Semiconductors (Pt II)
Eric Edwards GW8LJJ continues his exploration of diodes, explaining their differences and uses.
In Part 1 (PW October 2021) it was shown that it was possible for diodes to conduct without overcoming the barrier voltage (0.2V for Germanium, 0.7V for Silicon). Fig. 1 shows a typical example when using a backup battery and silicon diodes to supply a standby voltage for a CMOS RAM. Very little current is needed by the device in standby. The circuit has 3V supplied by a lithium battery and taking as an example, if the standby resistance of the CMOS is 1MΩ, and the current drawn is 2.5wA (0.0025mA or 0.0000025A), then the voltage supplying the CMOS will be 2.5V.
This means that there is a total voltage drop (3V-2.5V) of 0.5V, so each of the diodes will have a 0.25V drop, which is well below the barrier voltage for a silicon diode. Replacing the CMOS with a load (resistance) of 100Ω, the current passing through the diodes and the resistor to the negative contact of the battery will be 14mA (0.014A).
The voltages at the diodes will have the barrier voltages (0.7V) and the voltage across the resistor load will be 1.4V (3V-1.4V = 1.5V = 0.75V drop per diode). These are rounded voltage numbers but indicate that when current is increased the barrier voltage comes into force.
Rectifier Diodes
Germanium diodes can be used as rectifiers when ‘detecting’ or ‘demodulating’ a radio signal but Silicon types are used when higher power is required such as in mains power supplies. The rectifier passes the current in only one direction so that when AC (varying positive and negative) is presented to the anode of the diode it only allows the positive current to pass, Fig. 2. This is commonly known as half-wave rectification as it is only conducting half of the full waveform.
NB: I am explaining the current as the conventional flow, which is positive to negative. We have learned now that current is a flow of electrons (negatively charged particles) that are attracted (flow) to the positive particles, which are called protons. Nothing is exact as there are many other factors involved but for these explanations I will use conventional current flow.
The circuit needs a reservoir capacitor along with a filter (inductor or resistor and a ‘smoothing’ capacitor) to complete the conversion from AC to DC. To convert all of the AC current to DC, full-wave rectification is used. Fig. 3 shows two types of fullwave rectification. The top circuit is using a transformer with a centre tap, which is the DC negative of the power supply. This transformer works in the same way as a conventional transformer (no centre tap) but produces two voltages, opposite each other with respect (reference) to the centre tap. When the top end of the secondary is positive the bottom is negative. When the top is positive the top diode conducts and when the bottom is positive the bottom diode conducts. This combines the two voltages at the cathodes of the diodes providing a DC output (pulsed) and then filtered in the usual way with the capacitor and inductor network. Because there are two diodes rectifying both parts of the phase it is called full-wave rectification (positive and negative parts of the waveform), which means the AC content is 100Hz (100 cycles per second). The frequency of the AC component of a halfwave rectified power supply is 50Hz, or 50 times per second. These are referring to UK mains frequencies. For low voltage power supplies the filter inductor is normally replaced with a wirewound (W/W) resistor because the current is greater (amps) than with the higher voltage power supplies where the current is usually in Milliamps (mA). To use an iron core inductor (choke) it would have to be a lot bigger to handle the current so an open-to-air W/W resistor is used.
Bridging it
The bottom diagram in Fig. 3 is using a normal transformer (one secondary winding) and the four diodes create bridge rectification. An explanation of how this works appears in PW Feb 2021 (From the Ground Up, Capacitors and capacitance, Part 2). The main difference is in the use of the different types of transformers. The one in the top diagram of Fig. 3 uses a transformer with a secondary centre tap so the voltage across the total (end-to-end) secondary winding needs to be twice that of the one in the bridge rectifier circuit to have the same DC voltage output because it is using a centre tap as the 0V DC supply.
