zener diode to decrease high voltage....

[Joe L]

The Zener voltage dropping diode can be either in the ground leg of the bridge as Doug shows or the hot lead before the input capacitors.

The center tap of the transformer is unrelated to the Zener position and is connected to the junction between the first two capacitors because you get about 20db less ripple by connecting there. It is optional but recommended for this reason. Don't hook it up to ground!!!

There is no voltage doubling with this circuit. Of course, you do double the voltage capability of the two input capacitors by connecting them in series.

[Chuck H]

The center tap of the transformer... is connected to the junction between the first two capacitors because you get about 20db less ripple by connecting there. It is optional but recommended for this reason.

Really??? 20dB less ripple? I'm interested. Can you post a link explaining this?

The "full wave bridge voltage doubler" moniker is the name I've seen used for this this particular rectifier arrangement elsewhere. To all who seem intent on telling me it's not doubling the voltage... I know.

[Joe L]

Really??? 20dB less ripple? I'm interested. Can you post a link explaining this?

The "full wave bridge voltage doubler" moniker is the name I've seen used for this this particular rectifier arrangement elsewhere. To all who seem intent on telling me it's not doubling the voltage... I know.

I think I read it in "TUT" or one of my textbooks from my electronics school. In fact, I got out my "Electronic Communications" last night to see if it was there that I found it but it was very lacking in bridge rectifier theory. If I can locate it, I will let you know. In the meantime, it is easy to test if you have a 100 watt Marshall around. I have a couple I need to replace caps in and might throw one on the bench to confirm what I said.

And sorry about that doubler comment Doug. I consider you as on my short list of people that really "get it". Also, we seem to be interested in the same threads. Like I tell the guys I work with, if I disagree with you on a technical issue, it's purely intellectual and not personal. I'm 59 and still love learning, experimenting and discussing this stuff.

[Joe L]

Hmmm... I just modeled it in Proteus Isis and it makes no difference if it is hooked up or not as far as ripple is concerned. There is a difference DC voltage if the capacitors are missmatched so at this point I would have to say it is there for matching the voltage drop across the capacitors.

For now, ignore the 20db less ripple statement.

[Bruce / Mission Amps]

Right. Just make sure to secure the CT so it is not connected and insulated. And be sure to heat sink the zener on the chassis (if it's aluminum) or a heatsink. Here's a modified schem (with corrected diode polarity):

Chuck I think you have your zener diode drawn backwards... it should be: cathode band pointed to ground to lift the FWB from ground by the zener voltage.

[Chuck H]

Good catch. Thank you. I've been using zeners as gates and regulators so much I got it backwards. Hopefully the OP hasn't installed it yet.

[Bruce / Mission Amps]

It won't hurt a thing backwards... it will just act like a regular diode with a >.6v to 1v drop instead.

[Tom Phillips]

Hmmm... I just modeled it in Proteus Isis and it makes no difference if it is hooked up or not as far as ripple is concerned. There is a difference DC voltage if the capacitors are missmatched so at this point I would have to say it is there for matching the voltage drop across the capacitors. For now, ignore the 20db less ripple statement.

Bummer. I was getting all exciter there for an instant. Oh well another item for the list of things too good to be true.

I have searched my books for a discussion about this particular circuit form using the center tap to the mid point of the cap stack but have not found anything except for systems that needed two supply rails. There is often mention about how a FW bridge saves transformer cost by eliminating the need for a CT and the extra turns needed to implement a two diode FW rectifier. It seems unusual that designers decided to use the CT to save the balancing resistors. Fender uses both resistors (100k) and the CT in their 70 W UL Silverface series. (?)

Tom

[acorkos]

well....i installed a 40 volt/25 watt zener B+ dump from weber...0V from bridge to zener, floating CT, 220K's across the big 100uF caps....no change in B+. crap....what did i do wrong?

[trobbins]

A main application/preference for the configuration with CT taken to cap centre-point is when significant loads are applied to the centre-point (eg. PP stage screens that like 50% B+). The balancing resistor technique runs into problems if the mid-point loading is not benign, or is not protected from a short or heavy loading.

[jmaf]

My 2 cents:



I've done this in the past with three lower power, lower voltage zeners. Say, 3 x 10 V x 3 Watt.

[Tage]

Would there be any problem or advantage to using both the balancing resistors and the center tap connected to the midpoint?

[trobbins]

You'll have to explain that some more Chuck - are you referring to the schem in Bruce's post #20? If so, then I can't see any major issues with ripple and hum - the load is across the two series caps - the top cap has no change in rectified source waveform/ripple - the bottom cap has a lower rectified source voltage and I guess marginally higher source resistance - but I can't see any other interaction that is different.

Ciao, Tim

[Chuck H]

Sorry trobbins. I misinterpreted the Q and so deleated the my answer.

[acorkos]

My 2 cents: I've done this in the past with three lower power, lower voltage zeners. Say, 3 x 10 V x 3 Watt.

would this work with a 120 watt amp w/525 volts on the plates? the guy plays at pretty much full tilt...