[Altair (OP)]

Basically I have a need of 5 volts of continuous voltage but my source can only provide 50 mV of dc voltage. So can someone please tell how will it be possible, please keep in mind that the system should be as light as possible...

Also I was wondering whether an oscillator can be used to create dc into ac for the above given magnitude of current.

[peppercorn]

50mV source? - what is the source?
Also you'll need to give an idea of the current your load requires.
And you want 5v AC out - yes? (even though you've written 'continuous voltage')

...be aware that having 100 times greater voltage demanded means (at least) a 100x larger current draw from the source.

[CliffordK]

You should be able to build an IC clock circuit that would convert your DC to AC, or perhaps a simple crystal oscillator, then it should be able to be fed into a transformer to boost the voltage.  However, many circuits require a few volts to to work. 

What frequency do you need?  Most crystal oscillators run in the KHZ range, while household current is 50 hz or 60 hz.

Perhaps you could use a rechargeable lithium battery to start the circuit, then use the higher voltage generated to run the circuit, and keep the battery charged.

As peppercorn mentioned, you have volts (electrical potential), and amps (current), and power (volts x amps, or watts).  To a large extent you can convert from one to another, but, 50mv is very low.

Say you need 100 watts, at 50mv, then you would need at least 2000 amps which is a lot of current.

Ultimately your circuit design will depend on the volts, amps, and power you have or need.

[alancalverd]

I'd be interested to know how you get on with this project. The potential applications of an electronic oscillator that can switch 50 mV at several hundred amps are enormous: this is the sort of voltage you can generate with a simple thermoelectric couple like iron and copper welded together, and the available current depends only on the cross-section area of the weld.

The approach I would take is to wind the source conductor around an iron core, to generate a magnetic field (field strength depends on current, not voltage) and use a mechanical drive to oscillate a coil in the field.

[CliffordK]

and use a mechanical drive to oscillate a coil in the field.

Brushes are essentially a cheap, low power method of using mechanical power to produce AC.  As long as you can drive the motor, you can design a system to switch any voltage or current within your design limits. 

I think a clock and transistors can do the same thing with a solid-state design, but you need to feed them a few volts.

Please post a little more about your basic design specifications.

[alancalverd]

I've never liked commutators or sliprings but I guess the entire automotive and electricity generating industries must have a point. Assuming our friend has lots of amps available, e.g. from a thermocouple, 50 mV should produce a significant stator field in an ordinary alternator. Switching low voltages at high current is best avoided.

[evan_au]

Starting with the reverse question: What load do you need to power? eg a USB port can deliver 500mA at 5V DC = 2.5Watts.

Translate this back to the input: eg 2.5W at 50mV = 50 Amps. This would require very thick and short wires and carefully designed electrical contacts, or you will lose a lot of power due to resistance.

Common semiconductors have a bandgap voltage, which is the minimum voltage drop of diodes and bipolar transistors. This is around 0.7V (700mV) for silicon, which is far too high for your application. By using field-effect transistors, you can switch lower voltages, but they need a higher voltage to switch the transistors on and off. This could be obtained from the output voltage - or from a battery before the output voltage stabilises (as mentioned by CliffordK).

If possible, try to generate a higher input voltage, perhaps by connecting many 50mV sources in series, as it is easier to process a higher voltage at a lower current. This technique is commonly used for ThermoElectric Generators, as used in the Mars Curiosity rover. Each thermoelectric junction generates a very low voltage, but by connecting thousands of them in series, you generate a voltage high enough to use common semiconductor materials and circuits. Managing resistive losses is a real challenge in circuits like this.