Obtaining High HT Voltages with Standard Transformers
G A French, The Radio Constructor, May, 1967.
This appeard as part of the Suggested Circuits series and was number 198.
Home constructors whose interests are mainly centred on receivers and low-power amplifiers using valves normally work with standard mains transformers having HT secondary voltages in the range of 200-0-200 Volts to 300-0-300 volts. Occasionally, however, it becomes necessary to provide HT voltages at appreciable currents which are higher than can be obtained with transformers of this type. A typical example would, for instance, be given by the construction of a high-power PA amplifier.
In cases of this nature the constructor may find that the standard mains transformers he has on hand are not capable of providing the higher voltages required and he becomes faced with the possibility of having to buy a new mains transformer whose price may be relatively quite high.
Amateur transmitting enthusiasts frequently employ high HT voltages at high currents in their equipment, and may similarly encounter the necessity of purchasing expensive mains transformers with high HT secondary voltages when they already have a, stock of lower voltage transformers.
This months Suggested Circuit describes a method of obtaining high HT voltages which uses two standard low-voltage mains transformers having centre-tapped HT secondaries, and which thereby enables constructors to employ components which may already be on hand to meet a high voltage requirement. The circuit suffers from two disadvantages, the first of these being that the HT secondary of one of the transformers employed operates at a higher potential above chassis than would be the case in normal service. Constructors using the circuit have, therefore, to accept the fact that there is an increased risk of insulation breakdown in the transformer concerned although, if the mains transformer is a well designed component, this risk should not be too high. The second disadvantage is that a full-wave thermionic rectifier having a common cathode for the two diodes cannot be used in the circuit.
Readers making up the circuit should be familiar with simple power supply theory and practice as well as the precautions against shock which are necessary with high voltage equipment. Construction of the circuit should not be attempted by beginners who do not understand the principles involved.
Employing Two Transformers
Fig. 1. Obtaining a high rectifledvoltage from two mains transformers having relatively low HT secondary voltages. Heater windings are not shown. The two rectifiers may, in practice, be combined in a single envelope to form a conventional full-wave rectifier valve. The rectifier heater supply can be obtained from a suitable winding on either transformer.
Before proceeding to the circuit proper, let us first examine one simple method of obtaining an increased HT voltage from two transformers having relatively low voltage HT secondaries.
Fig. 1 shows two transformers, both of which have 200-0-200 Volt HT secondaries. These are connected so that the entire HT secondary of one transformer couples to the anode of one rectifier and the entire HT secondary of the other transformer couples to the anode of a second rectifier. Also, connections are such that when one anode goes positive during the AC cycle the other goes negative, and vice versa. The result is exactly the same as would be given in a normal full-wave circuit in which the anodes are fed from a 400-0-400 Volt HT secondary. Thus, the two transformers are able to provide an HT voltage which is twice that offered by a single transformer on its own.
The circuit of Fig. 1 is quite practicable if both transformers offer exactly the same HT secondary voltage. (This need not necessarily, of course, be the 200-0-200 volt figure shown in Fig. 1, which was chosen for purposes of explanation.) Ideally, the two transformers should be identical components, whereupon the maximum rectified HT current available from the rectifier cathodes is the same as that for which each HT secondary is rated. Should the two transformers be markedly dissimilar (as would occur if, say, one had a singificantly lower HT secondary current, rating than the other) the winding resistance and losses in the two halves of the rectifier circuit would become unequal and rectification efficiency would be lower on one half-cycle than on the other.
In Fig. 1, it should be added, heater windings and wiring have been omitted for simplicity of explanation. In practice, the two rectifiers could be combined in a single envelope as a conventional full-wave rectifier with a common cathode.
As may be gathered, the circuit of Fig. 1 is quite useful but it requires that, apart from offering the same HT secondary voltage, both transformers should be reasonably equivalent in other respects as well. Note, incidentally, that the end of each HT secondary remote from the chassis connection carries twice the alternating voltage that would appear if the transformer were employed under normal circuit conditions.
Fig; 2. An alternative method of fusing two transformers to obtain high HT voltages. Directly heated rectifiers can be employed instead of the indirectly heated types shown here. The heater windings on either or both transformers may be applied to equipment heaters in normal fashion.
In Fig. 2, which represents this months Suggested Circuit, two mains transformers are employed in an alternative arrangement which offers a higher rectified HT voltage than is available from either. The transformers do not have to be identical and both the HT secondary voltage and current ratings may be different. The rectified HT voltage is the same as would be given, in a conventional circuit, with a full-wave HT secondary offering a voltage equal to the sum of the two HT secondaries in Fig. 2. Thus, if one transformer in Fig. 2 had an HT secondary voltage of 200-0-200 and the other an HT secondary voltage of 250-0-250, the rectified voltage would be the same as would occur with a conventional full-wave rectifier circuit using a single HT secondary of 450-0-450 Volts. The maximum rectified current available is equal to the lower current rating of the two HT secondaries. If one transformer had an HT secondary current rating of 100mA and the other an HT secondary current rating of 60mA,. the maximum rectified output current would be 60mA.
The transformers are connected so that the secondaries are applied to the rectifiers in anti-phase. When the upper end of T1 HT secondary is positive during the AC cycle, the upper end of T2 HT secondary is negative, and vice versa.
There are several ways of examining the operation of the circuit of Fig. 2 and the explanation which now follows is, perhaps, as simple as any.
