Is there any harm in using an overspecified power transformer? - diyAudio
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Old 7th January 2015, 01:04 AM   #1
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Default Is there any harm in using an overspecified power transformer?

Say for example I need a transformer that will provide X voltage at Y amps.

Presuming I found a good deal on a transformer that was a good match for the voltage (X) but the rating for the amperage (Y) was something on the order of twice or three times Y, is there any harm? Do I need to take it into account when designing the power supply or can I simply 'forget about' the extra current capacity?

Thanks!
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Old 7th January 2015, 01:39 AM   #2
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That should work out fine. Your voltage will then be a little more stable under a load and usualy this will make the bass a little tighter. Back in years gone by one popular mod for
th Dh220 was to replace the stock transformer with one with the same voltage but a much
higher current rating.
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Old 7th January 2015, 02:00 AM   #3
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Quote:
Originally Posted by woody View Post
That should work out fine. Your voltage will then be a little more stable under a load and usualy this will make the bass a little tighter. Back in years gone by one popular mod for
th Dh220 was to replace the stock transformer with one with the same voltage but a much
higher current rating.
Thanks! Just wanted to be certain.
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Old 7th January 2015, 02:09 AM   #4
AJT is offline AJT  Philippines
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Quote:
Originally Posted by Wigwam Jones View Post
Say for example I need a transformer that will provide X voltage at Y amps.

Presuming I found a good deal on a transformer that was a good match for the voltage (X) but the rating for the amperage (Y) was something on the order of twice or three times Y, is there any harm? Do I need to take it into account when designing the power supply or can I simply 'forget about' the extra current capacity?

Thanks!

same voltage but more current capacity means that regulation will be very good indeed...and if you have space inside your box, why not?....

i once sought advise from an audio guru here, and his advise then was to get the biggest that will fit inside your box....
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Old 7th January 2015, 03:31 AM   #5
cwtim01 is offline cwtim01  Hong Kong
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If Y is small enough you may not need a soft start circuit, but if 3Y is big enough then a soft start maybe required to deal with the inrush current.
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Old 7th January 2015, 03:35 AM   #6
AJT is offline AJT  Philippines
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that depends, if you have just a pair of X caps, you may not need soft start, but if you have 3 pairs of X caps, then things would be different, you may need a soft start....

it is an actual basis....add to that, what type of mains switch will you be using...
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Old 7th January 2015, 04:32 AM   #7
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The way generalized rule of thumb is the charging spikes will ask ten times the power....A 500mA supply, will see the charging spikes at 5A. It is the short time-span that the supply has to bring up the discharged caps...depends.
So, if you have a humungous transformer, way over what you "need", you may not need a soft-start...watch the lights flicker as you power it up.




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Old 8th January 2015, 12:56 PM   #8
AndrewT is offline AndrewT  Scotland
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The capacitor input filter that converts the rectified AC to DC makes enormous demands on the transformer.

A transformer must be de-rated when feeding a capacitor input filter.
Post7 gives some numbers that can be used in P=IR for the primary and secondary heat load predictions (10times the current for one tenth of the time equates to ten times the heat load, it is not the same as driving a resistive load).

The effect of this de-rating is that the continuous DC current must be about half the rated AC current. And that will make the transformer run hot.
For reduced heat loading and increased reliability, I suggest you aim for continuous DC current to be about of the AC current rating.
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Last edited by AndrewT; 8th January 2015 at 12:59 PM.
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Old 9th January 2015, 08:08 PM   #9
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"Is there any harm in using an overspecified power transformer?"

If there were, the entire high end audio industry would collapse
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Old 11th January 2015, 04:32 PM   #10
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As a number of people have said already the inrush current is what to watch. NTC of say 5 amp or less on the primary will cover it . Toroidal transformer are bat about inrush . On the positive as has been said voltage regulation will be better.
Search the site and you will find the math that was done to determine how big the cap needs to be to regulate voltage based on current demand and how tight you want it to be.
Large cap need chokes or resistors to damp down the current charging spikes which can be very large but of short duration causing noise and damage to the diodes.
Any tool or concept must be used correctly for them to work properly. .
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