Splitting it
Using a bridge rectifier and a tapped secondary transformer it is easy to make a split supply. This is a power supply that has positive and negative DC outputs that can be used to supply the voltages for OpAmps, as an example. This is more convenient than having two separate power supplies, one for a positive and another for a negative voltage.
It can be seen in Fig. 4 how this is achieved. The secondary of the mains transformer is connected to a bridge rectifier as in a standard bridge rectifier power supply but the transformer has a centre tap, which is used in a full wave power supply using two diodes. The centre tap is the 0V DC output and the positive voltage regulator provides the required positive DC output while a negative voltage regulator provides the negative DC voltage. The reservoir capacitors are usually very high capacitance values, typically about 1000µF for a low power (1A) supply. The regulators used for plus and minus (positive and negative) 12V power supplies are standard 7812 for the positive and 7912 for the negative voltages.
The current draw should be a maximum of 1A for each voltage when using these, which is more than capable of supplying several OpAmps. The ‘smoothing’ is provided within the regulators so there is no need for the filter (inductor/capacitor) circuit.
Doubling it
From Wikipedia: A voltage doubler is an electronic circuit which charges capacitors from the input voltage and switches these charges in such a way that, in the ideal case, exactly twice the voltage is produced at the output as at its input. The simplest of these circuits are a form of rectifier which take an AC voltage as input and outputs a doubled DC voltage. The switching elements are simple diodes and they are driven to switch state merely by the alternating voltage of the input. Voltage doublers are a variety of voltage multiplier circuit. Many, but not all, voltage doubler circuits can be viewed as a single stage of a higher order multiplier: cascading identical stages together achieves a greater voltage multiplication.
Shown in Fig. 5 there is an AC voltage source powering the circuit, which is a voltage consisting of a positive cycle for one half of the AC voltage and a negative cycle at the other half. When the AC is positive on the top of the AC cycle, the current travels through the diode D1 and charges up capacitor C1 and when it is fully charged, C1 equals the same voltage as the input voltage (5V).
When the bottom half of the AC cycle is positive, the current travels through diode D2 and charges up capacitor C2. Once again, capacitor C2 charges up to the same voltage as the input voltage. However, because there is 5V across C1 and now 10V across C2 they both add to provide 10V so the voltage is doubled, or twice the input voltage, as can be seen in Fig. 6. When the top half of the input is positive, diode D1 is forward biased, which allows current to flow through it and charge capacitor C1. When the bottom half of the cycle is positive, diode D2 is forward biased and allows current to flow through it and charge capacitor C2.
Tripling it
Fig. 7 is a Tripler circuit, which means the DC voltage output is three times the (AC) voltage input. As with the doubler circuit, diodes and electrolytic capacitors are the only components used. Let’s take the top as the positive half of the cycle to charge C1 and C3 through the diodes D1, D2 and D3. This will charge the capacitors to the source voltage (5V).
When the phase reverses to place the bottom half at positive, D2 conducts (forward biased) and C2 is charged to twice the source voltage because it is added to the already charged C1. C2 will have 5V on its negative lead plus 5V from C1 positive charge.
No current can pass through D1 as it is reverse biased. Diode D3 is also conducting during this period, placing a charge into C3, and this is added to the other capacitor charged voltages bringing the total charged voltage to three times (tripled), Fig. 8. There are other multiples such as four times and more.
Decouple it (ten times)
The diagram in Fig. 9 shows ten diodes and ten electrolytic capacitors as a times ten voltage multiplier. Let’s start with a positive at the bottom of the AC input so that the current signal travels through D1 and charges C1, which places 12V across it because the negative terminal of C1 is connected to the negative (top) of the AC input voltage. When the polarity changes so that the positive is at the top of the input voltage, it places 12V on the negative side of C1 and because there is 12V on the positive side this adds so that there is now 24V from the positive of C1 to the bottom of the AC input voltage signal (anode of D1).
