# help with bridge rectifier

I have just completed a bridge rectifier which I want to use to power my arduino from a 9v ac power supply. I have fitted a 6800uf 10v capacitor across the pos and neg. when testing it I found that I have 19v dc across the pos and neg. any ideas to what I have done wrong.

I have just completed a bridge rectifier which I want to use to power my arduino from a 9v ac power supply. I have fitted a 6800uf 10v capacitor across the pos and neg. when testing it I found that I have 19v dc across the pos and neg. any ideas to what I have done wrong.

Most likely you did nothing wrong. A simple low power AC transformer has it's rated output voltage specified at it's maximum rated output current. When supplying less then it's maximum current rating it's output voltage rises. This is a function of it's transformer winding resistances and there is not much you can do about it. In your case you will need to get capacitors rated for higher then the unregulated 19vdc you are seeing at the output of the bridge rectifier.

I would also suggest that it would not be wise to connect it to your arduino as, if the transformer is capable of supplying much more current than the arduino needs it's output will probably be too high for the onboard regulator to cope with.

the transformer is 300m/a output and checking the output voltage with a multimeter shows 9v ac.

the transformer is 300m/a output and checking the output voltage with a multimeter shows 9v ac.

Is that with no load attached to the DC output? A full wave bridge with cap filter output should charge up to the peak value of the AC voltage, so DC output should be 1.414 X the rms AC voltage. But your seeing 19VDC?

If you have 9V AC, and that is what you measure, then you have a peak voltage of:-
9 * 1.414 = 12.726 volts.
This is a long way off 19V, are you sure you are not forgetting to put the meter back to DC when you measure what is across your capacitor.

That measurement is with no load connected as I dare not connect it up to my arduino board with 19v dc

If you are measuring 9V AC and 19V DC then there is something wrong with your measurements not your setup.
Sort that out first, maybe you meter is wonky.

Ihave just put my meter across the pos and neg of the bridge rectifier and I get 10v dc but across the pos and neg of the capacitor i get 19v dc so it seems my meter is ok. It must be something to do with the capacitor.

Ihave just put my meter across the pos and neg of the bridge rectifier and I get 10v dc but across the pos and neg of the capacitor i get 19v dc so it seems my meter is ok. It must be something to do with the capacitor.

That sounds like a wiring error, that caps leads should wire directly to the pos and neg terminals of the bridge rectifier. How can those two values be different?

Lefty

sorry my mistake I was measureing the wrong leg of the rectifier. still getting 19v, my eyes are failing me.

I think it is time to post a photo.

BTW if you are connecting a 10V electrolytic across 19V it is possible it might burst/explode. Wear eye protection or get a higher voltage rated capacitor.

I have used this bridge rectifier
http://www.ebay.co.uk/itm/220893700493?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
and this capacitor
http://www.ebay.co.uk/itm/270904036947?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I know what these parts look like, what I don't know is how you have wired them up.
Can you post a photo of what you have.

hope this helps

The 2 terminals to the right is the 9v ac in and the empty terminal to the left is the 9v dc from the bridge rectifier.

er a circuit diagram would be useful too

It looks like you are feeding AC back into the DC side. The AC is being connected to a 10K resistor and also a capacitor.
To me this doesn't make sense and is not the simple circuit you were saying it was.
So what is needed now is a schematic to see what you are actually trying to make.