IEEE Consultants' Network of Long Island

Magnetic Field Strength Test Coils - John Dunn, Consultant, Ambertec, P.E., P.C.

I was using a pair of these specially purchased test coils to make some AC magnetic field strength measurements and my results were not very repeatable. Ordinarily, I would blame that on my being somewhat clumsy in handling the test coils, but this was getting to be a bit more of a problem than I could account for by dint of personal ineptitude.

Each test coil came potted in an epoxy cylinder with two very thin wires emerging from one end. The calibration information for these two coils was that output signal versus field strength  was based on the central axis of that epoxy cylinder, the Cylinder Axis,  being in alignment with the field being measured.

  

Semiconductor Evolution - John Dunn, Consultant, Ambertec, P.E., P.C.

This is definitely a tale of yore, but it holds a good lesson anyway.

Our product used a push-pull power amplifier made with power darlingtons, type 2N6578, which were operated in class-C service. It was required of each side of the subject amplifier that its transistors achieve current cut-off before the opposing side began to conduct for the next half cycle of operation. The operating frequency was typically 25 kHz. These things really worked very nicely until around 1984 when we began having trouble.

We found that some 2N6578 devices were not getting to their cut-off state quickly enough. They would still be carrying current when the other side started to carry. This led to unwanted overlaps of current flows of the two sides of the push-pull amplifier and sometimes that led to catastrophic amplifier failures with all kinds of smoke and unhappiness.

LED Traffic Lights Problem - John Dunn, Consultant, Ambertec, P.E., P.C.

What price do we really pay for power efficiency?

I've heard good words said about LEDs as used in traffic lights. The LEDs are said give out as much light as the old incandescent lamps did while using less electricity to do it. So, since the LED traffic lights are power efficient, I guess they must be a great design.

On the other hand, the LEDs optical radiation pattern is quite narrow so that when viewed far enough off center, their glow can't be seen anymore.

While I was driving southbound on Route 110 in Farmingdale one very windy day, as I came to a set of LED traffic lights hanging from cables in front of the Telephonics driveway, those lights were rendered invisible from a distance by being steadily wind driven too far away from the vertical. The lights were just plain invisible until I got really, really, really close. This was a clear (no pun intended) detriment to public safety.

Headset Driver Oscillations - John Dunn, Consultant, Ambertec, P.E., P.C.

In an investigation into parasitic oscillation of some headset driver amplifiers, the equivalent circuit of a nominally eight-ohm headset was empirically examined using mimicry. The method is shown in the sketch below:    

 

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:  

  

Resistor Aging Rates vs. Temperature - John Dunn, Consultant, Ambertec, P.E., P.C.

Svante August Arrhenius (19 February 1859 – 2 October 1927) gave us Arrhenius' Law which teaches us that aging processes are accelerated by two-to-one for each 10°C rise of temperature.

Sometimes, we can see that law at work in a component's data sheet. Consider for example the following extract from a data sheet for chip resistors:

   

Opto-Coupler In Linear Service - John Dunn, Consultant, Ambertec, P.E., P.C.

We may examine the linear properties of an ET1104 opto-isolator as follows.

Note that there was no reason in particular for choosing an ET1104 to examine. There just happened to be one on hand. Also, the 1.33K to the LED was used only because it happened to come to hand at the moment. It could just as well have been a 1K.