I am doing my mini project on atmel AT89C51 . But I face problems in power supply. Since to test my project,I created a power supply of 5V for the microcontroller, motor driver ic L293D, and two dc geared motor. The circuit is:
230V > transformer (230/8v) > 8vac > bridge rectifier circuit > 1000microfard capacitor 25v( parallel). > voltage regulator L7805 > 220 microfarad,63v(parallel)
And the out is taken across this capacitor. I dont know whether I could use a 63v cap for 5v needs. I do because it was only available.
When this supply is given to microcontroller Vcc , it is not responding. And when I checked this power supply using multimeter, I noted that this 5vdc power supply also contains 10vac!
I also checked this with a 9v dc battery, same here too.
WHY A DC SOURCE OR A BATTERY CONTAINS AC CONTENT???
Is this causing microcontroller problems?
Best Answer
Cheap multimeters will read DC as approximately double on an AC range. Almost certainly, if your multimeter is cheap it will do this. Try measuring a battery and see what you get....
When measuring AC voltages, the signal is rectified and averaged to "create" a DC value. The circuit that converts an analogue signal to a digital number works with steady DC values and the rectifier and averager circuit provide this. So, if an AC signal were 10V RMS, after rectification and averaging it would yield a DC value of about half i.e. 5V.
If you fed 5V DC into your meter (on AC ranges), it would be rectified and smoothed and would still be 5V. So now you have 10 VRMS and 5 VDC both being processed to the same value. Of course, on AC, the clever little circuits know that they need to compensate the numbers they have read and, in effect, they double the numbers to make AC readings accurate but this has the effect of making you think that there are serious levels of AC on a perfectly stable DC supply.