The Crystal detector or diode rectifies the
radio wave so that it feeds a DC signal, varying at
an audio frequency rate to the earphones.
This audio is what we hear.
When the diode is forward biased it does not initially conduct until the voltage reaches a threshold value at A. There is a slight curve between A and B , and then a relatively straight line from B to C and beyond. The initial threshold voltage varies from diode to diode eg.
· Lead Sulphide (Galena) .3 volt· Germanium .3 volt· Silicone .6 volt· Silicone Carbide (Carborundum) 3 volt· Silicone on metal (Schottky) .4 volt
The curves AB and BC are much the same for all solid
state detectors theoretically there is little to choose
between them. To overcome the threshold voltage the
diode can be permanently forward biased so that it
starts operating no matter how small the radio signals
applied.
The potentiometer should allow about 1 milliamp to
flow from the battery. Measure the current flowing in
one of the arms from the potentiometer, adjust until
current just starts to flow. Alternatively listen on
the earphones to a weak station and adjust for maximum
loudness (hard to judge)
You may have seen circuits using multiple diodes, which
supposedly increase the output of the crystal set,
remember you are limited by what the aerial and earth
system can deliver to the crystal set. Each diode
added increases the resistance and voltage drop.
The earphones should correspondingly have a higher
resistance, an extra 2000 ohms for each diode added.
Note that ordinary circuits use one diode, a half wave
rectifier, using one half of the available radio wave.
The circuit below shows an example of full wave
rectification.
Wind another coil of say 60 turns on a former which
will just fit over the existing coil. Tap every 5
turns with one lead to the earphones from the centre
of the coil. The diodes have the same number of turns
between them and centre tap. One diode works on one
half of a cycle of radio signal, the other diode on
the other half cycle, there is only one diode
conducting at a time.
So you have built a crystal set but can't get any 2000 ohm earphones, don't worry there are
ways around this.
In the basic crystal set circuit, the detector is tapped down on the tuned circuit coil. This
matches the detector circuit to the tuned circuit at a point of reasonable balance between
selectivity and sensitivity, the tuned circuit looks like a resistor of say 40.000 ohms. The
diode and earphones together have a resistance of say 4,000 ohms.
The coil has 80 turns, tapped at 25 turns, the turns ratio is 80/25 = 3.2.
The resistance is transformed as 'turns ratio squared'
Turns ratio = 3.2
Resistance ratio = 3.2 squared = 10.24
The tuned circuit resistance is transformed down at the 25 turn tap by a factor of
10.24, i.e. from 40,000 to around 4,000 (the resistance of the detector circuit)
In the same way you can make a low impedance earphone look like high impedance
earphones. Suppose you have 150 ohm earphones, you need to match
these to 2,000 ohms.
The resistance ratio is 2,000/150 = 13.33,
corresponding turns ratio = square root of 13.33 = 3.65
An audio transformer is required which has a turns ratio of 3.65 to 1, a small power
transformer can be used such as 240 volts to 65 volts, 115 to 37, or a transistor radio
transformer.
Good quality earphones have their diaphragms placed close to the magnets. Some
have adjustable spacing. The low impedance earphones are likely to have thick
diaphragms, be insensitive and not sound good. Don't expect too much.
Moving coil earphones typically 8 or 16 ohms are very inefficient (about 1%) but
sound good. They are probably not a good choice for crystal sets.
Another variation on our crystal set..
Up to now we have varied the coupling between circuits by variable tapping and
coupling between coils. Figure 14 shows how we can couple two circuits using a
capacitor.
The two coils are kept separated, between the top of the two coils add a 150 pf
variable capacitor. This varies the amount of energy fed from the antenna side of the
circuit to the detector side of the circuit. Note that both sides of this capacitor are above
earth, it must be insulated from your hands and earth, if you put your hands directly on the
capacitor you will change the coupling so put a knob on the shaft.