The current now flows through D2 and charges C2 to 24V. The polarity of the supply is reversed so that the bottom is now positive and between the top input and the + of C2 is 32V. It flows through D3 and back to the top charging C3 to 25V. When the top of the AC input is positive, there is 12V at that point plus 12V on C1+ and 25V at C5 and the current flows through D4 and charges C4 to 50V. Reversing polarity places + at bottom such that current passes through D5 and charges. You can see how the voltage is building up during each charging of the capacitors.
At the junction of C1 and C3 there is 12V (derived from the rectified 10V AC) and where C2 and C4 join there is 25V so it has doubled. It has doubled again at the junction of C4 and C5, and at the end of this multiplier there is 100V DC, ten times the input voltage. This is high impedance as little current can be drawn because it is converted to voltage, meaning there is no power gain.
The voltage increases and the current available decreases. This is similar to a transformer where the voltage (or current) at the secondary may be higher but the power dissipation is the same in the primary and secondary.
Fig. 10 shows the voltage multiplier with a meter displaying the input AC voltage and another showing the multiplied DC output. A low impedance power supply will have a large electrolytic capacitor connected across the output but if a capacitor or any low impedance load was placed across this multiplier, the voltage would collapse because it is trying to draw current that is not available.
However, it can be used with a light load such as a photomultiplier tube where the anodes require very little current. The EHT (Extra High Voltage) is generated by the voltage multiplier connected to the main anode of the tube and the other 11 accelerator anodes are connected with a series of resistors (resistor chain). Voltage multipliers were commonplace in the CRT (Cathode Ray Tube) type television receivers to provide a high voltage, where 15kV or higher was obtained from a Tripler (or other multiples) connected to a tap on the line output transformer, which the television service engineer of years gone by will recall.
Controlling it
Current through a device can be controlled using a silicon controlled rectifier (SCR), part of the thyristor family. It is a threeterminal device with an anode and cathode, as in a conventional diode, but with a third terminal called a gate. This controls the current through the device and it can be considered a diode with a switch to turn it on (and off). As with diodes, an SCR is a unidirectional device as it can only conduct in one direction.
This is ‘triggered’ by a small positive current going into the gate to control a much larger current through the device (cathode
to anode). This is not to be confused with the amplification of the input current as with a transistor. The gate current is separate from the main current flowing through the device and is a control (signal) current. It is there to gate (turn on) the main current from cathode to anode (in electron terms) or in conventional current terminology, from anode to cathode.
It is annoying (to me) that the direction of current flow was not corrected many years ago. In Fig. 11 two body types of SCRs are shown with the connections on the TO220 case (part number 15/80H) easily seen. The other all-metal type (2N4170 or BT101 part number) may not have any markings to identify the terminals. It can be taken that the body is the anode, but it will be prudent to check with the manufacturer’s datasheet. The larger of the two tags is the cathode and the shorter one is the gate. If an ammeter is connected to the SCR as shown in Fig. 12 with a 5V DC power supply, no current will flow through the SCR and the ammeter until the switch is closed. When the switch is opened, the ammeter will still be showing current flow until the 5V is removed. Connecting the 5V again, no current will flow until the switch is closed again. The current when the switch is open can be turned off either by removing the 5V or reversing the polarity to the anode.
Dimming it
We saw that when a DC supply is connected to the SCR and an ammeter with the switch closed, current flows and still flows even when the switch is open. The only way to stop the current flowing is to either remove the power supply or reverse it. It not always convenient to reverse the supply but if the DC power were replaced with an AC power supply and the same test carried out, when the switch is closed current flows as in the last test but when the switch is open the current stops flowing. This is because the power supply has been reversed. In fact, it is reversing 50 times per second or at whatever AC frequency is being used.
This is the same as reversing the DC voltage on the last test. The advantage of using an AC power supply as the main source is that it allows the current to be varied and not just switched on and off. This is achieved by varying the DC voltage applied to the gate. Reducing this voltage reduces the main current flow and it can even be turned down very low to stop current flow altogether. But as the gate voltage is increased so is the main current thereby providing a variable current through the SCR, Fig. 13.
