# bridge rectifier help

Feb 7, 2012
10
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 not tried it with load on yet as I dont want to fry my arduino. am I right in thinking when I put load on the voltage will decrease.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,461
Your AC source has probably an open circuit voltage (RMS) of 14 V. The capacitor stores the peak value which is 1.4*RMS (minus ~1.2 V for the rectifier diodes). The voltage will drop if you load the circuit. However, I'd recommend to bring down the voltaeg to a level that is more digestible for the arduino. I'd use a simple linear regulator (type 78xx where xx stands for the voltage - I don't know what input voltage the arduino expects) plus some stabilizing 100nF capacitors. Its easy, look it up in the 78xx's datasheet.

Regards

Harald

#### davenn

Moderator
Sep 5, 2009
14,214
welcome to the forums

and speaking of the voltage out of the bridge rectifier. Asa rule of thumb, its a really good idea to have the voltage rating of the electro cap at twice the DC voltage out of the bridge. else the electro cap is likely to go bang.

Dave

Feb 7, 2012
10
Your AC source has probably an open circuit voltage (RMS) of 14 V. The capacitor stores the peak value which is 1.4*RMS (minus ~1.2 V for the rectifier diodes). The voltage will drop if you load the circuit. However, I'd recommend to bring down the voltaeg to a level that is more digestible for the arduino. I'd use a simple linear regulator (type 78xx where xx stands for the voltage - I don't know what input voltage the arduino expects) plus some stabilizing 100nF capacitors. Its easy, look it up in the 78xx's datasheet.

Regards

Harald

But i am testing at the terminals where the a/c comes in and i am getting 9v a/c surely this would be 14v as well on open circuit.

#### davenn

Moderator
Sep 5, 2009
14,214
But i am testing at the terminals where the a/c comes in and i am getting 9v a/c surely this would be 14v as well on open circuit.

is that with or without the bridge rectifier connected ?

D

Feb 7, 2012
10
that is with the bridge rectifier connected

Feb 7, 2012
10
attached is a scan of my circuit

#### Attachments

• vvv.jpg
134.2 KB · Views: 733

#### davenn

Moderator
Sep 5, 2009
14,214
OK ... Using a resistor divider is not a good way to go especially for supplying a uPC
there is absolutely NO voltage regulation and the uPC will NOT react well. Time for you to go to plan B .....

here is a really simple regulated 5V supply using the 9VAC plus or minus a few volts
that I have drawn up for you....

that should be really straight forward for you

cheers
Dave

#### Attachments

• PSU2a.GIF
3.6 KB · Views: 1,647
Last edited:

Feb 7, 2012
10
Thanks i will try this out

Jan 5, 2010
7,682
Or you could pick up a wall wart that outputs 5V at a flea market for $2 like I do #### davenn Moderator Sep 5, 2009 14,214 Or you could pick up a wall wart that outputs 5V at a flea market for$2 like I do

but there's no fun in that where's your sense of experimentation and adventure, Bob ?? hahaha

Dave

#### BobK

Jan 5, 2010
7,682
Well, I got tired of building power supplies when I was in high school, over 40 years ago. I still have a +- 12 V supply I built back then that uses zener diodes and a pass transistor for each supply and it still works fine. I am a programmer at heart (and by profession) so I spend most of my time using microcotrollers for inappropirate things.

Bob

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