tutorials:learn:powersupply:transformeracdc.html
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tutorials:learn:powersupply:transformeracdc.html [2010/10/26 01:31] ladyada |
tutorials:learn:powersupply:transformeracdc.html [2010/10/26 16:04] ladyada |
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| So lets say we have a current draw of 50 mA and a maximum ripple voltage of 10mV we are willing to live with. For a half wave rectifier, the capacitor should be **at least** = 0.05 / (60 * 0.01) = 0.085 Farads = **85,000 uF**! This is a **massive** and expensive capacitor. For that reason, its rare to see ripple voltages as low as 10mV. Its more common to see maybe 100mV or ripple and then some other technique to reduce the ripple, such as a linear regulator chip. | So lets say we have a current draw of 50 mA and a maximum ripple voltage of 10mV we are willing to live with. For a half wave rectifier, the capacitor should be **at least** = 0.05 / (60 * 0.01) = 0.085 Farads = **85,000 uF**! This is a **massive** and expensive capacitor. For that reason, its rare to see ripple voltages as low as 10mV. Its more common to see maybe 100mV or ripple and then some other technique to reduce the ripple, such as a linear regulator chip. | ||
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| + | You don't have to measure that formula, but you should keep the following in mind: When the current goes **up** and the capacitor stays the same, the ripple goes **up**. If the current goes **up** and you want the ripple the same, the capacitor must also **increase** | ||
| ===== Full wave rectifiers ===== | ===== Full wave rectifiers ===== | ||
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| {{:tutorials:learn:powersupply:xformerpack2_t.jpg|:tutorials:learn:powersupply:xformerpack2.jpg}} | {{:tutorials:learn:powersupply:xformerpack2_t.jpg|:tutorials:learn:powersupply:xformerpack2.jpg}} | ||
| - | Wow so this looks really familiar, right? From let to right, you can see the wires that come into the transformer from the wall plug, the transformer output has two power diodes on it and a big capacitor (2,200uF). You might be a little puzzled at the **two** diodes - shouldn't there be **four** for a full-wave rectifier? It turns out that if you have a special transformer made with a 'center tap' (a wire that goes to the center) you can get away with using only two diodes. So it is a full wave rectifier, just one with a center-tap transformer. | + | Wow so this looks really familiar, right? From let to right, you can see the wires that come into the transformer from the wall plug, the transformer output has two power diodes on it and a big capacitor (2,200uF). You might be a little puzzled at the **two** diodes - shouldn't there be **four** for a full-wave rectifier? It turns out that [[http://en.wikipedia.org/wiki/Full_wave_rectifier#Full-wave_rectification|if you have a special transformer made with a 'center tap' (a wire that goes to the center) you can get away with using only two diodes]]. So it really is a full wave rectifier, just one with a center-tap transformer. |
| These transformer-based plug-packs are **really cheap** to make - like on the order of under $1! | These transformer-based plug-packs are **really cheap** to make - like on the order of under $1! | ||
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| {{http://www.ladyada.net/images/metertutorial/maswart_t.jpg|http://www.ladyada.net/images/metertutorial/maswart.jpg}} | {{http://www.ladyada.net/images/metertutorial/maswart_t.jpg|http://www.ladyada.net/images/metertutorial/maswart.jpg}} | ||
| - | Yow! 14V? That's not anything like the 9V on the package, is this a broken wall wart? No! Its totally normal! Transformer-based wall adapters are not designed to have precision outputs. For one thing, the transformer, if you remember, is made of coils of wire. The coils for the most part act like inductors but they still have some small resistance. For example, if the coil is 10 ohms of resistance, then 200 mA of current will cause V = I * R = (0.2 Amps) * (10 ohms) = 2 Volts to be lost just in the copper winding! Another thing that causes losses is the metal core of the transformer becomes less efficient as the amount of current being transformed increases. Also, there is going to be more and Altogether, there are many inefficiencies that will make the output fluctuate. In general, the output can be as high as **twice** the 'rated' voltage when there is less than 10mA of current being drawn. | + | Yow! 14V? That's not anything like the 9V on the package, is this a broken wall wart? No! Its totally normal! Transformer-based wall adapters are not designed to have precision outputs. For one thing, the transformer, if you remember, is made of coils of wire. The coils for the most part act like inductors but they still have some small resistance. For example, if the coil is 10 ohms of resistance, then 200 mA of current will cause V = I * R = (0.2 Amps) * (10 ohms) = 2 Volts to be lost just in the copper winding! Another thing that causes losses is the metal core of the transformer becomes less efficient as the amount of current being transformed increases. Altogether, there are many inefficiencies that will make the output fluctuate. In general, the output can be as high as **twice** the 'rated' voltage when there is less than 10mA of current being drawn. |
| ===== Let's look in detail ===== | ===== Let's look in detail ===== | ||
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