no other supply is given only 220v dc is given....current can 50mA
6 years ago
sir can i have circuit diagram of it......
I recommend using a switchmode DC adapter, of the kind normally intended for use with 220 volts AC mains power, for example like a charger for a mobile phone (although usually those output 5 VDC, not 9 VDC). This is based on the same reasoning Max mentioned, i.e. the first stage in nearly all these switchmode DC adapters is just a rectifier followed by a big capacitor, and that stage will still work with DC. Although if it is a half-wave rectifier (just one diode), then I guess you'd have to get the polarity right (and it would not blow up if you got it wrong).There are a couple of other good reasons to use this kind of DC adapter. One reason is that it should isolate its low voltage DC output from its high voltage input, just like it was designed to do with mains power, and that would be nice.Another reason is I am guessing used switchmode DC adapters will be cheap and easy to find, almost anywhere in the world, especially chargers for mobile phones. I mean, I have no idea what the market is like for pre-owned phone chargers, where you live, but in my home country, the Former US, used DC adapters are cheap, or almost free, especially those little switchmode 5 VDC phone chargers.
Most cheap switchmode power supplies will accept a DC input. In fact, take apart any modern cell phone charger, laptop charger, computer PSU, etc etc etc, you will immediately see that after some simple filtering, the AC goes directly into a full bridge wave rectifier to be converted into DC anyway, then a large smoothing capacitor smooths out the ripple. Then that is "chopped up" with a pass transistor and fed into a smaller and lighter ferrite transformer and that's converted to a lower voltage which more circuitry can regulate before outputting the remaining power.
For 50 mA? You could almost get away with using a giant resistor divider! You could even do it the photonicinduction way, by getting a heating element like resistance wire connected across the power supply, and using a clip on the wire to adjust voltage output :P Though though would be extreamtly dangerouse and really just a proof of concept.
They actually make really small DC/DC converters for niche applications, but a tad bit costly. If that suits, there are a million solutions:
Yes, I would use a down switcher.