Help: Voltage Dividing

[exztinct01]

we can get +-12V from a single 12V transformer by voltage doubling
can we get +-21V from a single 42V transformer?

[Phoenix]

we can get +-12V from a single 12V transformer by voltage doubling

Well, we can get about 15.5VDC from a bridge-rectified 12V transformer (12VAC x square-root 2 = 16.96VDC - 2x0.7 diode drops = 15.56VDC).
A full wave voltage doubler will give 12VAC x2 = 24VAC x square-root 2 = 33.93VDC - 2x0.7 diode drops = 32.5VDC or +/-16.25VDC

Less for smaller transformers typically used in effects because of transformer losses, and higher unloaded of course.

can we get +-21V from a single 42V transformer?

If the transformer is a single winding, then in practical terms, no, not really. You could reference ground to the centre of 58VDC with a decoupled voltage divider, but you'd need to waste much more power in the resistors than is being used in the circuit. You could also do the same with decoupled zeners and current limiting resistors, but you still run into the same current wasting problems.

If the transformer is 42VAC centre-tapped (21-0-21), or dual 21V windings, then no problem, 42VAC x square-root 2 = 59.3VDC - 2x0.7 diode drops = 57.9VDC / 2 = +/-28.9VDC.

What exactly are you trying to do? Or are you just trying to better understand?

[PRR]

> +-21V from a single 42V

I smell a Power Amplifier.

Since the voltage divider typically wastes MORE power than the load it feeds, this is "stupidly" inefficient.

[R.G.]

I also smell some fundamental misunderstanding of transformers and power supplies.

Highly recommend "Power Supplies Basics" at geofex.com.

[Transmogrifox]

As mentioned previously a voltage divider circuit is not the right way to do this UNLESS you have decently symmetrical AC loads (very little DC ground current).  In this case the "V/2" is simply a bias for your amp driver IC while the divider capacitors charge and discharge large currents in equal amounts so you don't get a net DC shift.  I still don't recommend this because I can think of a lot of ways this can go wrong.  It simply points out one set of conditions under which such a "bipolar" supply might be practical.

I see a way of getting +/-25V out of a single-tapped nominal 12V transformer by using a pair of voltage multiplier circuits (2*17V - 2*diode_drops - 5V transformer losses).

My simulation of this circuit gives +/- 25V into 10 ohms to ground on each leg, just to get an idea of the amount of sag with load.  This all depends on the size of capacitors along with transformer impedance.  My model assumed a transformer sized to drive an 8 ohm speaker load at +/-20V (around 50 Watts).

This same circuit also has places you can get +/- 12V supply by taking outputs and ground reference at a different point.  Really, there's 2 ways to do it since it's a pair of voltage doublers.  The one way you have already identified.

[antonis]

I smell a Power Amplifier.

I also smell some fundamental misunderstanding of transformers and power supplies.

I simply smell smoke...

[ElectricDruid]

I smell a Power Amplifier.

I also smell some fundamental misunderstanding of transformers and power supplies.

I simply smell smoke...

Hey, it was werkin till it caught fire...

T.

[Rixen]

could use a virtual ground circuit buffered with a power stage, but wasteful at high power levels and overly complicated in most situations..

[exztinct01]

I'm just looking for a use to my 42-0-42 toroid 

[Phoenix]

I'm just looking for a use to my 42-0-42 toroid 

That'll give you approx +/-59V. Nothing practical you can do with it that'll give you much lower voltage.

[merlinb]

I'm just looking for a use to my 42-0-42 toroid 

Use a voltage doubler to get about 230Vdc and use it for a tube amp?

[antonis]

can we get +-21V from a single 42V transformer?

I'm just looking for a use to my 42-0-42 toroid

I think R.G. is (once again) right..


How many secondary wires (outputs) has your transformer..??

What is it's current (or power) rating..??

Maybe you should build a nice lab benchtop power supply....

[exztinct01]

can we get +-21V from a single 42V transformer?

I'm just looking for a use to my 42-0-42 toroid

I think R.G. is (once again) right..


How many secondary wires (outputs) has your transformer..??

What is it's current (or power) rating..??

Maybe you should build a nice lab benchtop power supply....

I know I said single output in my first post but without using that other lead, we can treat it as a single 42V right? If I only use two leads, one on the side and one in the middle, then middle one's the zero volt reference and that other lead is 42V in reference to that middle lead.

[Phoenix]

I know I said single output in my first post but without using that other lead, we can treat it as a single 42V right? If I only use two leads, one on the side and one in the middle, then middle one's the zero volt reference and that other lead is 42V in reference to that middle lead.

You can full wave rectify it, which will get you ~+58VDC, or -58VDC.
You can bridge rectify it, which will give you ~+117VDC, or +/-58.5VDC, or -117VDC.
You can full wave voltage double it, which will give you ~+234VDC, or +/-117VDC, or -234VDC.
And there are further multiplication options if you want higher voltages.

EDIT:
You don't want to use just half of the winding, because that will reduce the available current, you won't get the full VA rating of the transformer. If you just want 58VDC you can full wave rectify it, but don't just bridge rectify half of the winding.