actually we are trying to make a dc transformer,in which we want variable dc voltage at primary sidewe are doing this by using a rheostatwhich will change the value of current continously thus accordingly the value of voltage varyare we thinking right or wrong????
Once you have converted AC to low voltage DC you can use the volume control knob off a radio or amplifier to vary your DC voltage on the secondary side. You could use a ceiling fan adjuster or light dimmer to vary the AC voltage on the primary side.
What you need is a DC to DC converter, not a transformer. Vicor make the best ones. They will accept a range of input voltges, some of them will accept quite a wide range.
A dc transformer is impossible.Voltage transformation happens when there is an operating frequency and dc circuits has zero frequency. You may use voltage divider network for voltage reduction.
Transformers can work on DC Every gasoline engine generates a high voltage AC spark by using DC. This is done by quickly pulsing the coil of an Auto Transformer with DC. THese bursts of power cause a magnetic field to be quickly created and then just a quickly to collapse as the DC burst ends. In older cars this was done by a set of points in the distributor and were operated by a rotating cam. THe points acted like a simple switch turning the power ON OFF very quickly as the cam rotated to direct a spark to each cylinder in turn. It is a rapidly changing magnetic field that induces power into the secondary of the transformer. Your thinking is on track in that a transformer uses the variations in primary current flow to induce voltages on the secondary. Your technique is in error as varying DC with a rheostat is way too slow to be noticed on the secondary. You need a means to quickly chop up the current and create a magnetic field that then expands and collapses in order to induce power in the secondary. Due to the change in direction of the magnetic field as it expands then collapses the voltage produced in the secondary is AC. A very simple way to do this is to use a push button that you can tap on and rapidly pulse the power to the primary. You can then replace the push button with a Buzzer (or relay wired as a buzzer) to chop up the DC. In essence you are then making your own inverter which converts DC into AC. If you have questions send an E-mail and I'll explain .
In principal the AC (varying) part of a varying DC signal will be transmitted through a transformer, but in practice this is not much use unless the transformer is appropriate. Sometimes transformers are used with a DC bias (so varying AC signal superimposed on a DC signal), but it is avoided where possible. Example? SIngle ended transformer coupled amplifiers, commonly used in early transistor amplifiers and valve amplifiers. Varying a rheostat like that would result in very low frequency variations, and the lower the frequency the more unlikely it is to pass through a typical off the shelf transformer. Remember, only the variations can pass through. The overall current has to be low enough that the transformer's core (the iron part) does not become saturated. The DC part is blocked by the transformer. Normally a DC signal like this can be transmitted through a transformer by chopping it up into small parts, so it becomes more like an AC signal, then is passed through the transformer, and then can be converted back to an equivalent of transformed DC using a rectifier and filters. This idea is used in isolation amplifiers, DC - DC converters, chopper amplifiers, which you can look up for more detail. Transformers must have a varying magnetic field to work. DC fields only bias the core, so are usually considered a problem because the core must be designed (larger) for this condition. So depending on your goal, this is not a very valid thing to be doing, despite the examples above. I have to mention there are also magnetic amplifiers, where the transformer is powered by AC, and a DC bias (usually separate DC bias and varying signal windings) varies the saturation of the core. Thus the varying DC signal controls the amount of AC allowed to pass, and the AC output can be rectified and filtered to get a replica of the varying control signal with greater power.
