High Voltage Generation For Oven Working
In this content nosotros are going to run across close the circuit that aids for high voltage generation needed for performance of microwave oven.The magnetron needs a high negative voltage to last applied to its cathode for right operation. Before nosotros commence amongst a description of the high voltage circuit in addition to its operation, it is essential that nosotros indicate out the Dangers that be when working amongst such a circuit
Influenza A virus subtype H5N1 typical high voltage circuit used to generate the magnetron furnish is shown below.
CIRCUIT DESCRIPTION:
The circuit consists of a leakage transformer, commonly referred to every bit the High Voltage or Power transformer, which has 3 windings. The primary winding to which the mains furnish is applied. The high voltage secondary which steps upwards the mains input to 2000V A.C. in addition to a filament winding that produces a depression voltage amongst high electrical current furnish for the filament of the magnetron. The other components inwards the high voltage circuit are the high voltage capacitor in addition to high voltage diode. Influenza A virus subtype H5N1 compensative(bleed) resistor is added to the circuit, commonly incorporated inwards the tin of the high voltage capacitor, which discharges the capacitor when ability is removed.
It is essential that the abrupt final connections on the magnetron are secure. If these connections larn loose or oxidised the resistance of the articulation volition larn high. Then because of the high electrical current time period localised heating volition occur. This heating volition campaign farther oxidisation in addition to volition every bit good loosen the joints further, due to expansion, resulting inwards higher resistance inwards the joints in addition to farther heating of the connections in addition to possible arcing. The final result tin last damaged terminals or insulation cloth on the magnetron requiring its premature replacement in addition to harm to the connecting leads. When re-fitting the abrupt connectors to the magnetron afterwards service, ever ensure they are create clean in addition to tightly secured.
As previously stated the ability transformer filament winding produces a depression voltage A.C. It should nevertheless last noted that the HIGH VOLTAGE circuit is connected to ane filament connection. This way that all of the wiring in addition to connections inwards this purpose of the circuit volition receive got a high negative potential amongst honor to ground. The voltage across the filament 2.5 to 3V A.C., the negative potential from all parts of the filament circuit to reason is Connected to the high voltage secondary winding is the remaining circuitry, the high voltage capacitor, high voltage diode, bleed resistor in addition to of course of didactics the magnetron. The capacitor in addition to diode shape what is oft referred to every bit a 'Half Wave Doubler', it carries out D.C. restoration of the A.C. input from the transformer secondary.
LEAKAGE TRANSFORMER:
The type of transformer used inwards the high voltage circuit of a Sharp microwave oven is known every bit a leakage transformer. It is designed to receive got to a greater extent than magnetic flux leakage than the touchstone type of transformer, that is to say that non all of the magnetic flux produced past times the primary winding finds a magnetic path to impact the secondary windings. There is a magnetic shunt built into the assembly that allows the magnetic time period to receive got an option path.
As far every bit the performance is concerned, this has the lawsuit of allowing the voltage to vary inwards club to hold a fairly constant electrical current through the magnetron anode circuit. The reactance of the transformer effectively changes amongst the load.
It is of import where a magnetron is existence used that its anode electrical current is maintained at a constant value during performance or the output ability volition vary dramatically. The leakage transformer performance volition accomplish this.
If the high voltage A.C. across the secondary winding were to last measured it would last seen to vary considerably every bit the charge inwards the oven moved about the cavity on the turntable. This is inwards answer to the changing charge position on the magnetron. It would every bit good last noticed however, that because of the activity of the transformer in addition to one-half moving ridge doubler circuit, that the high negative D.C. voltage across the magnetron would stay stable, every bit does the anode current.
HALF WAVE DOUBLER:
The circuit below shows a simplified schematic diagram of the 'Half Wave Doubler’ circuit. To last to a greater extent than precise its activity on the input waveform is to a greater extent than consistent amongst a D.C. restoration circuit. The doubler label comes from voltage measurements made on this circuit amongst no magnetron load. Influenza A virus subtype H5N1 voltmeter connected inwards seat 'A' inwards the diagram below would present 2000V ac., in addition to -4000V D.C. inwards seat B. When analysing these results, it could last said that a doubling of the input voltage had occurred. However if the same measurements were made using an oscilloscope, it would last seen that the waveform at seat 'B' is the same every bit at seat 'A'. The circuit has shifted the waveform amongst honor to the naught trouble (ground) this is every bit good shown inwards the diagram below.
The argue the meter shows a doubling of voltage is that the same reference indicate is used, that is ground. One atomic number 82 of the meter is connected to reason when taking both voltage measurements. In the illustration of waveform 'A', the meter is assail the A.C. voltage range. This volition present the RMS value of ane half-cycle, amongst honor to ground, this would last unopen to 2000V.
In the illustration of waveform 'B' the meter would last laid to mensurate D.C. voltage, because the waveform is unidirectional, e.g. negative only. The meter would read -4000V, which is the RMS of the peak to peak voltage on this waveform, which is every bit good equal to the peak to peak value of waveform 'A'.
When the magnetron is connected to the circuit, it volition impact the waveform at seat 'B'. The alter is due to the electrical current existence drawn past times the magnetron. The magnetron volition laid out to bear at close -2,000 volts, it in addition to so acts every bit a shunt regulator in addition to clamps the waveform at that point. Now when nosotros cheque amongst a meter the voltage would last inwards the club of -2000V D.C.
