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Old 6th February 2009, 09:03 AM   #1
Atilla is offline Atilla  Norway
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Default Confused about a transformer's power rating

I'm designing a small PSU for a headphone amplifier, I just need a single secondary that can handle currents in miliamp range.

I've small PCB with IEC socket, line filter, inrush limiter and fusing with an EI transformer in the end, salvaged from old network equipment. It's a very nice board, that I'd like to use, but I'm not sure of the power rating of the transformer.

It has 2x115 primary windings for 115/230V operation and 18V secondary winding. Voltage without load is 21V. I get about 26Vdc after rectification on that.

The only reference to what it can handle is the label that says to use a 125mA slow-blow primary fuse. And since it was driving quite massive circuit, I'm assuming it should be able to drive 3 low-power opamps and 3 buffers, which power the headphones.

Am I right?
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Old 6th February 2009, 10:10 AM   #2
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Should be fine, just see if the transformer gets too hot too keep your finger on when driving the full load.

Doing some rough theory:

125mA derated to 75% is 93.75mA
230 x 0.093 = 21.6VA
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Old 6th February 2009, 10:49 AM   #3
AndrewT is online now AndrewT  Scotland
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Hi,
based on Ritchie's (good) estimation your transformer could be anywhere between 9VA and 25VA.
The 21/18 -1 (17% regulation) indicates it is towards the higher end of this range. Assume 18VA for an example.

18VA and 18Vac gives 1Aac on the secondary.
For DC duty after a capacitor input filter the maximum continuous current must not exceed 500mAdc.

Your 2channels of 3 opamps and 3 buffers will need around 2 * [3*6mA +3 *15mA] ~ 126mA quiescent current. This is 25% of maximum continuous rating for an 18VA secondary.

What is your real quiescent current?
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Old 6th February 2009, 10:56 AM   #4
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A slightly more scientific approach is to look up the typical VA rating of the core in a table. From one I found at http://www.jogis-roehrenbude.de/Transformator.htm :
Core ... VA rating
EI48 ... 5
EI54 ... 10
EI60 ... 15
EI66 ... 20
EI78 ... 35
EI84a ... 50
EI84b ... 75
EI106a ... 100
EI106b ... 140
EI130a ... 230
EI130b ... 280
You can find the core dimensions in a table on that page as well. In general EIxx has xx millimeters on the longest side. The /a and /b versions differ in core thickness.

With multiple secondaries, you have to judge by the wire diameter. Smaller transformers use higher specific currents (A/mm2), see the "Stromdichte" graph on the above page (http://www.jogis-roehrenbude.de/Stromdichte-T.jpg) and the wire gauge tables. For example, in a 20VA transformer, assume 3.5 A/mm2 from the graph; an 18V 1A winding might use a 0.29mm2 wire i.e. around 0.6 mm diameter.
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Old 6th February 2009, 10:57 AM   #5
Atilla is offline Atilla  Norway
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I'll need to calculate that, based on the op-amps I choose. I'll check for OPA134, since I'll probably experiment with those. The buffers will be BUF634. I intend to use a PIMETA PCB : http://tangentsoft.net/audio/pimeta/ . BTW, that is a total of 3 opamps and 3 buffers for both channels + ground. It's an interesting circuit.

Based on the input above, the trafo should be around 16 to 18 VA, let's assume the lower.

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Old 6th February 2009, 11:09 AM   #6
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by wine&dine
Smaller transformers use higher specific currents (A/mm2), see the "Stromdichte" graph on the above page (http://www.jogis-roehrenbude.de/Stromdichte-T.jpg) and the wire gauge tables. For example, in a 20VA transformer, assume 3.5 A/mm2 from the graph; an 18V 1A winding might use a 0.29mm2 wire i.e. around 0.6 mm diameter.
that looks like a really useful resource, but I can't read it.
How do we translate into English?
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Old 6th February 2009, 12:34 PM   #7
Bakmeel is offline Bakmeel  Netherlands
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Altavista Babelfish?
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Old 6th February 2009, 12:50 PM   #8
Atilla is offline Atilla  Norway
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Google translate did a decent job on this page.
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Old 19th February 2009, 05:16 AM   #9
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May I know what language is that link at post#4

Thanks in advance

Gajanan Phadte
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Old 19th February 2009, 07:21 AM   #10
Atilla is offline Atilla  Norway
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It's in German.
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