If the ammeter were replaced by a ‘bulb’ (incandescent lamp) of suitable voltage, the current could be varied through it, thereby dimming the lamp. Another use is to control the speed of a motor. This dimming or speed control is done at 50Hz (UK mains) because it is turning on at one half of the mains cycle and off at the other half. This may not be a problem although some flicker may be seen with the lamp dimmer.
Another use for an SCR is to protect an overvoltage in a power supply and used in this way it is called a ‘Crowbar’, Fig. 14. In the diagram the input to the 5V regulator is supplied from a 12V power supply via a fuse, which is rated for the circuit current requirements. If the voltage output from the regulator increases to an amount set by the Zener diode circuit, in this case 6V, the SCR will be triggered and a heavy current will flow through it thereby ‘blowing’ the power supply fuse.
This is a drastic way to protect a power supply, hence the term crowbar. The fault can then be located and repaired and with the fuse replaced the circuit should work again as normal without damage to any circuit components or the SCR and associated components.
Triacs (and Diacs)
A Triac is a three terminal device that can be used as an AC switch. This is similar in characteristics to but differs from the SCR in the sense that it conducts in both directions, so it is bidirectional when it is triggered by a positive or negative signal at the gate. The terminals are labelled MT1 (Main Terminal), MT2 and Gate.
Diac (Diode for Alternating Current)
A Diac is represented as a pair of back-toback diodes and is similar to a pair of Zener diodes in action as it only conducts when a certain voltage has been exceeded momentarily (called the breakover). It can be turned off by reducing the voltage below its avalanche breakdown. It is also known as a ‘transistor without a base’. It is bidirectional, so it can be turned on or off with either positive or negative polarities.
The diode I used in my test has a 2A rating and a trigger of 32V. The main application of a Diac is in a Triac triggering circuit. When it is connected to the gate terminal of the Triac and a suitable voltage is applied to the Diac, the Triac will conduct. When the voltage across the gate decreases below a predetermined value, the gate voltage will be zero and the Triac will be turned off.
A use of the Diac/Triac combination is with a mains lamp dimmer Fig. 15. This is a better method of the use for a lamp dimmer or motor speed controller etc because it uses both halves (positive and negative) of the AC waveform and the frequency is then 100Hz per second, so there is much less flicker (if any). One connection of the lamp is to the live lead of the household mains and the other to the MT2 terminal of the Triac. This also has a resistor (10kΩ in the diagram) connected to the lamp and Triac with the other end connected to a 1MΩ potentiometer. This value is not critical but should be quite high to provide a linear dimming of the lamp. The slider, which is connected to one lead of the Diac, is also connected to the other end terminal of the potentiometer because it is used as a variable resistor and not a potential divider so only the centre and one end is needed. There is also a capacitor connected from there to the neutral lead of the mains. The other end of the Diac is connected to the gate and the MT1 of the Triac is connected to the mains neutral lead. The purpose of the capacitor is to allow it to be charged through the potentiometer and when the required charged voltage is reached the Diac conducts and triggers the Triac.
Transorb
Transorb, Fig. 16, is a common name for a Transient Voltage Suppression (TVS) Diode and is used to protect sensitive electronics from voltage spikes. There are unidirectional (only conduct in one direction) and bidirectional types and similar to Zener diodes but are designed to handle very high peak currents and therefore they do not fail as would a standard Zener diode, and don’t need resetting or replacing after passing a heavy peak current as they will return to normal open state when the voltage spikes are not there.
As with Zener diodes they have voltage ratings and the correct type should be used in a circuit.
References
• Radio Communications Handbook, 14th Edition
• ARRL Handbook, 1995
• Electronics Engineer’s Reference Book, 6th Edition
• Circuits, Devices and Systems, 4th Edition Transistor, Thyristor & Diode Manual. RCA