Need to oversize trafo?

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PederBass

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Joined 2004
#1
Hi!
Searched for the following but no luck, so therefore I ask u nice people this:

Is there any point at all to use a trafo in a ps that is heavily oversized?
I mean for instance using a 100VA where the only real need would be for a 15VA... (2x15 0.5 A needed for an all IC-preamp)


Thanks!

/Peder
 
#4
PederBass said:
Hi!
Searched for the following but no luck, so therefore I ask u nice people this:

Is there any point at all to use a trafo in a ps that is heavily oversized?
I mean for instance using a 100VA where the only real need would be for a 15VA... (2x15 0.5 A needed for an all IC-preamp)


Thanks!

/Peder
Click to expand...

Hi,

Please note that in a ps, the diodes conduct only for 10-20% of the time, depending on the load. So if you take a bigger xformer, which means a lower internal resistance, the ps caps are charged quicker than with a "normal" xformer. Is that Good? Quicker charging means sharper pulses means more radiated emi and also means more/stronger higher harmonics on the supply line.
For me, that is Not Good.

For preamp and other low level apps that need a fairly constant and small current, I would recommend a xformer with physically separate windings (to block mains borne noise etc) and not too low internal resistance so that the charge pulses remain relatively flat and you don't get all those high-level harmonics in your equipment. It really would be a pity to spend more money only to get down in quality, wouldn't it? A 20VA or so would be OK for your app IMHO.

Jan Didden
 
#5
Re: Re: Need to oversize trafo?

janneman said:
Please note that in a ps, the diodes conduct only for 10-20% of the time, depending on the load. So if you take a bigger xformer, which means a lower internal resistance, the ps caps are charged quicker than with a "normal" xformer. Is that Good? Quicker charging means sharper pulses means more radiated emi and also means more/stronger higher harmonics on the supply line.
For me, that is Not Good.
Click to expand...
Johannes, the size of the smoothing package has stronger influence of the current peaks but if you compare a small 1-10 VA tranformer there is truth in what you say.
 
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