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toroidal trafo help
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Old 7th August 2010, 01:13 AM   #1
jayache80 is offline jayache80  United States
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Default toroidal trafo help

Hi there,

I'm building a small power supply for powering preamps. The goal is to get +/-15VDC. I've never wired a transformer before, much less a toroid one.

Here is the data sheet/catalog for the trafo:

Mine is on Page 9, Part Number 62033, 7.0VA. I attached just that page to this post.

Input: 115Vrms
Output: supposedly 30Vpp, or 15Vpp per secondary (about 20Vpp per secondary open circuit)

Per the diagram, I wired the two primaries in parallel. It states that I should get open circuit voltage at secondaries of 2x20.3V. I interpreted this as 20.3Vpp per secondary.

However, I probed it and I am getting 60Vpp on each secondary which is way too high. Any idea what I am doing wrong?

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File Type: pdf talema trafo 62033 7VA.pdf (77.1 KB, 27 views)
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Old 7th August 2010, 02:43 AM   #2
infinia is offline infinia  United States
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However, I probed it ....
Interesting, you only have an O-scope?
AC so use V RMS.
Measure the input and compare to 115V AC RMS. cross check with a DMM
Sounds like you got the secondaries in series with 120 input.
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Old 7th August 2010, 04:33 AM   #3
sawreyrw is offline sawreyrw  United States
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The RMS voltage is 20.3 volts no load. The peak to peak voltage is 2*sqrt(2)*20.3=57.4 volts.
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Old 7th August 2010, 04:41 AM   #4
wintermute is offline wintermute  Australia
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toroidal trafo help
Hi Jayache80,

When you say you wired it in parallel could you give which coloured wires for the secondary you wired where?

If you are doing a standard +- supply You should have (based on the pdf) red and brown wired together. and the measurements should be taken across green to red/brown and blue to red/brown

red/brown becomes the zero volts reference and your bridge is connected to green and blue the + and - outputs of your bridge become + and - X volts with respect to the 0V line (ie red/brown).

It sounds like you are measuring from green to blue based on the voltage you are seeing.

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Old 7th August 2010, 11:04 AM   #5
AndrewT is offline AndrewT  Scotland
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the 62033 transformer is a 7VA 115/230Vac:15+15Vac

If you wire one primary to the mains the secondary is 15Vac + regulation.
For a small transformer like this regulation may be as high as 20%.

Now wire the other primary in parallel with the first.
If you get this paralleling of primaries right you have doubled the transfromer rating from 3.5VA to 7VA.
If you get the primary wiring wrong the best that will happen is that the fuse blows quickly and the transformer is undamaged.
Build a mains light bulb tester and use it to protect your projects and yourself.

Now the the secondaries.
When fully loaded with a resistor across each secondary the output voltage will be 15Vac when the input is 115Vac.
If the secondaries are unloaded then the secondary voltage will be higher. In this case expect ~18Vac when mains is 115Vac.

If the mains is different from nominal the secondary will vary in direct proportion to the mains voltage.

18Vac is a sinewave where the peak voltage of the waveform is 18vac * sqrt(2) = 25Vpk.
If you were to display this waveform on an oscilloscope you could measure the peak to peak waveform and see 50Vpp for this 18Vac secondary output.

Notice the difference between Vpp, Vpk, Vac, Vrms. If the AC is sinewave then Vrms=Vac.

Be careful with mains voltage. It kills.
regards Andrew T.

Last edited by AndrewT; 7th August 2010 at 11:07 AM.
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Old 7th August 2010, 11:51 AM   #6
runebivrin is offline runebivrin  Sweden
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You're getting 60V peak to peak, which is the same as 30V peak, which is the same as 21.2V RMS, which is pretty much what is to be expected.
Mains transformers are always specified at RMS values. Rectified this will be about 28V, which is readily regulated to 15V DC, even if it might be unnecessarily high.
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Old 9th August 2010, 08:37 PM   #7
jayache80 is offline jayache80  United States
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Hello everyone, thank you all for your helpful replies.

infinia: it turns out I wired it correctly. However, using a DMM set on RMS like you said would have hinted to me immediately that I was reading the datasheet wrong. (I was expecting 20.3 Vpp, not 20.3 Vrms).

wintermute: I wired the mains in parallel by putting both black and violet on Hot, and both yellow and red on Neutral. Yes, Red+Brown would be connected and taken throughout the circuit as Ground, so measuring from Green to Red/Brown (Ground) would be my positive supply, while measuring from Blue to Red/Brown (Ground) would be my negative supply.

Swreyrw, AndrewT, and runebivrin: thanks, you're all correct.

Thanks again!
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