IEEE Consultants' Network of Long Island

Bounceless Switching Without Mercury - John Dunn, Consultant, Ambertec, P.E., P.C.

We needed to make a bounceless switch for step load testing of some power supplies and the written specifications for this testing called for the use of mercury switches  for their bounceless switching properties. However, we didn't have any such switches on hand nor could we find them for sale anywhere, mercury now being out of favor for safety reasons.

Therefore, we made a bouncless switch for ourselves this way:   

 

Our power supplies passed their tests.

PWM Chip Slow CLK Fall Time - John Dunn, Consultant, Ambertec, P.E., P.C.

An industry standard PWM chip in this particular switchmode supply was driving a type 4013 D-type flip-flop using that PWM chip's clock (CLK) output. The 4013's response to this drive was sporadic. The 4013 would toggle properly on each rising edge of CLK, but would often toggle improperly on the falling edges of CLK which would mess things up quite badly.

The reason turned out to be that the falling edge of CLK was very much slower that the rising edge and with just a little bit of noise on that falling edge, would upwardly cross the threshold voltage of the 4013 clock input "C" and cause the unintended toggling. Note that the 4013 has no input Schmitt trigger which might have prevented this trouble.      

 

A Sneak Noise Source - John Dunn, Consultant, Ambertec, P.E., P.C.

I once chose a TO-99 packaged, low-noise op-amp, the OPA2111 when that device was a product of Burr-Brown. Today, that device is being sold by Texas Instruments.

I got some really nice results with it, but I was just plain lucky to have avoided trouble with that part from a noise source I hadn't reckoned with, ambient light.   

The OPA2111 uses a pair of junction FETs for its differential input stage. The trademark for this feature  is now Difet ®.

According to a Burr-Brown tech-rep with whom I spoke way back then, those two FETs were quite large, occupying approximately 50% of the chip's area all by themselves. Being large, those FETs were highly sensitive to ambient light that could sneak into the package via the glass beads that held the eight leads of the TO-99 can.

Bypass Capacitor Boobytrap - John Dunn, Consultant, Ambertec, P.E., P.C.

We all know of the technique of paralleling capacitors of different capacitances to achieve wideband bypassing, but here is a look at a hazard of which to be aware.

Consider two layers of some circuit board, the following being sort of a typical case:  

  

Logic Vcc Power-On Switching Problem - John Dunn, Consultant, Ambertec, P.E., P.C.

Some troublesome digital chips were not starting up properly if their +5 Vcc was applied with too slow a rise time. (Hey, sometimes things like this can happen, right?) The minimum required rise time for that +5 Vcc rail was determined to be 45 mSec.

This was a problem which later resolved by the chip vendor, but while the problem still existed, a circuit was needed to delay Vcc application to the troublesome chips until the power supply voltage had risen to a particular threshold value, and not before, and to then apply the already risen power supply voltage to the chips with a relatively fast rise-time as sketched here: