12AC to +/- 12DC transformer

Buckethead

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Summary
What type of transformer is good for converting low voltage 60Hz AC to low voltage dual polarity output
I can't seem to find a transformer that simply converts a low voltage 9V-12VAC to a center tapped output to get +/- 9v/12VDC. It seems all power transformers have a 120VAC input voltage and the lower voltage transformers are designed for high frequency switching. What is a low voltage to low voltage low hz transformer called or do you know where I can find a selection. Looking for <1A output current.
 
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I can’t find anything, like you. Two transformers? Perhaps step up the 12Vac to 120V, then 120V back down to 24V with a centre tap?
 

berkeman

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I did get some hits on 12V 1:1 transformers, and again you could parallel two of those and stack the outputs with rectifiers. So you need around 12W total power out?
 

Buckethead

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2 transformers might be OK except the extra board space and cost might be prohibitive. Still might be OK. I couldn't find much with your search. What part(s) did you find?
 

Buckethead

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I can’t find anything, like you. Two transformers? Perhaps step up the 12Vac to 120V, then 120V back down to 24V with a centre tap?
Two transformers could work, but dont want to do the step up. I'm surprised this is such a rare need.
 
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It seems to me I have used a center tapped 24V transformer as a "step up autoformer": You excite one side of the secondary with 12V and define the center tap as ground. Of course you do have to worry about the voltage generated on the primary. Actually I guess the higher the rated output voltage the better.......
Also DC to DC converters are cheap and some of them "float" so you could change polarity.
 

Buckethead

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It seems to me I have used a center tapped 24V transformer as a "step up autoformer": You excite one side of the secondary with 12V and define the center tap as ground. Of course you do have to worry about the voltage generated on the primary. Actually I guess the higher the rated output voltage the better.......
Also DC to DC converters are cheap and some of them "float" so you could change polarity.
Yes, I may have to go the DC to DC convertor route and skip the transformer altogether.
 

berkeman

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Yes, I may have to go the DC to DC convertor route and skip the transformer altogether.
That's probably a better approach, especially if you are mounting this on a PCBA or in a small enclosure. Just rectify the 12VAC input and feed that to a DC-DC converter that makes +/-12V at the output. What is the application? If you are powering analog circuitry with the +/- outputs, you may want to follow the DC-DC outputs with low dropout linear regulators to eliminate the ripple.
 

tech99

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If you have available a low voltage AC supply with one side grounded, you can connect a diode to it and obtain half wave rectification. Then you can connect another diode to it, with reverse polarity, and obtain a second DC supply with the oppsote polarity.
 

davenn

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Henryk

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You don't need a transformer. You said you have 12 V AC source and you want to get +/- 12 V DC. A simple voltage doubler with two diodes will do it.
 

phyzguy

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You don't need a transformer. You said you have 12 V AC source and you want to get +/- 12 V DC. A simple voltage doubler with two diodes will do it.
I agree. Why would you need a transformer? Hook up two half-wave rectifiers, back to back. One will generate +12V DC, and the other will generate -12V DC.
 
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First you are looking for the wrong thing. A transformer is AC only. You need a power suppley and there are supplies that go from 12 volts to an isolated 12 volts. You then connect the 12 volt out postive termonal to the 12 volt input ground to get a + and - 12 volt supply
 

davenn

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First you are looking for the wrong thing. A transformer is AC only.

Yeah he knows that 😉

You need a power suppley and there are supplies that go from 12 volts to an isolated 12 volts. You then connect the 12 volt out postive termonal to the 12 volt input ground to get a + and - 12 volt supply

I cannot even make sense of that ?
 

phyzguy

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Look at the attached Half-Wave_Circuit.png. D1 conducts on the positive half cycle, and D2 conducts on the negative half cycle. With no capacitors, you will get positive and negative going pulsed DC, as shown in Half-Wave_No_C.png. When you add capacitors, you get +/- 12V DC, as shown in Half-Wave_C.png. Actually, because of the diode drops, you only get +/-11.3V DC, but any circuit will have to deal with this. Half-Wave_Ripple.png shows the ripple that results with the R and C values shown. you can adjust this by sizing the capacitors given the load.
Half-Wave_Circuit.png
Half-Wave_No_C.png
Half-Wave_C.png
Half-Wave_Ripple.png
 

Svein

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  • 12VAC is (more or less by definition) 12V RMS AC.
  • 12V RMS sine wave means ≈ 17V peak
  • Most 12V transformers I have encountered are actually 12.6V (a holdover from the tube age)
  • The diode drop of 1N4001 varies from 0.8V at 0.1 A to 1.3V at 10A
  • A better rectifier would be a Schottky-barrier diode (has a lower forward voltage drop)
 

Baluncore

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Most 12V transformers I have encountered are actually 12.6V (a holdover from the tube age)
The 6.3V and 12.6V vacuum tube filament voltages came from the lead acid batteries used, each cell had an emf of 2.1 volt.

But a 12V transformer will be designed to produce an unloaded voltage 5% higher than 12V to allow for winding resistance. When the transformer is fully loaded the voltage will fall to 12V from 1.05 * 12 = 12.6V, which appears to be a coincidence.
 

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