Simple Voltage Converters

This subproject deals with simple circuits that are voltage converters. These circuits are not really useful anymore but do help demonstrate some simple circuit techniques. They mainly help to reinforce the topics of 555 timers, diode clamping, voltage double, and zener regulation. A few other circuits create their own oscillators and they will be discussed.

These are circuits that use 555 timers to generate a square wave for the voltage conversion. The generated square wave is put through external circuitry such as a voltage clamping circuit or a voltage doubler. These simple techniques were introduced in ELEG309 but are reinforced in these circuits.

These circuits used clamping to change the output voltage. The same tecnique could be used for both increasing and decreasing the voltage based on the position and orientation of the clamping diode. These circuits use very few components and the external components used are very common and easy to find. They only require a few resistors, capacitors, and diodes along with the 555 timer.

Circuit clamping negative.

Circuit clamping positive.

The operation of these circuits is quite simple and straight forward after looked into. The 555 timers along with R1, R2, C1, and C2 in the first circuit determine and create the square wave oscillation. The part of the circuit that actually provides the conversion is C3 and the diode going to ground. What happens is during the positive portion of the square wave the diode is forward biased and thus C3 is between the output of the 555 and ground. This allows C3 to charge up to approximately the output voltage. Then when the output becomes zero the diode turns off and the capacitor discharges. Since it was charged positive it now disharges and looks negative. The second diode is oriented to only allow for the negative waveform to go through. Then finally C4 acts as an envelope detector or filtering capacitor to create a constant DC output.

Looking at the second circuit you can see a similar setup just different positiong and orientation of the output diodes and capacitors. This time the first diode is oriented to VCC instead of ground. This means that when the output is zero the diode is on and charges the capacitor. Then when the output is positive the voltage of the capcitor and the waveform add together. Then they go through the output diode which ensures positive output and once again are smoothed by the capacitor.

So the capacitor size and oscillation frequency along with applied load all are important in determining the exact output voltage. This is because the larger the load the large the capacitor you need to maintain voltage. Also, the faster the oscillation the more constant the output voltage.

Like the clamper circuit, this circuit also uses the same sort of 555 timer setup along with some extra capacitors and diodes. These can work as also voltage triplers and quadruplers if used corrctly. This is shown in the following circuit.

Circuit showing a voltage doubler.

The basic operation of the voltage doubler is similar to the clamper in some ways. Using the below figure a descriptoin of operation from Sedra and Smith will be given.

Looking at the first capacitor and diode, one may notice that is is nothing but a clamping circuit. This works in exactly the same way as the earlier clamper circuits, allowing the voltage to get clamped around itself so infact you get double the negative voltage. So in fact the earlier circuits which were used to clamp the voltage actually are voltage doublers just with capacitors and oscillations rates set to not double the voltage.

Now looking at the above doubler circuit one can see that the same principle is applied. This time however, the diodes are going to VCC instead of ground. So, as before they once again increase the voltage, this time it should be effectively doubled. There is then a cascade of these doubling stages which can be used to increase the voltage further. However, each time the voltage is increased the avaliable current is decreased.

Besides the 555 timer circuits which used the 555 timer to generate the oscillation there were several other circuits which created their own oscillation or switching. Each will be briefly discussed but details of operation will not be gone into as they more complex. Notes on some of the operation were also lost at some point during research.

This first circuit uses transistors and an inductor which were not seen in the 555 timer circuits. By reading the description next to the diagram one can see that it creates an oscillating signal. This signal is then amplified by the npn and it creates a switching supply. The switching output is then rectified and regulated by the output stage.

The following picture shows the output signal just before the rectifying diode. The diode blocks the positive half so you only have to the negative output which is then smoothed by the capacitor and regulated by the zener.

This is a second circuit which also creates its own oscillation, this time with an inverter chip.

This is an interesting circuit in the way it operates. The first two inverter, a and b along with the capacitor and resistor create an oscillator. The output of these two then go through 4 inverters in parallel. This gives extra current driving capacity to give a better output. The chip used was a CD4069 from National Semiconductors which is the same as a 74C04. This is the same as the chip used in the circuit. However, a 74ls04 was used it didn't function at all. Looking at the specs the 74ls04 has a 5v max input and also it doesn't have the same current capabilities as the 74C04. Here is a picture of the output of the oscillator.

This oscillating signal then seems to go through what appears to be an adjustable clamping circuit. The reason it is assumed to be adjustable is the transistor would control if the diode to ground is on or not. There also appears to be a voltage doubler structure which leads to the output of the circuit. The zener helps to control the switching of the transistor. Unlike previous circuits this one seems to be very constant with the output voltage as it appears to be somewhat regulated.

The previous converters were all DC to DC thus they had to create their own oscillation or switching. This circuit converts AC to DC just like the last stage of the DC converters. Here is the diagram

This circuit works because of the zener diodes. The zeners are placed parallel to the recitfiying diodes and the capacitors thus limiting the voltage to the zener voltage. So the AC signal comes in and is clamped to the zener voltage on both the positive and negative sides by each set of zeners. Then the rectifying diodes remove any of the unwanted voltage from the signal and finally the capacitor smooths out the signal.

Note the voltage scales. The top signal is the voltage at the zeners and the bottom is the AC input. The top signal goes to the zener voltage at the peak and goes lightly negative as the zener works as a normal diode when forward biased.

This circuit however could be extremely dangerous because there is no isolation from the AC line. If too much current was drawn and the zener were to cut out of zener mode and become an open it could cause the powered device to get a large voltage across it causing damage. Another problem is that the resistors being used at 5W resistors because of the heat generated. Each sees about 45mA disipating 3.675W each which produces a lot of heat. So this circuit is both ineffiecent and dangerous but shows a neat use of a zener diode.

Previously I made a very simple AC to DC voltage converter. Unlike the circuit which pulls straight from the power line this one has an isolation transformer which actually steps up the voltage slightly from 120 to 140V. Then it goes through a full wave rectifier. Finally there are two very large capacitors which smooth out the signal and provide the DC voltage. The adjustability is provided by the placement of a variac on the input of the supply. The variac is also called an autotransformer. It works by having a primary coil and then a wiper which can be moved and allow the number of turns on the second to change thus adjusting the voltage.

As can be seen from the above circuits these are not really practical. One point is that a circuit which doubles the voltage in essence uses the same techniques as one that decreases it from 12 to -8. The circuits using the 555 timers are inefficient and are not regulated. The ideas in them are useful and important but better voltage converters are out there. The circuit which converts the AC to DC from the powerline is a dangerous circuit. There is no isolation. Also, if the zener were to cut out of break the powered circuit could get a very large AC voltage at the input and break it. With transforms that step down the voltage this problem is somewhat removed.

Some more practical conversion circuits are using power regulators, charge pump, buck, boost, buck boost, and flyback converters among others. Switching power supply design can be looked up online for more information on these topics. The flyback converter will be discussed more in the strobe section.