5V power supply problem?

Question

Hi,I designed a power supply board which is powered by a 12V transformer connected to a full bridge rectifier. The rectifier drops ~2 volts and considering the voltage of the supply main etc I get 9.5 - 10V output from the rectifier.

The output of the rectifier I connected to a 7805 IC and I get 5V Output. I connected a 220 uf cap accross the + & ground terminal and get ~5.25V. I also have a 24v rectified source, the 12V and 24V ground are connected.

I connect this supply to a board a) which contains 2 microcontrollers, 1 16x2LCD,board b) which contains 4 5V relays , 4 LM339 ICs and 8 3094 power transistors.

The problem is there is a very heavy dip in voltage when I connect to each of the boards, each microcontroller dips nearly 1V so when I put in my second microcontroller the voltage dips as low as 3V and the case becomes worse when I connect the supply to the other source.

I need to have common ground thro' the 2 boards.

How can I prevent the voltage from dipping down and maintain a constant 5V?

Can any one please give me a solution to this problem?

Answer 1

You should get 15 volts DC from the bridge.

You probably need a large cap across the output of the bridge rectifier. Try 20000uF to 50000uF (rated at 20 volts or higher). Measure the AC voltage at that point, it should be less than 0.5 volts. If not, make the cap larger.

Or your supply is being overloaded. Check the current out of the regulator. It should be within the ratings of the transformer and of the 7805, and of the diodes.

Check the temperature on the 7805, it may need a heat sink. The part will shut down if it overheats, and that may be what you are seeing.

Check the wire size you use for grounds and +5. use #20 at least.

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Answer 2

You need a capacitor across the output of the rectifier.  You are applying what is known as "pulsating DC" to the regulator, and that won't work.

Put an ADDITIONAL 220uF cap across the output of the rectifier to ground.

Actually, I wonder if you need the 220uf across the regulator output . . maybe something smaller would work, like maybe a 10uf tantalum.  BUT . . .do what the specs for the 7805 say.

AND . . .I was wondering if the 7805 has enough power capacity to handle all that you're driving.  You need to measure the total current draw on the 5V line.  You might try inserting a voltage sensing resistor (1 ohm/2 watt) in series with the 5V line and measure the voltage drop across the resistor, then figure current using Ohms Law.  Or you could just break the 5V line ans insert an ampmeter in series.

Makeu sure you are not exceedign the apability of the 7805.

Answer 3

You're only getting 9.5 to 10V from a full bridge rectifier? That cannot be right.

You should be getting 12V (ac) from the transformer alone. You feed this into the rectifier and then a big electrolytic capacitor across the +ve and -ve terminals to smooth the DC output of the rectifier.

Do you have an oscilloscope to check waveforms? Only it is sounding like one of the following problems:

1)  Missing electrolytic capacitor, or bad connection to it, or it is open/short circuit.

2)  Not getting full rectification for the rectifier, a half-wave rectification might give about the voltage you are getting. Check wiring, soldering and finally check for a faulty rectifier.

3)  Are you overloading the transformer with a near-short circuit, or circuits that are trying to draw far more current than the transformer can supply?

Good luck finding the problem and I hope it helps...

Answer 4

You have to smooth the output of the rectifier, before it goes into the regulator, otherwise you will be feeding chopped DC to your circuits, and your microcontroller really will not be happy about that.

You need a capacitor across the rectifier output. Value will depend on what current you are drawing, but I would say in the vicinity of 1500 - 4700uF.