well, at least he's worked his way down from 25 KV to a kilovolt to
158 volts. That's some progress.
Good engineering practice means (to me) anticipating failure mechanisms, and noting that the device to be protected is the FET, NOT the relay, so judiciously placing the catch diode near the FET costs you nothing and adds insurance.
Design however you like. I design my chips for no failures and I've succeeded. You've admitted you often have "revs". The only "revs" I have are spec changes... the customer says, "What I asked for in the original spec is not what I really wanted". ...Jim Thompson
Is there any violation of dV/dt occurring? From memory these devices don't like sharp rise times.
air speed meter that had low weight
says nothing
occasionally.
A pitot / static tube system is ok if you can compensate for temperature and local barometric pressure.
?-)
thing,
and reliable.
related to it (ice forming).
piezo sensors,
are very small.
Icing can foul almost any sensor, including ground speed radar.
?-)
Well, assuming infinite Q and zero self-capacitance for the loop, we have a 6.7 volt peak and a radian frequency of 1/sqrt(1uH*20pF) = 223 Mrad/s, so the peak dV/dt would be about 1400 V/us. The datasheet typicals are a few ns to switch 30V, so I think it's pretty safe.
Even if it weren't, there isn't nearly enough energy in that tiny little kick to blow out the gate or avalanche the device.
Cheers
Phil Hobbs
In order to get a 50V swing across a 27 nF cap with half an amp flowing, you'd need a pretty big dL/dt.
I really doubt you'd get that from a gear tooth--the air gap is too big.
Also, of course, if your variable-reluctance sensor has a Q of 50, you're automatically averaging over the positions of at least 50 teeth, which is generally not what you want. (It also wouldn't work at all unless you were within a couple of percent of the nominal shaft rotation rate.)
Parametric circuits are really fun, though, so it's worth playing around with.
Cheers
Phil Hobbs
If you drive the gate from some reasonable source impedance, like 50 ohms, you get a lot of Miller feedback into the gate. That puts a plateau in the falling-edge gate drive which further limits the peak drain spike.
No PCB trace is going to create a spike hazard for a 2N7000 and a 12 volt supply. Diode forward recovery could, I guess, but that's improbable too. Big slow diodes will have a lot of capacitance.
It would be a challenge to come up with a realistic model of a circuit that does in fact blow a 2N7000 from PCB parasitic inductance. Jim can do that and impress us.
Some of the variable reluctance pickups can make 100 volts peak from a gear tooth at high speed, but not into 27 nF. Our VME tach module has an input network, sort of like a tape head equalizer, that accepts signals like that. The voltage tracks frequency over a huge range, up until eddy currents and capacitance kill the slope.
I knew those old NAB preamp app notes would be useful for something someday. ;)
Cheers
Phil Hobbs
Den fredag den 22. november 2013 17.46.16 UTC+1 skrev John Larkin:
I know a guy that build ECUs, the crank sensors are usually VR. Capacitive coupled with a current limiting resistor into a smith trigger gate held in the "eye" of the hysteresis with a large resistor (there's a trick to that) seems to be pretty much bulletproof from ~zero to tens of kilohertz
-Lasse
Sure. Select a cap large enough to cover down in the hole and then put a R in series to keep the Z flat through out.
Jamie
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