Modifying a 240V / 50 Hz switch mode power supply for 120 V / 60 Hz

Most switch mode power supplies can handle both 120 or 240 Volts just fine. They usually either have a switch to select the input voltage or are simply designed to cover the whole range to begin with. But what if one needs to operate a switch mode power supply, that is exclusively designed for 240 Volt / 50 Hz, on 120 V / 60 Hz?

Operating a 240 V / 50 Hz switch mode power supply on 120 V / 60 Hz is not necessarily impossible even if the power supply has no input voltage selector or explicitly demands a higher voltage. A little change of the input circuitry does the trick.

WARNING: This article is for information purposes only. Only absolute experts who understand all security related aspects of this modification should consider doing this on their own risk. Improper precautions may cause fire, serious injury and / or death.

The first stage in a switch mode power supply is a rectifier. The mains voltage is being rectified and smoothed.

The first stage in a switch mode power supply: Bridge rectifier

The first stage in a switch mode power supply: Bridge rectifier

In case of my Peaktech 1540 a bridge rectifier and two 3.3 µF capacitors take care of this part. The Peaktech 1540 is one OEM name of the Manson SPS9400. Other names include Jaycar Powertech MP3090 and BK Precision 1692.

Inside view of a Peaktech 1540 switch mode lab power supply

Inside view of a Peaktech 1540 switch mode lab power supply

Close-up view of the High Voltage Section of a Peaktech 1540 switch mode lab power supply. Clearly visible is the bridge rectifier (w. heat sink, bottom left) and the two 3.3 uF smoothing caps (top left).

Close-up view of the High Voltage Section of a Peaktech 1540 switch mode lab power supply. Clearly visible is the bridge rectifier (w. heat sink, bottom left) and the two 3.3 uF smoothening caps (top left).

What one essentially needs to do in order to use a 240V / 50 Hz switch mode power supply with 120 V / 60 Hz is to find a circuit that replaces the circuit above at 120 V / 60 Hz and acts equivalent. Luckily, a Delon voltage doubler circuit can do this perfectly.

This simple voltage doubler turns the 120 V AC mains into about 340 Volts DC

This simple voltage doubler turns the 120 V AC mains into about 340 Volts DC

Basically, the diodes make sure that C1 and C2 get charged alternately. One capacitor gets charged during the positive half-wave of the mains cycle and the other capacitor gets charged during the negative half-wave. For the DC output side of the circuit both capacitors appear in series and therefore with twice the voltage compared to C1 and C2 individually.

Care needs to be taken to pick the right size capacitor values for C1 and C2. If the values are too big, the diodes might not like the high currents and create some magic smoke [1] and refuse further service. If the capacitors are too small, the energy stored in the caps may not be enough for the following stages in the power supply.

I personally usually look up the overall capacitance of the next stage in the power supply and try to use about twice that capacity. That compensates for the charging rate difference due to the different frequencies. The original bridge rectifier effectively doubles the European 50 Hz mains and re-charges the original caps at a rate of 100 Hz. The Delon voltage doubler preserves the mains frequency, in this case 60 Hz. So essentially the capacitors need to ‘wait’ a bit longer until they get a new load of electrons.

CAVEAT: It is important to understand that the modified device will draw approximately twice the current at 120 V / 60 Hz than it would draw at 240 V / 50 Hz. Important parts of the device, such as power connectors / wires and EMI-Filters, may not be designed for such high currents. Failure to modify those parts may result in fire or explosion causing property damage, personal injury, or death.

Links and Sources:
[1] Wikipedia, Magic Smoke http://en.wikipedia.org/

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