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Old 20th February 2012, 02:40 PM   #1
epilot is offline epilot  United States
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Default Transformer current measurement

Hi guys,

I have got an step down transformer for my LM3886 power amplifier. it is a center taped transformer, having a secondary rated as 20V 0 20V, So after rectifying it gives me 56V total or 28 0 -28. The seller told me that the said transformer is a 6A one (whats is this current? is it surge current or anything else plz), but I am suspect if he was right or not, so How can I measure this factor by myself please?
The transformer gets hot when I connect it to my said LM3886 power amp. the power amplifier I have designed has a load of 4 ohms.

thanks a lot

Last edited by epilot; 20th February 2012 at 05:34 PM.
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Old 20th February 2012, 02:48 PM   #2
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6A is the rated current, i. e. it should deliver that current forever at 20 VAC output voltage and without burning averything around.
In your case I'd check everything is O.K. in the circuit; if so I'd buy another transformer (larger).
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Old 20th February 2012, 03:33 PM   #3
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Does it have a power rating mark of 240W? Then it's current is correct. Otherwise you can use the thumbrule of 1kg per 50W ... so it should weigt around 5Kg.
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Old 20th February 2012, 03:34 PM   #4
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If the transformer is, in fact, rated for 6 amps. it's 6 amps RMS. In this application the transformer RMS current will be much larger that the DC load current. This is because the transformer only conducts a small percentage of the time and it has very high currents while it conducts.
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Old 20th February 2012, 03:39 PM   #5
DF96 is offline DF96  England
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6A means 6A into a resistive load. A DC PSU is not a resistive load, so the transformer gets hotter. Fortunately, Class B audio amps handling normal music don't draw full current all the time so the transformer is cooler. The two effects to some extent cancel out, but how much depends on how loud and how clipped is your music. Modern CD with everything at full level and you shouldn't draw much more than 2A continuous DC from a 6A secondary. Decent recording with high dynamic range and you can draw 6A on peaks and 1A the rest of the time.

Having said that, is your amplifier oscillating? Does the amp get hot too even with no signal?
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Old 28th February 2012, 03:38 AM   #6
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any transformer plugged in dissipates heat. supply voltage passing thru the coils which offers resistance hence the heat. for transformer sizing (va etc.) consider this link: Transformer Sizing, Transformers on Sale!.
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