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Q & A: Converting AC to DC

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Most recent answer: 04/25/2008
Q:
how to convert ac current to dc current?
is there any conversion factor?how to select an autotransformer for dc motors?
- santhosh (age 13)
neyveli
A:
The key component in an ac/dc converter is a rectifier bridge, typical made of 4 rectifying diodes. These let current pass more easily in one direction than the other. If the two ac input wires are connected by diodes pointing toward one of the outputs and away from the other output, then the sign of the voltage between the output leads won't switch back and forth.  In principle, for modest loads (devices through which the dc current flows) on the output the average dc output current will be the same as the average absolute value of the a input current. The same relation applies to the voltages. That means that if the input ac voltage is, say, 10 V (rms), the average output voltage will be 10 V * 2*sqrt(2)/pi= 9.0 V. So the rms to average dc conversion factor is 0.90.
    Under very light load, the output voltage can drift up to the peak input voltage 10V*sqrt(2)= 14.1V. Depending on the application, it may be good to use capacitors to smooth out the output voltage, so it won't go up and down.

I'm not sure I understand the last part of the question well enough to answer.

Mike W.

(published on 10/22/2007)

Follow-Up #1: wind powered battery

Q:
I am building a 12 volt wind generator using a 12volt 6watt bicycle generator it supposedly puts out 18-20vac I assume that vac means volts alternating current. Then my research leads me to believe that to charge automotive type batteries I will need covert to DC and limit the voltage. This is an experiment and hate to have to limit power since this is my goal to produce as much power as I can. Next I am not sure it will turn fast enough to reach the 18-20 vac. With all of that said any help you can give will be greatly appreciated the more specific the better thank you very much ,Steve
- Steve (age 50)
Alsea,Oregon ,USA
A:
You're right that you will need to convert to dc, presumably using a full-wave rectifier made of 4 diodes, able to handle whatever current your generator supplies. The maximum voltage supplied by the system would be enough to overcharge the battery in the long run. You could  put a protective diode across the battery, one that starts conducting at around 13 V, to protect the battery from gradual overcharging, which can lead to explosions. To be on the safe side, you could use two such diodes in parallel. You'll lose very little power until the battery is charged. If there's some danger that the generator can make so much current that the battery would be in danger of exploding from charging too rapidly, you could put fuses in the lines from the rectifier  to the battery. The fuses should be rated for whatever the maximum recommended charging current is for the battery. Since my small home charger, which isn't very fast, can put out about 10 Amps, my guess is that 10 A fuses would be very conservative. Most likely, as you guessed, the problem will be in getting enough power, not getting too much.

Mike W.

(published on 04/25/2008)

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