Dual positive voltage supply question

[Austin]

I want to make a dual positive voltage supply with a center tapped secondary and a bridge rectifier. Basically like this:

Is this right?

[Gingertube]

Austin,
Draw it up twice.
On the first one put + at the top of the winding and - at the bottom of the winding and trace the current path.
On the 2nd one put - at the top of the winding and + at the bottom of the winding and trace the current path.
Check both the above for any "short" of a winding.
I did that and it looks fine to me.

You could perhaps have taken a short cut, the circuit is the same as used for typical + and - supplies in a low voltage op amp power supply, all that you've done is shift the 0V point to what is usually the -ve rail. so you get a positive rail in the middle and a second positive rail of twice the voltage at the top. Is that what you wanted?

There is one thing to watch out for - most High Voltage trannies with a centre tapped winding are rated for alternate operation of the 2 winding halves on opposite half cycles. With this arrangement both half windings (either side of the centre tap) will be providing current on both half cycles so you have to divide the transformers secondary current rating by 2.

Cheers,
Ian

[Austin]

Austin,
Draw it up twice.
On the first one put + at the top of the winding and - at the bottom of the winding and trace the current path.
On the 2nd one put - at the top of the winding and + at the bottom of the winding and trace the current path.
Check both the above for any "short" of a winding.
I did that and it looks fine to me.

You could perhaps have taken a short cut, the circuit is the same as used for typical + and - supplies in a low voltage op amp power supply, all that you've done is shift the 0V point to what is usually the -ve rail. so you get a positive rail in the middle and a second positive rail of twice the voltage at the top. Is that what you wanted?

There is one thing to watch out for - most High Voltage trannies with a centre tapped winding are rated for alternate operation of the 2 winding halves on opposite half cycles. With this arrangement both half windings (either side of the centre tap) will be providing current on both half cycles so you have to divide the transformers secondary current rating by 2.

Cheers,
Ian

Yep two positive voltages is what I was trying to do, I am trying to experiment with direct coupling.

Update: It works!

I guess I will know if my power transformer can't put out enough current if it gets too warm as it is a mystery what rating it is.

I thought a full bridge rectifier was supposed to make better use of your transformer enabling you to use a smaller power transformer for the same load? It originally had a single 5y3gt.

Now I get more than 800 volts with tho with no load from ground to the second positive.. Even half of this is going to be at the safety margin edge for el84 and a 12ax7- also the two filter caps I had on hand to add are only rated at 300 volts.. They didn't blow when I powered them up so hopefully they will make it. Is it true they can sound like a gun going off if they blow?

[Tage]

I would be cautious of how much current you draw from the middle node on that supply. I asked about this method in another thread and got some good feedback, http://music-electronics-forum.com/t29364/ , around post 23?


Is this supply for a future project? if so I would go with some 450V-500V caps, if it going to be part of an amplifier that gets worked hard, I like to have things a little over rated, because yes a big electrolytic blowing makes a lot of noise and a mess.

Hope things go well, Tage

[Gingertube]

Austin,
When they blow they make one hell of a mess which is difficuilt to clean up. And the capacitor guts stink. Do change the cpas to 450V rated caps.
Tage,
This circuit arrangement gives 2 "individual" supplies one stacked on top of the other. The circuit discussed in your linked thread was a voltage doubler arrangement. This means that the current draw limitations on the bottom supply do not hold for this circuit.
Cheers,
Ian

[Tage]

Ah yes I see the difference now, I like this set up better then may have to use it in the future.

[Austin]

I would be cautious of how much current you draw from the middle node on that supply. I asked about this method in another thread and got some good feedback, http://music-electronics-forum.com/t29364/ , around post 23?


Is this supply for a future project? if so I would go with some 450V-500V caps, if it going to be part of an amplifier that gets worked hard, I like to have things a little over rated, because yes a big electrolytic blowing makes a lot of noise and a mess.

Hope things go well, Tage

This is an amp I am working on now, it is an old console conversion. I have been playing guitar and listening to mp3s with it and just using it as a learning tool.

One thing I didn't consider is the fact that I don't have the same values of filter caps in the top and bottom, is this going to be a problem? edit: the 330 volt output is supposed to go to the preamp

[Tage]

Well idealy the are the same, then your total capacitance is the same as half of one, kinda the opposite of series resistors. But I have seen some Ampegs that have different values in series, usualy just a difference of 10uf or so. I'm sure someone else can get more technical, but I like to use even rated caps and balance resistors across both caps, just seems to be the way things are done, although I understand working with what you have may not allow it.

Then again with the layout of this power upply I don't think that you need to be too concerned about the voltage ballance?

[Austin]

Well idealy the are the same, then your total capacitance is the same as half of one, kinda the opposite of series resistors. But I have seen some Ampegs that have different values in series, usualy just a difference of 10uf or so. I'm sure someone else can get more technical, but I like to use even rated caps and balance resistors across both caps, just seems to be the way things are done, although I understand working with what you have may not allow it.

Then again with the layout of this power upply I don't think that you need to be too concerned about the voltage ballance?

Right, the voltage balance doen't matter; just an unmoving reference point that can supply current. These are just the caps I already had but I really didn't think the voltage would be this high because it was only about 275 before, with all the tubes in.