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Bipolar voltage supply?

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ClydeCrashKop

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I was thinking about swapping polarity to change motor direction and wondered if I could get bipolar output from a full wave bridge rectifier by using one of the AC wires as a center tap. So I tried it and got what I was hoping for.

Bipolar supply.jpg

This was the pattern from an AC terminal to the positive and negative terminals of the full wave bridge rectifier.

Full wave rectifier split.JPG

I thought This is great! All I have to do is smooth it out with a couple capacitors.

This is what I got!???

Bipolar voltage doubler.jpg

Searching online, I found that this is a not very common way of making a voltage doubler. Out of pages of voltage doublers, I only found this one using a full wave bridge rectifier. And they didn't use the AC wire to get bipolar.

Voltage doubler.jpg

While searching, all of the bipolar power supplies used a center tapped transformer. Single output transformers are much more common. Why isn't this a well known, common circuit? Is there something wrong with it?
 

dknguyen

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I assume those are open-circuit voltage curves. Did you actually test how it performs with a load?

The double capacitor method to split DC unipolar supply (not just this one, but any DC unipolar supply) into a bipolar supply has high output impedance and limited current capability. You don't actually need the diode bridge center-tap using this method.

As for the the voltage doubler effect due to the presence of the center-tap...you'd just use a transformer with the proper turns ratio to get what you need instead of half of what you need and then finagling with the capacitive divider. This gives you better current capability.

Your circuit is basically a capacitive charge pump and these have worse current capability than transformers or inductive step-up/step-down circuits. But, unless you plan to work directly off mains voltage, your circuit requires a transformer anyways. Your circuit takes a perfectly good transformer with it's step-up/down and high current capabilities and forces it to operate in an inefficient charge pump circuit.
 
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ClydeCrashKop

Well-Known Member
Most Helpful Member
I was just playing and didn't expect to create a voltage doubler.
I put a 50 ohm load on one 25 volt side and it dropped to 22 volts. (900 mA wall wart).
I will think about that capacitive charge pump until I understand it. Thanks.
You are right. Anything I would make with reversible motors wouldn't be mains powered unless I make one of these.

Robot unplugs.gif
 

AnalogKid

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While searching, all of the bipolar power supplies used a center tapped transformer. Single output transformers are much more common. Why isn't this a well known, common circuit? Is there something wrong with it?
Not wrong, just not optimal. The half-wave output has a higher ripple voltage and ripple current compared to a center-tapped transformer/full-wave circuit. For the same ripple voltage amplitude at the output or into the downstream voltage regulator, the bulk filter capacitor must be much larger.

For the same output power in the two different circuits, the transformer is the same size. This means the cost trade-off is the addition of a centertap vs. doubling the size of two big filter capacitors. That's an easy one.

ak
 

gophert

Well-Known Member
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I was thinking about swapping polarity to change motor direction and wondered if I could get bipolar output from a full wave bridge rectifier by using one of the AC wires as a center tap. So I tried it and got what I was hoping for.

View attachment 116105

This was the pattern from an AC terminal to the positive and negative terminals of the full wave bridge rectifier.

View attachment 116106

I thought This is great! All I have to do is smooth it out with a couple capacitors.

This is what I got!???

View attachment 116107

Searching online, I found that this is a not very common way of making a voltage doubler. Out of pages of voltage doublers, I only found this one using a full wave bridge rectifier. And they didn't use the AC wire to get bipolar.

View attachment 116108

While searching, all of the bipolar power supplies used a center tapped transformer. Single output transformers are much more common. Why isn't this a well known, common circuit? Is there something wrong with it?

Your circuit is not common because there is an easier way to achieve the same result...almost.
These two circuits have the same output by simulation
70ECD1E0-6F7E-4DF7-8101-5BADE04486F3.png

Output of left circuit (ac source and two resistors...
74E8863F-F6D4-46A6-A7D3-DF5B52427A5B.png

Output of right circuit...

975CAAC1-CFE3-4BD3-AA77-698D6925403C.png
 
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