Let us commence at an instant during the AC cycle when the upper end of T1 HT secondary is at positive peak potential relative to chassis. Under this condition rectifier V1 is capable of conducting, whereupon this positive peak potential becomes available at its cathode. At the same instant the upper end of T2 HT secondary is at peak negative potential relative to its centre-tap, with the result that the voltage between its upper end and the centre-tap is added to that at V1 cathode. The voltage at the positive output terminal at this instant is in consequence, equal to the voltage appearing across the upper half of T1 HT secondary plus the voltage appearing across the upper half of T2 HT secondary. At the peak in the next half-cycle it is the lower end of T1 HT secondary which is at peak positive potential relative to chassis, and it is the lower end of T2 HT secondary which is at peak negative potential relative to its centre-tap. This time V2 conducts and, once again, a positive potential equal to the sum of the voltages in each half of the HT secondaries appears at the positive output terminal.
The output terminals of the circuit of Fig. 2 may be applied to either a choke input filter (in which the first smoothing component is a choke) or to a capacitor input filter (in which the first smoothing component is a reservoir capacitor). When the rectified output is applied to a choke input filter, the voltage at the positive output terminal will consist of rectified positive half-cycles in just the same manner as would be given by a conventional full-wave rectifier circuit. The arrangement of Fig. 2 is, indeed, a modified full-wave circuit, the only change being that the transformer windings are in effect split, with the rectifiers appearing at the junctions, of the two sections instead of at their ends. If the output terminals are applied to a capacitor input filter, the reservoir capacitor will tend to charge up to peak rectified voltage in the same way as occurs with a conventional full-wave rectifier circuit.
To find the peak inverse voltage applied to each rectifier, We may examine the circuit at an instant in the AC cycle when the upper end of T1 HT secondary is at peak positive potential relative to chassis. Under this condition V1 conducts and may be looked upon as a short-circuit. The inverse voltage applied to V2 is then the peak voltage appearing across the entire HT secondary of T1 plus the peak voltage appearing across the entire HT secondary of T2. This is the peak inverse voltage to which each rectifier is subjected. This peak inverse voltage is 1.4 times the RMS voltage ascribed to each of the HT secondaries with the result that, if we were to use the 200-0-200 Volt and 250-0-250 Volt HT secondaries mentioned just now, the peak inverse voltage applied to each rectifier would be 1.4 x (400 + 500), or 1,260 Volts. This is the same peak inverse rating that we would obtain with a conventional full-wave rectifier circuit using a 450-0-450 volt HT secondary, and in which the peak inverse voltage would be 1.4 times the RMS voltage across the entire HT secondary. This peak inverse voltage figure applies when either a choke or capacitor input filter is employed.
With the circuit of Fig. 2, the insulation between the HT secondary of T2 and the transformer metalwork and other windings is subjected to higher potentials than would occur in normal service. This is the first disadvantage referred to earlier. T2 HT secondary centre-tap would normally be at chassis potential but it is now at rectified positive output potential (during peaks, with a choke input filter) and so the transformer insulation has to withstand this added potential. The constructor using the circuit has to accept the fact that the risk of transformer breakdown is increased in consequence. In practice, it would be desirable to use a reliable and well-made component in the T2 position.
Thermionic rectifiers are assumed in Fig. 2 and it will probably be found most convenient, with practical transformers, to use the rectifier heater winding on one transformer for one rectifier and the rectifier heater winding on the other transformer for the second rectifier. Since the cathodes of V1 and V2 are not at the same potential it is not possible for these two diodes to be combined in a single envelope as occurs with a conventional full-wave rectifier circuit, and this represents a second possible disadvantage with the circuit. Instead of thermionic rectifiers, a chain of silicon rectifiers suitably bridged by parallel resistors and capacitors may alternatively be employed, these following standard high voltage practice.
If it is intended to fuse the circuit when a choke input filter is employed, a fuse may be inserted in series with the anode of each rectifier. The fuses may be similarly inserted in series with the anodes when a capacitor input filter is employed, but it has to be remembered that the fuses will now also have to carry the ripple current in the reservoir capacitor and may need a higher rating than with a choke input filter. Also, it will be necessary to use thermionic rectifiers and to ensure that the AC mains is only switched on when these are cold. If the mains supply were switched on with the rectifier cathodes at operating temperature the resultant charging current in the reservoir capacitor would blow the fuses. The fuses could similarly blow if silicon rectifiers were employed with a capacitor input filter.
Fig. 3. Adding a full-wave rectifier to Fig. 2 enables a second and lower output voltage to be obtained. The manner in which V3 may be heated is discussed in the text.
An interesting modification to the basic circuit of Fig. 2 is illustrated in Fig. 3 and this could be particularly useful for amateur transmitting equipment in which earlier stages are operated at a lower HT voltage than the final stage. In Fig. 3, a high HT voltage is obtained from the combination of the two transformer HT secondaries as before, this voltage being designated HT+2 in the diagram.
At the same time a lower HT voltage, indicated as HT+1, is obtained from a conventional full- wave rectifier, V3, coupled to the ends of the HT secondary of T1. V3 and T1 HT secondary form a standard full-wave rectifier circuit, and the HT+1 output voltage is that to be expected from the voltage rating of T1 HT secondary. The peak inverse voltage applied to the diodes of V3 is 1.4 times the RMS voltage across the entire HT secondary of T1, as is given with any normal full-wave circuit. A separate heater winding is required for V3 although, if this valve is of the type in which a high potential is allowed between heater and cathode (as in the EZ81, for instance), it may be found possible to run its heater from a common 6.3 Volt heater line at chassis potential.
An advantage of the circuit of Fig. 3 is that the HTsecondary current rating for T2 need only be sufficiently high to satisfy the requirements of the HT+2 output. The HT secondary current rating for T1 must be equal to or greater than the sum of the HT+2 and HT+1 currents. Thus, the circuit arrangement of Fig. 3 is capable of enabling both high and low HT voltage requirements to be satisfied, whilst being relatively economic in HT secondary current ratings.
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