IEEE Consultants' Network of Long Island

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.      

 

Nines Complement Arithmetic - John Dunn, Consultant, Ambertec, P.E., P.C.

To subtract one number from another, you can do the usual arithmetic algorithm, but you can also use an addition process instead that can be handy for checking subtraction results when working by hand. This avoids doing the identical process with which you got your first result lest you inadvertently repeat making an error you might have made the first time.

Consider the following examples:

8718 - 4732 = 3986  This is completely standard arithmetic.

Now, replace the "4732" with its nines complement which in this case means "5267". Each digit of this new number is that which if added to the corresponding digit of the old number, totals to nine. Ergo, 4+5=9, 7+2=9, 3+6=9 and 2+7=9.

Dot Matrix Average of Dots - John Dunn, Consultant, Ambertec, P.E., P.C.

The frequency of usage of letters in a six-hundred word essay was used to estimate the average power dissipation of an alpha-numeric, 7 x 5 dot matrix display using LEDs.

Each letter has some particular number of dots turned on. The letter "A" for example, activates seventeen dots. Then from the essay, each letter was found to have some frequency of usage which again, for the commonly used letter "E", was 13.54%, but for the rarely used letter "Q", was only 0.03%.

The average usage in ordinary text turned out to be of 15.6 dots per character.      

RMS of A Square Pulse Train - John Dunn, Consultant, Ambertec, P.E., P.C.

You can find the root-mean-square (RMS) value of a unipolar square pulse train as sketched below using an average responding voltmeter, typically, a digital multimeter and a scope.

The RMS value equals the average value divided by the square root of the waveform's duty cycle:

 
While there are true-RMS meters that can conveniently yield this waveform's RMS value, such meters have crest factor limitations which become of concern for low values of duty-cycle. For this waveform, this method has no such limitation.

 

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:  

Quartz Crystal Spurious Modes - John Dunn, Consutant, Ambertec, P.E., P.C.

An equivalent circuit for a quartz crystal has two resonant frequencies, one at a parallel resonance and one at a series resonance, where the parallel resonant frequency is always slightly higher than the series resonant frequency.

Shamelessly setting the component values of this equivalent to yield the proper values of parallel resonance, series resonance and equivalent series resistance of a specific crystal, an impedance calculation versus frequency was performed and an actual crystal was tested with the following results: