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Episode 35 -- October 10th, 2016 -- Multimeters and Oscilloscopes, Diodes, Transistors, Amplifiers, Oscillators, AM Modulation
Listen to the episode 51.4 MB MP3
Multimeters
Check the voltage of a battery using DC Volts
Check the AC voltage from a wall outlet VERY CAREFULLY on the AC Volts 200 volt scale
Check diodes and resistors
Check fuses and light bulbs using the resistance scale. If a fuse or light is good, it'll read zero ohms. If it's broken, it'll read INF or infinite ohms.
The screen is a graph. The grid (almost always there) is made up of major divisions (usually around 1 cm) and minor divisions (usually 2 mm). 10 mm = 1 cm, so 5 small dots between each major dot.
The horizontal axis of the graph represents the flow of time, from the the earliest on the left to the most recent on the right, and always continuing with new information coming in from the right.
The verticle axis represents voltage on one (or more) of the verticle inputs. Sometimes a probe will be used for accurate measurements which diminishes the voltage by exactly 10x. Sometimes 1x. Sometimes it isn't accurate enough to know.
An alternating Current signal such as a sign wave will show up on the oscilloscope screen exactly as it appears in the textbooks, a wavy, curvy, symmetrical line.
Verticle input (maybe 1, 2 or 4) is connected to the point in a circuit where you want to see a signal. Ground is almost always in common.
Horizontal Time Base
1 second to microseconds (MHz) per division or even nanoseconds (GHz) or picoseconds (Terrahertz) per division.
So if you have a sine wave which takes exactly 4 major divisions and the timebase of your oscilloscope is set for 1 nanosecond per division, that sine wave takes 4 divisions for a single cycle or 4 nanoseconds or one quarter of a Gigahertz or 250 MHz. My home ham radio repeater operates at 224.700 MHz, very close to that frequency.
Diodes and rectifiers and LEDs
A rectifier is the same thing as a diode, but used for converting AC to DC, usually in Power Supplies. While it is certainly true that a diode is also used to detect an amplitude modulation signal and convert it to audio, it does this by stripping off half the signal so that an earphone will hear only the audio (because the earphone can't physically vibrate enough to represent the high or very high frequency signal, it only vibrates according to the waves at which it resonates, audio.
P-doped (Annode), N-doped (Cathode), depletion region in the middle
Anode and Cathode (on test)
Annode to plus and cathode to minus is forward bias.
Reverse bais no current flow (unless really big!)
Cathode has a stripe (on test) Stripe to minus!
So, Put the stripe towards minus, as though it were an arrow, pointing in the direction of "hole" flow. (oppose it the flow of (-) negatively charged electrons).
Rectifiers and Bridges
a single diode can be used as a half-wave rectifier to convert AC into <50% duty cycle DC.
4 diodes in a bridge configuration ("Bridge Rectifier") make a full-wave rectifier to convert AC into <100% duty cycle DC.
A rectifier is just a fancy word for a diode being used to convert AC into DC.
Reversed biased and Zener Diodes (but not on the tech test)
Zener Diodes
Zener Diodes are designed to have a precise breakdown or zener voltage (reverse bias) for use as a voltage regulator for small loads. Typical forward bias is 1 volt. Reverse bias is the Zener voltage (V-sub-Z), typically some voltage you want such as 3.3 volts.
Typical use is in a "Classic" Zener Voltage Clamping circuit with a zener diode (reversed biased) and a series resistor before the load to "regulate" down to the zener voltage.
Shotkey Diodes
handles power better
Works at higher frequencies
A Schottky diode is often used at the output of a power supply to insure against damaged due to reverse connections.
Photodiodes change bias based on the amount of light present
Power Supplies for Grow Lights -- Sine Waves prevent harmonics!!
Diodes are very often used to limit damage from unexpected spikes in voltage. Transient-voltage-suppression (TVS) diodes are specialty diodes, kind of like zener diodes — lowish breakdown voltages (often around 20V) — but with very large power ratings (often in the range of kilowatts). They're designed to shunt currents and absorb energy when voltages exceed their breakdown voltage. Used with flyback transformers or motors during startup.
2 diodes, either annode to annode (NPN) or cathode to cathode (PNP)
Operation in switching circuits
Use the base-emitter circuit to control the emitter-collector circuit
A photocell changes its resistance based on the amount of ambient light. A tiny photocell -for example- can control whether electricity flows or not based on how bright the area is. Or whether or not an LED is shining on it. You may want to do this to completely isolate two different circuits.
Output circuits of a small Arduino or
Rasberry Pi can control high powered LEDs or Relays. Inegrated Circuits such as small computers are very delicate and can only switch up to 10 ma (10/1000 or 1/100 of an ampere). But an LED requires 20 ma.
Operation in amplification circuits
But the tricky part is that one can be used to control the other. And since the other can have a lot more power going through it than the first, you can use a tiny voltage change to make a large voltage change by a large quanitty. Here, for example, is a very simple audio amplifier circuit.
So a tiny base-emitter voltage can control an enormous emmitter-collector circuit so you can amplify a microphone signal to use in making a 1,000 watt FM transmitter.
Field Effect Transistor (FET, JFET)
A Juction Field Effect Transistor has a Source, gate, and drain
Closest in general operating characteristics to a Vaccum Tube
MOSFET - Metal Oxide Semiconductor Field Effect Transistor
Operation and unique applications;
construction
Characteristics
UJTs "A unijunction transistor (UJT) is a three-lead electronic semiconductor device with only one junction that acts exclusively as an electrically controlled switch. The UJT is not used as a linear amplifier"(Wikipedia)
Amplifiers -- makes a signal louder. Makes an RF signal bigger.
Attenuators -- makes a signal quiter or smaller. Resistors can attinuate.
Oscillators -- an Oscillator is merely an amplifier specifically designed with its output coupled to (connected to) its input so that it feeds back upon itself rather like when you twang your electric guitar too close to the speaker. Oscillators are designed to produce sustained pure sine waves. In fact, the only ways to make AC is either with an Alternator or an oscillator. Inverters are oscillators. (Generators make DC).
Mixers -- When you simply connect the output of two oscillators at different frequencies, they will mix and produce both the outputs of each oscillator plus the two of them added together (A+B) as well as the difference between them (A-B). So a mixer simply connects the output of two different signals (perhaps one is an oscillator and one is a signal in which you're interested) and filters out all but the one you want.
Filters -- 3 kinds
Bandpass
High Pass
Low Pass
Superheterodyne Receiver -- In a superheterodyne receiver, the signal from the antenna (around .2 - 50 microvolts - or millionths of a volt) is amplified in the radio frequency (RF) amplifier stage. The output of the RF stage is one input of a mixer. A Local Oscillator (LO) is the other input. In most cases, this first local oscillator is a variable frequency oscillator (VFO) (that is, it either has a tuning knob, usually a variable capacitor or it is a digital frequency synthesizer so it can produce whatever frequency is asked of it by a microprocessor so that your receiver tunes different frequencies. If there are different bands, they are also chosen here. A mixer is a small circuit with two inputs, A and B (or whatever you'd like to call them) and it produces two outputs, A+B and A-B. Since, in a receiver, we don't care about the A+B signal, it is filtered out. The A-B output of the mixer is at the Intermediate Frequency (IF) often around 10.7 Mhz, depending on to what particular frequency of a band to which you've tuned. A second mixer stage converts the signal to a second IF frequency, typically 455 KHz. There's often an automatic gain control which listens at the IF and if the signal gets too loud, it tells the RF Preamplifier to quiet down for a moment and then it goes back to receiving. The signal is then demodulated (demod). The modulation technique is irrelivent -- it might be AM, FM or SSB. The simpleist would be to just use a single diode as a decoder and then send it to an audio amplifer and a speaker to hear AM signals. Just a tiny bit more complicated than that, you could mix a carrier with a Beat Frequency Oscillator (a simple audio tone) to output CW as a tone or SSB as a voice.
AM -- Amplitude Modulation
Frequency Modulation
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