Transformer size and load - no load regulation

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JAY X

Well-known member
Joined
Jan 9, 2009
Messages
683
Hi All,

Never paid much attention to this detail.... ::) 

A year ago i bought a  24VA planar encapsulated transformer unit with 18-0-18 secondaries, it works good but it is a bit too big for my 1u rack box. So i'm going to get a lower wattage model : 6VA. 2x18v  2x167ma  2x24,3v unloaded
(My supplier only has 24VA or 6VA). Its unload voltage regulation(after datasheet specs) shows 6v difference from loaded condition. ¿is that unload regulation CRITICAL or dangerous?

When i used 30w toroidal transformers there was a 2-3 volts difference between load unload voltage, but under load the voltages where ok 18v, 17,6v or so.

The power supply i built (with the 24VA transformer) is made with lm317/337 regulators, 2200uf filter caps.
Now i realize that bigger transformers have better load/unload voltage regulation.

The circuit to feed is small signal, two dip10 DPDT relays and opamps LM833P only and draws 40ma. So by power cappability 6va is clearly enough, but voltage regulation worries me..

¡Thank you for your help!
JAY X

 
Hi,

Ok, after some search, it is the PSU, with its caps and regulators that make all the work of regulating the secondary current to a constant value, independently of load. So, no problem using small transformers.

JAY X
 
your regulator needs how much voltage above output voltage to work?

xfmr might get hot at 75% regulation,

6 watts goes to circuit, 2 watts heats copper,
 
> xfmr might get hot at 75% regulation

Yes, but presumably not dangerously hot.

Small objects have larger surface/volume ratios. The small volume tends to mean less-fat copper, higher loss; the larger surface works to shed the heat.

In commercial reality the small iron WILL run hot and saggy.

However he complains that the previous iron was "a bit too big". He's made his too-small bed, he's gonna lie in it (or wise-up and bump the box up).

Anyway his proposed load may be SO low that load current losses are trivial. (It may still run warm from low inductance and significant imaginary vampire current with real I^2R losses.)

My immediate concern is that 24.3V times 1.414 is 34.3V. Before our 10% allowance for high wall voltage and preferably 10% more for real-bad-day wall spikes. The common regulators are rated 35V. I know from experience that they may take 36V for decades, but it's really too close to the lip of the cliff to be Good Practice.

I'd shop for 2*15V. The unloaded voltage is nearer 20V, which leaves some room to get to 15V, though 12V may be better.

Actually unless the relays are pigs, I'd do 2*12V or even 2*10V, C-R-C filter, some thousands of uFd will be plenty clean for '833s.
 

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