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Aug 22, 2012, 02:59 AM
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Analog BJT fail

Yesterday's circuit failed catastrophically, because the voltage emitted by the NPN wasn't high enough to completely turn off the PNP. That was a lot of work.

There's a lot of rationale in building h-bridges instead of using an LF353 or high speed op-amp. The h-bridges can switch 3x faster than an LF353.

A few more ideas came along, to minimize transistor count. The trick is to use all NPN's, but they can't go all the way to the rails. While working at 12V, they died at 30V.

Finally gave up on component minimization & blasted the problem with a transistor mountain, still insisting the h-bridges should be controlled by 1 input & using an NPN to invert the control signal.

That failed miserably. While the rise time was a spectacular 1us, the fall time was longer than the world's worst op-amp at 100us. Virtually no power went to the speakers. It's unknown what killed it. It wasn't the inverting NPN. It wasn't the 100k resistors. The last suspect is shorting the bases of each push/pull pair of transistors to save on resistors.

Whatever the reason, the circuit is going to be replaced by LF353's. H bridges are impractical to mass produce. The LF353 can get 3us rises & falls for a lot less money. The wavelength is 41us, so the 2us difference shouldn't affect anything.

Today had many lessons.

#1 Working at 30V has a lot of traps for young players not present at 12V. 10k pullup resistors won't do. 30V across 10k is 0.1 W or 44% more than an 0402 resistor can handle.

Spent forever tracking down a problem where the h-bridge would work at 12V, but only momentarily at 30V, before it experienced a short. Your instinct says a BJT is shorted, but the 10k 0402 resistors were burning out from 30V. Only 100k would do.

#2 The Rigol crashes if bidirectional trigger is selected. You have to be very careful to not highlight bidirectional trigger. Recovering it requires hitting start/stop after power cycling, before it does the 1st trace.

#3 The Rigol failed to save some images. It seems to have corrupted the file system.

It's running v2.05 of the firmware, but it's hard to believe the problems were fixed in 2.06. Upgrading it to 100Mhz required downgrading from 2.06 to 2.05, because it requires reflashing the firmware & there's no downloadable 2.06.
Last edited by Jack Crossfire; Aug 22, 2012 at 05:14 AM.
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