24VAC half bridge rectified = xx DC???

[supak111]

I was wondering what would the output be from a 24VAC after a half bridge rectifier? Would it be around 12v DC?

I can always add a cap on the DC side to help with the ripple. Thanks

PS I think I found my answer sorry. I think it would be 24vac x 1.414 / 2 = ~17vdc (-.7v?)

 

[supak111]

Because its a half-bridge-rectifier not full.

PS if it was full it would be 24vac x 1.414 -1.7v(diode drop) = ~32vdc lol I think...

 

[davenn]

yeah, I shouldn't be answering Q's so early in the morning when I haven't woken up properly

 

[supak111]

Haha actually I just found some good info on half bridge output voltage. I guess you shouldn't divide by 2 (or multiply by .5) I would be multiply by .3185 I guess. Here is what I found:

"Let's say the AC in is 12.6 volts RMS. To get peak we multiply 12.6 by 1.414, which equals about 17.8 volts. But the average (or measured) voltage DC is peak times .3185 equals about 5.67 volts. This is what is called pulsating DC. Pure DC, such as from a 12 volt auto battery, has none of the "ripple" and will be a real 12 volts."

If thats the case than my 24vac x 1.414 x .3185 - 0.7v-drop = 10.1vdc? Humm I'm not sure whats correct now.


.

 

[(*steve*)]

You will actually get a slightly higher voltage from a half wave rectifier, but the ripple will be at the same frequency as the AC (it will be double for a bridge rectifier) and it will be capable of supplying less power.

 

[supak111]

For my application I don't need much power, few watts, my main concern is to change the 24vac to around 10-14v dc.. PS this 24vac transformer is running of a main so it has plenty of power, its whats inside most of your home furnaces.

 

[(*steve*)]

It's not the transformer that's the problem.

Google a bit for differences better full and half wave rectification.

To get 10 to 14 volts you'll need (at its simplest) a filter cap and a regulator.

What is the nature of your load?

 

[supak111]

I know the difference between half and full bridge rectifier, I was thinking if I use half I will have lower voltage closer to my 10-14v. If I use full bridge I would have too much voltage 20+v and will have to add more components.

I know I'll add a filter cap but I don't really think I'll be adding a regulator. The load is a 12v relay nothing special

 

[(*steve*)]

You clearly don't know the difference between full and half wave rectification (besides the very basics).

Either will give you about 33VDC from 24VAC.

Can you get a relay with a 24VAC coil? You might get away with a resistor or a 20V zener diode in series with the 12V relay coil, but depending on the relay current it might get quite hot -- you would need to calculate the power disipated and choose one of an appropriate power rating (and in the case of a resistor, a correct resistance).

Is the idea that the relay is energised whenever there is power, or is it more complex than that?

 

[Alec_t]

Although the average voltage of half-wave rectified 24VAC is around 10VDC, the voltage on the smoothing cap will be close to the peak voltage, namely around 33VDC as Steve said.

 

[BobK]

And it is half wave, not half bridge. A half bridge is an entirely different thing.

Bob