# Power transformer rating

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#### rsumperl

Greetings all,
If I have a power transformer that states the secondary as 440vac @ 150va, is that telling me I can draw 340ma?

Thanks,
Ray

#### 20to20

Greetings all,
If I have a power transformer that states the secondary as 440vac @ 150va, is that telling me I can draw 340ma?

Thanks,
Ray

Yes, but I'd look real close at that 150"v"a to see if it isn't actually 150"m"a.

#### rsumperl

Checked twice, definitely 150va

#### paulb

But it doesn't mean you can draw 340mA DC after rectification.

#### TheGimp

It may also be the input VA rating of the transformer so figure 90% of that for safety sue to core losses, IR loss, etc.

Capacitive input filter will get you near 622V with SS rectification. So figure 217mA max.

Inductor input filter will give lower voltage at higher current.

#### rsumperl

So, if I do a choke input, do you think 300ma possible?

#### TheGimp

At what voltage?

Sim it in PSUD II and see what you get.

#### Merlinb

For ordinary rectifiers (capacitor input) the RMS ripple current will be about 1.5 times the DC load current. You shoudl be safe up to 230mA continuous DC.
If its a class AB amp then you can design for higher maximum currents if you think it won't be continuous.

#### Tom Bavis

What's the DC resistance of primary and secondary?

#### rsumperl

Tom, the resistance of the primary is 2 ohms and secondary is 30 ohms.

#### Tom Bavis

300-350 mA seems reasonable for that winding resistance.

Simulated in PSU Designer (5 Hy / 30 Ohms into 220 uF) I get 307 mA RMS in the transformer secondary at 300 mA DC load, 385V. About 2.7W copper loss in the secondary, 2.4 in the primary.

#### rsumperl

Tom,
Do you think occasional peaks of 500ma DC could be had? I realize that running perhaps 250-300ma steady state current creates heat, but I'm guessing the worse that will happen with peaks of 500ma will be reduced voltage. The transformer will be feeding a bridge and then into a 500v voltage regulator.

#### Tom Bavis

The numbers above are for choke input. Cap input would be good for a little over 200 mA continuous. Peaks above that will be OK - use "stepped current source" in PSU designer to see how much drop will occur.

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