Help with cleaning of AC please

[Megahertz]

I have one capacitor type circuit with bridge and zener to bring down the voltage from 200VAC to 12V DC. I then connect the PIR module after a 7805 which provides trigger to my PIC based on any human presence. The problem is that when this circuit is run from an 12V dc supply, PIR works fine, but when connected to an AC supply, the PIR re-triggers the PIC and the trigger kind of starts oscillating. After some research, I found out that the supply needs to be very clean else this could happen, which kind of confirms my findings as well, as when the circuit was powered with 12V there was no problem. So can someone please suggest some capacitor values which can help clean the AC signal and avoid any ripple going through to the PIR module.

[AvionicsMaster1]

The datasheet at http://www.sparkfun.com/datasheets/Components/LM7805.pdf recommends a .1uf input cap and a .33uf output cap. Even if it's something close to those values you should have a fairly well filtered power supply. You don't say what PIC you're using but the value of a cap connected from power to ground at the chip socket is usually specified in the datasheet. If you'll show a schematic someone else might see something more specific.

[Graham]

Hi there You did not state the value of the reservoir capacitor after the bridge / zener part of the PSU. If the o/p of this is applied to the 7805 without some large value capacitor to act as a reservoir then the 7805 will drop out when the full wave rectified waveform drops below the min input value to power the 7805 which needs about 1.5volts above the o/p (5V) to sustain the o/p at 5v. Try a 100uF 35 vwk alumin capacitor. Tantalums can sometimes be a little iffy. They have been known to fail catastrophically under certain conditions, like smoking failure.

[AvionicsMaster1]

Sparkfun PIR module pulls 1.6mA @ 3.3V. If his PIR is placing that much of a load on a power supply something is wrong with the circuit. I'm assuming the 7805 is after the 12v zener and according to the datasheet the caps should provide clean enough power. Usually the caps are twice the circuit voltage so a 25V in and a 10v out outta do it.

If the ringing is because of the sensor you could put a capacitor from your PIC input pin to ground to damp the triggering. A schematic and some code would be nice.

[Graham]

HI
THE 0.1UF INPUT AND 0.33UF ON THE O/P OF THE 7805 ARE FOR STABILITY AND LOCAL DEVICE DECOUPLING. IF THEY ARE NOT PRESENT CLOSE TO THE 7805 INSTABILITY MAY OCCUR. IT IS ESSENTIAL THE INPUT SUPPLY HAS TO HAVE A LARGE RESERVOIR CAPACITOR TO HOLD UP THE PULSATING O/P FROM THE BRIDGE RECTIFIER /ZENER COMBO. OTHERWISE THE RECTIFIED AC WAVEFORM WILL DROP TO ZERO EVERY HALF CYCLE.
Graham (Sorry, I left the caps lock on, still in microcode mode).

[Ramius]

Actually the values listed are reversed. .33uf is on the input and .1uf goes on the output.

6841 Page 22

Ed

[AvionicsMaster1]

I, O. They're just vowels. Thanks for pointing that out though.

[Graham]

Hi
The circuit on page 22 fig. 7 assumes that you feed it with a smoothed DC filtered supply. The circuit on page 26 fig.18 shows a negative output supply using a positive regulator but shows the correct components, i.e. a bridge rectifier, reservoir cap. and output decoupling cap for stability. The circuit on page 22 fig. 9 maybe the one that is confusing you. It's confusing me too! A 470 microfarad capacitor straight from the 120 volt AC mains onto the regulator. That really would reduce the ripple, ( when the breaker or fuse went BANG!).

[pedja089]

That isn't 120Vac, it is 120Hz... And that is test circuit for Ripple Rejection test.

[Graham]

Hi
Thank you for pointing out the error. Although in defense of my mistake, the circuit does come under the heading of "Typical Applications' and is scarce on detail. But..... it does read 120Hz which I miss-read as 120volts, the US domestic outlet voltage, nominally! Sorry.