question about transformer current rating with a multitap secondary

[dietnews]

hi,

i'm sorry if this question has already been answered. if it was, i wasn't able to find it.

i found a Stancor P-8621 power transformer which i would like to use to make a regulated power supply. The transformer's secondary has taps for 24V,26V,28V, and 30V. the transformer is rated at 15A RMS on the secondary.

i want to use it in a 30V/10A regulated power supply with a full wave bridge rectifier, a filter capacitor, and two LM338K regulators. i know that Idc = 0.62 X Iac which comes out to 9.3A DC.

So i think i need a little more current to make this device safe at 10A. i know dont need to use the 30V tap on the secondary because Vdc = 1.41 x Vac.

So here is my actual question: if i use one of the lower voltage taps on the transformer's secondary (such as the 24V tap), would i increase the transformer's current rating? And if so, by roughly how much? Can i just divide the VA (450) by the secondary voltage i am going to use in order to find the current rating?

 

[DUG]

Current rating may be tied to wire diameter, not total VA.

The lower your AC, the larger the filter cap has to be to maintain headroom for the regulators.

The higher the current spikes through the transformer and diodes.

You may end up with diminishing returns going in that direction.

9.3 vs 10 7% difference.

All you need is a 3ohm 600W resistor to try it. (and a fan)

 

[Elvee]

i want to use it in a 30V/10A regulated power supply with a full wave bridge rectifier, a filter capacitor, and two LM338K regulators. i know that Idc = 0.62 X Iac which comes out to 9.3A DC.

So i think i need a little more current to make this device safe at 10A. i know dont need to use the 30V tap on the secondary because Vdc = 1.41 x Vac.

I wouldn't be too worried about 7% of the current missing: first, your relation is a crude approximation, and the 0.62 factor could be smaller or larger, and unless you intend to use this supply constantly at 10A, such a small difference is not problematic.

Anyway, what you could do is to insert a small choke between the bridge and the filter caps: it will reduce the rms/average ratio, and will have other beneficial effects, like reducing the stress on the diodes and caps, and reducing the DC voltage before the regulators. It will also help the smoothing.

 

[dietnews]

thanks for the comments. i'll do some reading on power supply chokes and see if i can scrounge one up to try out

i don't anticipate running this power supply at 10A constantly but i figure it will happen every once in a while and its better safe then sorry. its going to be a bench supply for testing QRP RF amplifiers.