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Transformer

It doesn't matter with your intended application. You've already found the proper wire to connect together. This is because you connected the two windings in series in the correct phase combination so that the voltages add. You will get the same effect if you connect Red and Blue. If, for example, you connect the Green and Blue wires together, you will get little or no voltage between the remaining two wires.

There's more than one way to get what you want. You can tie Green and Yellow together and use that as the common ground. Four diodes will give you + and - outputs with respect to the common ground. But the on-load outputs will be close to 12V, leaving little head room for buck (step-down) regulators.

There are regulators with dual outputs and you can use separate regulators for +ve and -ve. Many switching regulator ICs can be configured for either positive or negative outputs from a positive input. You could use a bridge rectifier to get 24V DC (higher with a light load) from the series connected secondary and use two regulators, one with +ve output and the other with -ve output. I'll leave it to those with more experience with switching regulators to recommend specific ICs.

One thing you should keep in mind is that switching regulators generally have poorer regulation and noise characteristics than linear types.

Reply to
pimpom
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Hi,

I am playing around with a transformer. (I have enough experience to make this probably safe...)

Anyway some facts about the transformer

1-phase 50-60 Hz Type TBAE 30 and the manufacturer is Muuntosähkö.

There are two secondaries 12V/1.25A

On the transformer is 6 wires. Two of them naturally for the 220V.

Now we have left 4 wires. Transformer lists them as pairs

Red/Yellow Green/Blue

My intention is to build controllable dual power supply +/-0 to +/-12V. And the question is how to I know in which direction the windings are? I think there is no middle point in the winding because there are four wires - right?

I connected Green and Yellow together and meter showed dual the voltage between Blue and red and the circuit breaker didn't blow..

Anyway is this right way to make a central tap, or should I get another transformer? How about which of the wires to connect? Is there some kind of phase shift depending which are connected and does it matter?

Then a question about regulators. I would like to use switching regulators so that heat would not be a problem. Do I need one positive switching regulator and other one negative switching regulator or can one handle both situations? Any part numbers?

Thanks beforehand.

And good links to dual power supplies would be nice. Either to buy or build.

Best Regards Kari

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PIC - ARM - DISPLAYS - RELAYS - MODULES - CONVERTERS - I2C - SPI -
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http://www.byvac.com   (I am just a satisfied customer)
Reply to
Kari Laine

Most, if not all, modern regulators have current limiting as well as thermal protection. In many cases, you can set the current limit yourself with just a single resistor.

Reply to
pimpom

Thanks - so I got it right.

Ok thanks for this info. I start fishing datasheets. Any type codes highly appreciated. I think I try to use separate for + and -, so that if I blow something it is easier to fix.

Have I understood right that regulators are current limiting so I don't need extra circuit for it? I try to build it so that even if I short the out the power supply it won't break.

OK , I am thinking for switching variety because it does not require so much cooling. Actually I might try with both because this is a learning project.

Kari

--
PIC - ARM - DISPLAYS - RELAYS - MODULES - CONVERTERS - I2C - SPI -
KEYPADS - ACCESSORIES
http://www.byvac.com   (I am just a satisfied customer)
Reply to
Kari Laine

Hi, Kari!

I would encourage you to start out with simple linear regulators and use heat sinks. But as Pimpom notes, there probably won't be enough headroom to get 12V DC, at least under much of a load, from this transformer.

You can get an idea of the transformer's actual regulation by measuring its output with a resistive load. Better yet, you can build the rectifier and filter stage (which you will need with whatever transformer and regulator you end up with), and measure the output voltage from that.

If you want to pull (say) 1 amp at 12 volts, then the load resistor would be 12/1 = 12 ohms, and it would have to withstand 12 * 1 = 12 watts. If you don't have a 12 ohm, 12 watt resistor, try a parallel combination of 10 x 120 ohms, etc. If you only have 1/4 watt resistors, this would give only

10 * 1/4 = 2.5 watts. That's probably good enough for a quick test. Have the meter reading the unloaded output voltage, then momentarily shunt it with the parallel array, read the meter, and remove the shunt, all in a second or so. (Watch your fingers... may get hot!) For a test like this, you can increase the resistor power rating enormously by submerging it in an oil bath. (At these low voltages, a water bath would also work fine, but I hesitate to suggest that... a water bath on the benchtop is not a really good idea!)

If the loaded voltage will still give enough headroom for your regulator, go for it! If you want to try anyway, look for "low drop-out voltage" regulators. (I don't have a part number, sorry.) Or, you may want to test at (say) 0.5 A and if that works you can at least get started with the transformer you have.

Also, note that most linear regulators come with current limiting as a standard feature.

Best regards,

Bob Masta DAQARTA v5.10 Data AcQuisition And Real-Time Analysis

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Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI DaqMusic - FREE MUSIC, Forever! (Some assembly required) Science (and fun!) with your sound card!

Reply to
Bob Masta

Ok I will do that. I have a look what the local supplier has.

Yep this is good advice.

Ok thank you! This is how to make sure what the supplied voltage is at certain current. I think I have those power resistors somewhere - will have to dig...

Yep I got the transformer for free and I think those regulators are not very expensive. I think the housing of the gadget will cost more.

Yep.

Thank you Bob, which reminds me that I will order the Daqarta now. What I have had little look on it. It is very nice and the help is superb! For everyone needing only audio frequencies price and quality can't be exceeded. And for a hobbyist learning electronics it is nice tool until one knows what kind of an "real" oscilloscope one needs later on. It is great to learn what one can do with an oscilloscope. Schools should be interested - it is quite a difference to blow up a $5 sound card than $5000 tektronix :-)

Or have I gotten something wrong ?

--
PIC - ARM - DISPLAYS - RELAYS - MODULES - CONVERTERS - I2C - SPI -
KEYPADS - ACCESSORIES
http://www.byvac.com   (I am just a satisfied customer)
Reply to
Kari Laine

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