# Biasing voltage

#### peaac

Joined Sep 17, 2011
11
Ok so in my past post I implemented a full bridge rectifier with a simple 1k load and got a rectified sinewave (I did not use a cap to smoothen it.)

The AC voltage was at 24 volts and obviously the output rectified sine's peak is a few volts lower.

I am wondering how I can center this output. I mean let's say that the out put is now 0-20v peak-peak (rectified sine) how can I add a -10v bias so it is -10-10 peak-peak? I know it sounds a bit against the norm but I am just playing with the circuit to learn...

Thanks.

P.S. you can see the last post here... http://forum.allaboutcircuits.com/showthread.php?t=61121 maybe you want to look at the schematic...

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#### crutschow

Joined Mar 14, 2008
24,714
Why do you say the peak is "obviously" lower? The sine output's peak is actually 1.4 times the RMS (ignoring any diode drops).

You can generate a pseudo ground using an op amp circuit as long as the current difference between the plus and minus currents is small.

But if you already have a minus 10V bias then why you want to center the output?

#### peaac

Joined Sep 17, 2011
11
Why do you say the peak is "obviously" lower? The sine output's peak is actually 1.4 times the RMS (ignoring any diode drops).

You can generate a pseudo ground using an op amp circuit as long as the current difference between the plus and minus currents is small.

But if you already have a minus 10V bias then why you want to center the output?
I'm not talking about RMS, and also not ignoring the diode voltage drops... so if the peak is 24V, then the rectified output is around 20V. (Not RMS, hence peak-peak)

And No, I do not have a -10. I want to have a -10. The output is a rectified sine wave (lower halves transferred to the top of time axis) from 0 to around 20v. I want to center it so it is from -10 to 10. I want to know if there is an easy way to do this.

#### peaac

Joined Sep 17, 2011
11
I'm not talking about RMS, and also not ignoring the diode voltage drops... so if the peak is 24V, then the rectified output is around 20V. (Not RMS, hence peak-peak)

And No, I do not have a -10. I want to have a -10. The output is a rectified sine wave (lower halves transferred to the top of time axis) from 0 to around 20v. I want to center it so it is from -10 to 10. I want to know if there is an easy way to do this.
I just had an idea... could a voltage divider with a 1k, 2k and -15v connected to the circuit work? (I would connect the output of that one to the the 2k resistor on this one.)

#### peaac

Joined Sep 17, 2011
11
I just had an idea... could a voltage divider with a 1k, 2k and -15v connected to the circuit work? (I would connect the output of that one to the the 2k resistor on this one.)
Nope, didn't work... lol... any ideas?

#### crutschow

Joined Mar 14, 2008
24,714
I didn't know you were not talking about RMS voltage. Normally AC voltage is given in RMS so when you gave no suffix as to the type of voltage from the transformer, I assumed it was RMS. If you are stating peak voltage then it should be stated as 24Vpk.

You ignored my pseudo-ground approach. That's the only viable way to get a positive and negative voltage from one voltage. Otherwise you need to generate a negative voltage with another winding or transformer.