IEEE Consultants' Network of Long Island

Coil Shielding Choices - John Dunn, Consultant, Ambertec, P.E., P.C.

The choice of which conductive material to place near to or far away from a coil can affect that coil's electrical behavior. The effect on a particular coil of wire of placing a non-ferrous metal (a twenty-five cent coin in this case), placing nothing at all and a placing small piece of ferrous metal is illustrated as follows: 

Type-K Thermocouples - John Dunn, Consultant, Ambertec, P.E., P.C.

Thermocouple tables are published from which you find the relationship between millivolt (mV) measurements and thermocouple temperature, typically in degrees C. That relationship between temperature and mV, in the case of a type-K thermocouple, happens to be approximated as an 8th-order polynomial.

The GWBASIC program below uses that polynomial to find an approximation of thermocouple mV output given a specific temperature in degrees C. This can be useful in making simulations where you really don't want to have to deal with a real device sitting on your workbench at multi-hundreds of degrees.

Some tabulated results from this program are as follows:  

The Neon Lamps - John Dunn, Consultant, Ambertec, P.E., P.C.

The story you are about to read is true. Only the names have been omitted to spare somebody the embarrassment. 

A whole array of neon lamps were provided at the bottoms of panels in a rack of military electronic equipment. Each lamp was supposed to turn on and off at the action of an SPST switch.

The circuit and the physical arrangement that were used are sketched here.

Diode Dynamic Impedance - John Dunn, Consultant, Ambertec, P.E., P.C.

A diode can be used as a variable attenuator element based on variability of the diode's dynamic impedance versus its bias. An estimate of that dynamic impedance can be found from the basic diode equation as follows: 

 
 

Flashlight Batteries and Bulbs - John Dunn, Consultant, Ambertec, P.E., P.C.

It's a small thing perhaps that so many flashlights become unreliable with use, but it's been a pet peeve of mine for quite some time.

Many flashlights depend on the tip of the bulb to make contact with the positive end of the upper flashlight cell as sketched below on the left. This works okay for a while, but eventually, the metal of the bulb's tip deforms by flattening out while the positive tip of the flashlight cell also deforms by becoming concave as sketched below on the right.

Then the flashlight doesn't stay lit as I hold it. I have to shake a little it to get light or I have to twist the light bezel back and forth a little bit.

The JFET and the VOM - John Dunn, Consultant, Ambertec, P.E., P.C.

I once had this junction field effect transistor (JFET), a 2N4391 I think it was, to which I attached a Simpson 260 VOM (volt-ohm-milliammeter), positive lead to the drain and negative lead to the source while in the ohms mode on the R x 1 scale. There was some mid-scale meter reading which meant that with the gate floating as it was, the device was carrying some kind of drain-to-source current.

This was nothing to get excited about, but then I noticed something. The meter deflection would vary in repeatable ways as I moved my hand to various positions in the vicinity of the JFET as it lay there on the table top, connected up as it was.  

Which Is The Positive End of A Diode? - John Dunn, Consultant, Ambertec, P.E., P.C.

"Which is the positive end of a diode?" is a seemingly silly question, but guess what. I've seen the answers to this question ( Yes, I do mean to be plural, not singular.) cause intense arguments among folks who should have known better.

The two opposing answers are illustrated here:   

 

The ETPL - John Dunn, Consultant, Ambertec, P.E., P.C.

In the further avowed interest of cost savings at the company that had lost the services of The Sculptor, the purchasing department often bought components from one particular semiconductor company who always seemed to be the lowest cost supplier. The name of that semiconductor company I shall not state, but it is no longer in business, thank heavens. These purchases were made in spite of it being repeatedly pointed out that purchased items kept coming in non-compliant to specifications or just plain defective.

The purchasing department's impregnable defense to such input was "The vendor says they meet the specs." even though nothing could have been further from the truth. Thus, as each defective item caused this or that problem, each of those problems was duly entered into the official Engineering Technical Problem Log, the ETPL.

Beware of Loop Gain - John Dunn, Consultant, Ambertec, P.E., P.C.

It is sometimes suggested that an op-amp can be made less susceptible to electromagnetic interference (EMI) by connecting a capacitance directly between the op-amp's non-inverting and inverting input pins, but doing so is something I very much counsel against.

Here's one example why:  

 

Collector Current Measurement - John Dunn, Consultant, Ambertec, P.E., P.C.

Imagine needing to measure the current flowing in some transistor's collector where that collector cannot be connected to very easily for one reason or another, say its being up at a few hundred volts for which a differential amplifier made to do that measurement would be quite costly. 

 
Measuring instead the emitter current and the base current at the same time is a possible way to overcome this problem as sketched above.

The pair of 2K resistors sample the currents, the four 100K resistors subtract the base current value from the enitter current value and the low common mode voltage differential amplifer buffers the result.