Sorta OT- Full Wave Rectifier Question

[Paul Marossy]

If I have a non-centertapped transformer and I full wave rectify the secondary using four diodes, I should see 1.414 times the voltage of the secondary, correct?

If I measure 10 volts less than the voltage at the secondary before it is full wave rectified, what would that mean? The rectifier appears to be doing its job and creating DC... :?

[niftydog]

If I have a non-centertapped transformer and I full wave rectify the secondary using four diodes, I should see 1.414 times the voltage of the secondary, correct?

more or less...(minus the drop accross the diodes [-1.4V])

If I measure 10 volts less than the voltage at the secondary before it is full wave rectified

not sure I follow, could you expand on this?

Not doing my trick and measuring DC while the DMM is set on AC?

[Nasse]

Do you have reservoir caps after full wave rectifier, I think the voltage is different without them

And low voltage small power transformers often have voltage that drops with some load

But sounds mysterious

[Paul Marossy]

niftydog-

Well, what I mean is that I measure 95 VAC at the transformer "secondary" (really the primary, but this is in a back to back configuration), but after it is "rectified", I measure 85 VDC. If I measure for AC, it measures something like 185V. None of this makes any sense at all, except for I might have possibly damaged one of those rectifier diodes while soldering? I guess I should disconnect it from the rectifier and measure it. That would indicate if there is a problem with one of those diodes, right?

I don't have the filter caps in yet, and I know that it has an effect on the voltage, but doesn't it drop some when they are installed? I don't believe that it would increase the voltage, would it?

[Boofhead]

You only get DC = 1.4 times AC if you have the caps in.  The transformer AC voltage is in RMS, the caps effectively grab the peak of the waveform which is 1.4 times the RMS. That's why you get 1.4 times - so no caps no 1.4 times.

Without the caps it's a completely different story the waveform is a full-rectified sine wave, which is a series of positive going humps.  When you use your meter to measure the DC of this waveform it will take the average of the humps.  The relationship between the AC RMS and the DC average of the humps turns out to be a factor of 0.9.  In your case 0.9 * 95VRMS = 85.5V DC AVERAGE which is exactly what you are seeing.

[Paul Marossy]

Oh, now that you've explained it that way, it makes perfect sense to me now. Thanks for enlightening me on that!  

[Lonestarjohnny]

Hey Boof, Your the first guy I've seen in awhile that know's his stuff about tranny's with no center tap, Good answer !
also with out the cap's your dealing with a floating ground which is not nice to your average Electrician, once you add the cap's then you have a a reference to ground.
I've been had a couple of time's by tranny's that are not centertapped, you can get some pretty darned wierd reading's from them when thing's are'nt right circuit wize.  :roll:
JD

[Paul Marossy]

I also did take into account that with no ground reference, I might get some odd readings. I just wasn't sure what I should expect...  :oops:

[niftydog]

with no ground reference, I might get some odd readings

but the potential difference between the two points will always be the same, regardless of whether it's floating or not. Your reference is wherever you put the black lead of your DMM. If you want to call that ground, then there's your ground reference. Placing caps in the circuit does not "create" a ground reference - it's always been there, it's just not referenced to a fixed point.

as for the other stuff, this is where a CRO comes in handy, to give you the full picture!

First off, your DMM tells lies (sort of!). It's also good for folks to be aware that unless you have one of those expensive "true RMS" meters you'll be getting erroneous readings for non-sinusoidal AC waveforms.

So, 95VAC as measured by your DMM equates to a peak of around 134V. (95 x sqrt2 = about 134)

Rectified, and less the diode drops, then averaged by the DC setting on your meter you're reading 132.6 x 0.637 = 84.5VDC.

For the folks playing along at home;

For a full sine wave the relationships between peak, RMS and average values are;

RMS = 0.707 x peak value
average = 0.637 x peak value

nb; 0.707 = 1/sqrt2

The same relationships apply for a full wave rectified sine wave.

Hence what Boofhead (great screen name dude!) was saying about the 0.9 factor. 0.637 / 0.707 = 0.901