I have an AC to DC transformer I got out of a radio. I removed the rectifing (spelling?) circuit. So why is it that the transformer still only responds to AC input and not DC? I basically want a transformer that can take low voltage (aroung 6-12v DC) and step it up to 400VDC. Any suggestions or help?
You will never be able to increase voltage by using DC in a transformer.
You cannot use a transformer to change DC voltage. Transformers work because the expanding and collapsing magnetic field in one coil generate current in the second coil. DC steady voltage current creates a stable magnetic field and so does not generate current in an intersecting conductor. To step up (or down) DC voltage, you have to convert it to AC (old vacuum tube car radios used a multivibrator to create an AC using DC voltage) run it through a transformer to change the voltage and then recify it to get back to DC voltage.
The way a transformer works is by magnetic fields in one side (the primary) causing current flow in the transformer's other side (the secondary). For a magnetic field to generate a current in a coil of wire, it must be moving. This could be physically moving like in a generator or a field that is expanding and collapsing like that cause by an alternating current. Only alternating currents can cause a voltage increase in a transformer. The ratio of voltage in to voltage out is equal to the ratio of turns in the transformer coils (primary, secondary). To go from 10 VAC to 400 VAC requires 40 times as many coil turns in the secondary as the primary. But, the first thing you have to do is convert the 10 VDC to 10VAC. To do that you'll need and ocillator circuit. (Also called an inverter.) Wikipedia has more information and diagrams.
Transformers don't work with DC inputs. Transformer physics require an input that changes with respect to time. That's why they work with AC. When you apply a DC input to a transformer, it looks basically like a low resistance, depending on the size and mass of the wiring. In order to provide DC as an input to this thing you have, you need a circuit to chop the DC and turn it into AC before it is input to the transformer.
The transformer only responds to AC voltage and is basically a short to DC voltage. In order to achieve your desired result, you'll need to configure a voltage multiplier circuit and it's much better to keep your AC input, installing capacitors in line with 110 Vac input and in parallel across your resistive load. Performing this feat with 6-12Vdc would involve switching circuits across capacitor banks and little or no available current.
