Constructing My First Power Supply: Investigating AC Components

[chaoseverlasting]

Ive recently completed a project in microcontrollers (my first project with uc's actually) and I built a power supply which gives me 12v and 5v dc.

I use a transformer to step the voltage down to 12v AC from mains and then the 12v AC is converted to 12v dc using a bridge rectifier with a capacitor filter.

From this point I take two outputs, one goes to a voltage regulator IC (LT7805) which gives me pure 5v dc. I still get 12v dc from the other output, but I also end up getting about 17v AC.

Why is that? Is there anyway I can block the AC component and route it to ground?

 

[Fenn]

Where are you measuring this 17VAC from? The 12VDC output? With respect to ground? Is there any ground before the rectifier?

 

[chaoseverlasting]

There is no ground before the rectifier, however the whole circuit uses a common ground but the 12v supply is not grounded there. And yes, I am measuring the 17v AC from the 12v output.

 

[negitron]

12 VRMS = ~ 17 Vpeak (12 * 1.414, for a sinusoidal voltage). You're probably measuring the peak value held by the cap.

 

[vk6kro]

It is just a function of your multimeter.

Most just rectify the incoming AC to read it as DC. But if it is DC anyway, they will still measure it.
Try putting a capacitor in series with one of the meter leads.

A better way is to look at it with an Oscilloscope. This will tell you the whole story straight away.

 

[chaoseverlasting]

How am I reading the peak value held by the cap? If I put a capacitor in series, won't it block the dc component only?

 

[vk6kro]

Exactly. That is what you want if you want to just read the AC component.

If you have a filter capacitor (in the power supply) that is fully charged and has no load on it, it shouldn't have any ripple (AC) on it at all.
As you draw current from it, the ripple will increase. Fortunately, regulators will remove small levels of ripple and give clean output

If you want 12 volts out, you will have to use another regulator (as well as the 5 volt one you are using to get 5 volts). This time a 12 volt one. 7812 or similar.

Or, if your transformer can give about 9 volts, you could use that instead of the 12 volt winding to give a peak voltage of about 12 volts on the filter capacitor.

 

[berkeman]

Ive recently completed a project in microcontrollers (my first project with uc's actually) and I built a power supply which gives me 12v and 5v dc.

I use a transformer to step the voltage down to 12v AC from mains and then the 12v AC is converted to 12v dc using a bridge rectifier with a capacitor filter.

What value of fuse do you have in the Hot lead going into the transformer? Is the power switch in series with the Hot lead? Are you using a 3-prong power cord, with the Earth connection bolted to the metal chassis of your power supply? Are all AC mains connections inaccessible by human fingers and dropped metal stuff (like paper clips) when your package is closed up?

 

[chaoseverlasting]

What value of fuse do you have in the Hot lead going into the transformer? Is the power switch in series with the Hot lead? Are you using a 3-prong power cord, with the Earth connection bolted to the metal chassis of your power supply? Are all AC mains connections inaccessible by human fingers and dropped metal stuff (like paper clips) when your package is closed up?

There is no fuse, the power switch is in series, I'm not using an Earth connection and all AC connections are inaccessible. :D Safety check again?

 

[berkeman]

There is no fuse, the power switch is in series, I'm not using an Earth connection and all AC connections are inaccessible. :D Safety check again?

You need a fuse in series with the Hot lead. You also need to be using double-insulated construction technniques (and a double-insulated rated transformer) if you are not using a 3-prong AC power plug and metal/grounded chassis/enclosure.

Please use industry-standard, UL-approvable construction techniques, even on your personal projects. It's important to get in that habit...