Electronics Instructor Q & A


Q. I have the analog meter that comes standard with my course. In Lesson 1408 it says that on the ohms ranges, the + lead is the black one and the – lead is the red one. Isn’t this backwards? I thought red was always + and black was always - .

A. To make the red lead + and the black lead – on the ohmmeter ranges would require an extra set of contacts on the range selector switch. This would make the meter more expensive than necessary and the extra cost would be reflected in the cost of our course. The extra contacts would also make the switch more complicated, which would reduce the reliability of the meter. When we chose this meter, we felt that our students would be able to understand this difference. If you wish, you can simply unplug the leads from the meter and swap them around when you are using a resistance range.


No Negative Solution

Q. In my lessons, the op amps are shown with both a positive and a negative power supply. What do you do if you only have a single power supply available, such as a single battery?

A. There is several ways around this problem. The simplest is to use a power supply “splitter.” This consists of two equal-valued resistors, which are connected, in series across the power supply. The resistors form a voltage divider, so that half the supply voltage is dropped across each resistor. The common “ground” for the signal is the point where the two resistors are connected together. This works only if the current is relatively low.

For higher currents, a transistor can be used. Its bases are connected to the output of an op amp. The inverting input of this op amp is connected to the emitter of the transistor. The non-inverting input is connected to a resistor “splitter.” The transistor acts as a current amplifier. A third way to solve the problem is to use an oscillator, and then rectify and filter its output to provide a separate negative supply. There are op amps that are designed to operate on a single power supply. The LM 324 is an example of this. The IC has four independent op-amps in a singlepackage. We use this IC in our microprocessor lab lessons on Digital-to-Analog converters.


Functional Junction

Q. I have heard that there are solid-state cooling devices. What are they and how do they work?

A. These are called “Peltier Junctions.” Unlike conventional cooling devices, they do not use mechanical pumps and refrigerants. There are no moving parts. Basically, certain materials are used to make a PN junction. The junction is forward biased. The current carries heat away from one side, which becomes cooler, and transfers to the other side which becomes warmer.

Peltier devices are not at all as energy efficient as conventional mechanical cooling devices and large amounts of current are needed to make them work. They are also more expensive than mechanical systems, for the same amount of cooling power. But their reliability and lack of moving parts makes them useful in some applications. For example, they are used in nuclear submarines, where electrical power is abundant, and the fact that they make no sound is a great advantage.


Delaying Action

Q. I do not understand what “propagation delay” is.

A. When the input(s) to a gate or flip flop change, it takes a certain amount of time, usually measured in nanoseconds, for the output to respond to the change. This is called the gate or flip-flop’s propagation delay because the input - signal is delayed in going through the gate. Most of the propagation delay is caused by the internal capacitance of the transistors inside the IC. It takes a little time for them to charge or discharge in response to the changing voltage on the input of the device. The propagation - delays of the gates and/or flip-flops in a digital circuit determine its maximum operating speed.


PINning it Down

Q. I was reading a magazine article the other day and it mentioned PIN diodes. What are PIN diodes?

A. PIN diodes have a P-type semiconductor and an N-type semiconductor, as do ordinary diodes. But between the P and N material, there is a very thin layer of undoped or “intrinsic” semiconductor. This is what the letter I stands for in “PIN”. Undoped semiconductor is a good insulator. The I layer is put between the P and N layers to reduce the capacitance the diode. The reduced capacitance makes the diode start to conduct and stop conducting faster than regular diodes. PIN diodes are used in circuits that handle radio frequency and high-speed digital signals.



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