We tin run across from the diagram on the previous page that the magnetron is pulsed on in addition to off at mains frequency hence the RF output volition last introduce for solely 50% of the time. The RF unloosen energy is pulsating at 50Hz in addition to is solely introduce during the negative half-cycle of the mains supply. This way nosotros solely larn close 50% efficiency from the magnetron, the final result is that the output ability is solely one-half of the ability consumed past times the oven. However, this pulsing helps to foreclose harm occurring to the magnetron caused past times overheating.
OPERATION:
The circuit has ii dissimilar fourth dimension constants. When the input signal produces a forrad bias potential on the diode, the full resistance is the charge resistance inwards parallel amongst the diode forrad resistance, hence the full resistance is low. When the diode is contrary biased, the full resistance is the charge resistance, which is high.
Time = Resistance x Capacitance
Using the higher upwards equation, it tin last seen that when the diode is conducting the fourth dimension constant is really brusk in addition to when the diode is non conducting in addition to so the fourth dimension constant becomes much longer. The capacitor charges speedily through the diode in addition to discharges piece of cake through the charge resistance.
To sympathise the performance of the circuit assume that the capacitor is initially uncharged. During the offset quarter cycle the diode is forrad biased. Current volition straightaway time period via the diode in addition to the capacitor is charged. The polarity of the accuse is shown inwards the diagram below. The degree of the accuse volition last a trivial nether the peak value of the input.
As before long every bit the voltage on the input side starts to fall, the diode volition larn contrary biased, amongst the accuse on the capacitor belongings the diode off. The input circuit volition straightaway render the positive plate of the capacitor to ground, hence the other plate of the capacitor volition last taken negative. In fact the negative plate of the capacitor volition ever stay at a voltage equal to input less the peak value. When the input approaches its side past times side positive peak whatsoever lost accuse volition last replaced.
With the magnetron connected in addition to drawing electrical current the circuit volition hold a voltage nearing the peak to peak value of the input waveform. The output does nevertheless alter moving ridge shape due to the activity of the leakage transformer in addition to the electrical current consumption of the magnetron.
As already stated the accuse on the capacitor is effectively added to the input waveform, to plough over an output that is shifted upwards or downwards depending on the polarity of charge. In the illustration of a microwave oven the shift is negative, every bit a negative potential is needed for the cathode.
The accuse gives us the deviation betwixt input in addition to output. If the capacitor starts to discharge, the diode activity ensures that on the side past times side positive peak the lost accuse is replaced. In other words if the capacitor accuse is reduced past times an increased load, the diode electrical current increases. This fact way that diode electrical current volition last equal to the electrical current existence drawn past times the charge on the doubler circuit. In the illustration of the microwave oven the charge is the magnetron, in addition to hence diode electrical current volition last the same every bit the magnetron anode current.
There is ane farther indicate to last considered piece dealing amongst this circuit, that is the value of the capacitor. The larger the value, the to a greater extent than electrical current available for the magnetron to depict on. Therefore, the value of the capacitor volition impact the ability output of the oven. The larger value the greater the output.
When replacing capacitors inwards the high voltage circuit of a microwave oven it is really of import that the same value is used every bit the master copy ane fitted during manufacture. It should last noted that dissimilar models amongst like output powers, may receive got dissimilar values of capacitors, hence dissimilar transformers volition last used. Also models amongst dissimilar output powers may utilization the same magnetron, amongst the output existence controlled past times the capacitor value.
From the security expression it should every bit good last noted that a bleed resistor is built into the capacitor, which volition discharge the capacitor when the ability is removed. In some models the bleed resistor is built into the diode. To foreclose a potentially hazardous province of affairs developing, when replacing the capacitor or high voltage diode, ever utilization the right type past times purpose number.
When checking these components, if it is institute that the bleed resistor has gone high inwards value or opened upwards circuit, the element should last regarded every bit faulty in addition to replaced.
ASYMMETRIC RECTIFIER:
The Asymmetric Rectifier or Short Protector, every bit it is sometimes called, is included inwards the circuit to foreclose harm to the high voltage transformer in addition to high voltage capacitor, inwards illustration of a brusk circuit occurring on the magnetron or high voltage diode. The brusk protector volition campaign the trouble fuse to blow inwards the higher upwards error conditions.
As tin last seen inwards the diagram at the top that the device volition non bear inwards whatsoever direction, hence has no lawsuit on the circuit nether normal operating conditions. The peak inverse breakdown voltage of D1 is 6kV in addition to D2 is 1.5kV. Under error weather condition when a brusk circuit occurs the peak inverse voltage of D2 volition last exceeded, this volition campaign the diode to drib dead brusk circuit. This inwards plough volition campaign a really high forrad electrical current through D1, causing it to drib dead brusk circuit also. This volition campaign a dead brusk to seem across the high voltage transformer secondary, which volition blow the trouble fuse, removing ability to the high voltage transformer.
HIGH VOLTAGE DIODE:
We receive got seen that really high voltages are generated inwards the high voltage circuit, in addition to hence during performance the high voltage diode is subjected to high peak inverse voltages. This way that a peculiarly constructed diode is needed to withstand such voltages. However it is non possible to industry a unmarried diode amongst the required characteristics, this is achieved past times connecting 20 diodes inwards series. Therefore the peak inverse breakdown voltages of each of the diodes are added, which provides the required overall peak inverse breakdown voltage. This needs to last borne inwards heed when carrying out resistance checks on high voltage diodes.
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