L7809 outputting AC

[kamiwaza]

Has anyone ever experienced this? I'm running a 12V transformer into an integrated diode bridge. The output legs of the bridge have NO AC on them. They feed the regulator, which in addition to 9VDC is also outputting about 20VAC. How can this the possible? There don't appear to be any shorts between the AC side and the regulator.

Cheers

kamiwaza

[doitle]

Can you post us an example of what your system looks like in schematic form? A diode rectifier bridge SHOULD be putting out time varying AC. There should be quite a bit of ripple which should then be partially filtered by a cap between the AC pin and ground of your 7809... Then there should be one more cap from the 7809 output to ground helping to filter even more ripple out.

[kamiwaza]

Here's the schematic:



The diode bridge definitely has no AC on the outputs, just 15V DC. A little low, maybe?

[earthtonesaudio]

Are you making your measurements with respect to earth ground?

[doitle]

Ah alright with the capacitor in place then there should not be much ripple from the rectifier.

What do you mean it is outputting 9V DC AND 20V AC? I'm trying to understand the problem more thoroughly.

[JKowalski]

The diode bridge definitely has no AC on the outputs, just 15V DC. A little low, maybe?

15VDC is exactly what it should have. A good rule of thumb is to multiply the AC value by 1.3 to get an approximate for the DC value coming out of the rectifier with an adequate filter cap.

Do you mean you have a 20VAC waveform centered around 9VDC??? That doesn't seem possible, considering that there should be no voltage higher then about 17V anywhere in the circuit. (Save the mains, obviously). Maybe you should double check your measurements. How exactly did you go about measuring the 20VAC?

[zyxwyvu]

Are you making your measurements with respect to earth ground?

This is also what I thought first. Are you sure your ground cable is connected to what you're measuring with?

[kamiwaza]

By earth ground I take it you mean the '-' leg of the diode bridge/ground pin of the L7809. Yes, the measurements are in respect to that. I am using an AMM, when using the DCV setting I get 9V between the output and ground pins on the L7809, and when I change to ACV I get around 20V. Of course this is a huge mystery to me because this value appears nowhere else in the circuit, and I have no idea how an L7809 could produce AC.

I also think that the 15V from the diode bridge is a little low; I calculate it should be just under 17V for full wave rectification of 12ACV.

[JKowalski]

I also think that the 15V from the diode bridge is a little low; I calculate it should be just under 17V for full wave rectification of 12ACV.

You have to consider this: When you full wave rectify something, it's still the ac signal, just with the bottom half flipped around the 0v. When you multiply the 12VAC signal by 1.44, as I believe you did, you get 17V. However, that is the peak DC value - that's the maximum voltage that the recitfied AC signal can reach. The filter capacitors try to retain that peak value - but they can't get it to perfectly flat DC. The regulator helps with that part.

When you measure that ripple-y DC/AC signal, your meter does not measure the peak DC value - it measures the average DC value. Since your rectified filtered DC is probably going at 120Hz cycling between 15 and 17 volts (it depends on how big your filter capacitor is, and the load on it) the meter on DC setting measures the average DC value, which should fall somewhere inbetween.

Additionally, the diodes in the rectifier have a voltage drop across them, which further reduces the voltage that can come out of them.

15 volts is normal, don't worry.