<Tr> <Td> Risk of electric shock </Td> <Td> Supplies with transformers isolate the incoming power supply from the powered device and so allow metalwork of the enclosure to be grounded safely . Dangerous if primary / secondary insulation breaks down, unlikely with reasonable design . Transformerless mains - operated supply dangerous . In both linear and switch - mode the mains, and possibly the output voltages, are hazardous and must be well - isolated . </Td> <Td> Common rail of equipment (including casing) is energized to half the mains voltage, but at high impedance, unless equipment is earthed / grounded or doesn't contain EMI / RFI filtering at the input terminals . </Td> <Td> Due to regulations concerning EMI / RFI radiation, many SMPS contain EMI / RFI filtering at the input stage consisting of capacitors and inductors before the bridge rectifier . Two capacitors are connected in series with the Live and Neutral rails with the Earth connection in between the two capacitors . This forms a capacitive divider that energizes the common rail at half mains voltage . Its high impedance current source can provide a tingling or a' bite' to the operator or can be exploited to light an Earth Fault LED . However, this current may cause nuisance tripping on the most sensitive residual - current devices . </Td> </Tr> <Tr> <Td> Risk of equipment damage </Td> <Td> Very low, unless a short occurs between the primary and secondary windings or the regulator fails by shorting internally . </Td> <Td> Can fail so as to make output voltage very high . Stress on capacitors may cause them to explode . Can in some cases destroy input stages in amplifiers if floating voltage exceeds transistor base - emitter breakdown voltage, causing the transistor's gain to drop and noise levels to increase . Mitigated by good failsafe design . Failure of a component in the SMPS itself can cause further damage to other PSU components; can be difficult to troubleshoot . </Td> <Td> The floating voltage is caused by capacitors bridging the primary and secondary sides of the power supply . Connection to earthed equipment will cause a momentary (and potentially destructive) spike in current at the connector as the voltage at the secondary side of the capacitor equalizes to earth potential . </Td> </Tr> <P> If the SMPS has an AC input, then the first stage is to convert the input to DC . This is called rectification . A SMPS with a DC input does not require this stage . In some power supplies (mostly computer ATX power supplies), the rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically . This feature permits operation from power sources that are normally at 115 V or at 230 V . The rectifier produces an unregulated DC voltage which is then sent to a large filter capacitor . The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks . These pulses have significant high frequency energy which reduces the power factor . To correct for this, many newer SMPS will use a special PFC circuit to make the input current follow the sinusoidal shape of the AC input voltage, correcting the power factor . Power supplies that use active PFC usually are auto - ranging, supporting input voltages from ~ 100 VAC--250 VAC, with no input voltage selector switch . </P> <P> An SMPS designed for AC input can usually be run from a DC supply, because the DC would pass through the rectifier unchanged . If the power supply is designed for 115 VAC and has no voltage selector switch, the required DC voltage would be 163 VDC (115 × √ 2). This type of use may be harmful to the rectifier stage, however, as it will only use half of diodes in the rectifier for the full load . This could possibly result in overheating of these components, causing them to fail prematurely . On the other hand, if the power supply has a voltage selector switch, based on the Delon circuit, for 115 / 230V (computer ATX power supplies typically are in this category), the selector switch would have to be put in the 230 V position, and the required voltage would be 325 VDC (230 × √ 2). The diodes in this type of power supply will handle the DC current just fine because they are rated to handle double the nominal input current when operated in the 115 V mode, due to the operation of the voltage doubler . This is because the doubler, when in operation, uses only half of the bridge rectifier and runs twice as much current through it . It is uncertain how an Auto - ranging / Active - PFC type power supply would react to being powered by DC . </P>

Which circuit is the first stage of a dc power supply