Switching power supply is not really expensive. Pls look at the latest charger of mobile phone, e.g. from Nokia. Mobile phone charger (or known as OFFLINE switching regulator) is usually able to deliver charging current of more than 500mA. At beginning I thought it was expensive. But we can buy it at a price not more than USD2.70 (remember, USD2.70 is RETAIL price, so the cost price will be more lower). More importantly, it provides isolation like AC-transformer based regulator. The circuit is simple and you can find dedicated IC to do the job for you. You can find this kind of circuits in On Semiconductor website.jayxflash said:I already said, I need 30mA. The circuit is for a sensor of movement. it's pretty expensive to build a switching source. that's why i need something simple like this. but still something functional...
nicleo said:Switching power supply is not really expensive. Pls look at the latest charger of mobile phone, e.g. from Nokia. Mobile phone charger (or known as OFFLINE switching regulator) is usually able to deliver charging current of more than 500mA. At beginning I thought it was expensive. But we can buy it at a price not more than USD2.70 (remember, USD2.70 is RETAIL price, so the cost price will be more lower). More importantly, it provides isolation like AC-transformer based regulator. The circuit is simple and you can find dedicated IC to do the job for you. You can find this kind of circuits in On Semiconductor website.
Probably my sentence in previous post caused little misunderstanding. The charger still has one transformer, but it's very much smaller compared to conventional step-down 50Hz/60Hz power transformer. As a result, the size of the charger becomes smaller and lighter. The transformer (with ferrite-core) is not connected to 240VAC or 110VAC directly (that's why it's called OFFLINE, probably). I don't have the schematic. In fact, I'm also looking for one. I opened one charger (from Nokia) before. The charger was made in China. The circuitry was really simple. If I have a digital camera, I will capture a snapshot and let you see. But unfortunately, I don't have. However, I found one (almost) similar circuitry in On Semiconductor website ([url]https://www.onsemi.com/site/products/summary/0,4450,NCP1200A,00.html[/url], but the one in On Semiconductor was more complex.samcheetah said:im really impressed by the size of the chargers that are avialable these days. can you give some specific links for those small switching mode chargers. and can you explain how does it provide isolation like transformer based supplies. i havent ever opened a Nokia charger but i do have a little idea about whats in there. do you know which ICs are used in most of those chargers
You are right that we need special tool to open the Nokia charger. As my mobile phone was lost and the charger is no longer useful, I used hammer to 'open' it. If I'm not mistaken, isolation from high voltage (240VAC/110VAC) is to protect our appliance (e.g. the Nokia mobile phone) from damaged. For the Nokia charger, before the high frequency transformer, there are few components, e.g. high voltage diodes, high voltage PWM controller, etc. These components are not protected, but it does not matter, as the main design objective is to protect the mobile phone from damage during charging. Besides, high voltage (few times higher than AC main voltage) diodes/rectifiers and PWM controller or switcher (you can find in On Semicondutor) are not expensive nowsaday. If those high rating components are used and if the spike at main power is not very high, then those components before the isolation transformer will still be safe.samcheetah said:i do have a little knowledge of switchers and im very interested in making one. i have read that basically switchers directly rectify the mains voltage without stepping it down as in linear power supplies. then that rectified voltage is switched at a very high frequency. and then it is rectified down to the needs of the application. the high frequencies allows the use of small transformers and the switching methods help in increasing the efficiency of the switcher. but how is such a switcher isolated from the mains. is it because of that high frequency switchmode transformer? well if thats true then the high frequency oscillator and rectifier at the front end are not isolate from the mains. is that true????
nicleo said:I used hammer to 'open' it
Alright ... we're almost out of your topic.jayxflash said:I undertood that the schematic posted in the first topic, is something "classic". I'm not interested in safety at all, I just want to have 5v from 220v AC, in the CHEAPEST possible way. the circuit will function in a plastic case, so there's noo need for separation and etc.
samcheetah said:until i get that special tool from somewhere, you work on getting a photo of that charger. and when you do that, just upload it here. i really want to see whats inside that charger.
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