# Question about linear voltage supply transformers and current

#### hunterage2000

Joined May 2, 2010
487
Hi,

If you have a chassis mount transformer with specs:

Primary V = 230Vrms
Secondary V = 24Vrms
S = 6VA

Does this mean the maximum rms current that can flow through the transformer is 250mArms?

Just wondered if you used this transformer to get a rectifed 25.9V DC input to a LM317 linear regulator. you could only get a maximum load current of 250mA DC before the output voltage drops?

Joined Jul 18, 2013
23,624
The Va of the transformer applies to both primary and secondary, IOW if you have a 6va version then yes, the load placed on the secondary is limited to 250ma.
Max.

#### Dodgydave

Joined Jun 22, 2012
10,050
At best the current will be 250mA, less losses, a Lm317 needs 10mA operating current, so that leaves 240mA load.

#### AlbertHall

Joined Jun 4, 2014
11,504
http://www.sowter.co.uk/rectifier-transformer-calculation.php
The above website shows the calculations for various rectifier configuraions. If you use a bridge rectifier and reservoir capacitor then the available output current is about 62% of the allowed secondary current. So in this case the maximum current would be 250mA * 0.62 = 155mA.

#### hunterage2000

Joined May 2, 2010
487
I see. I'll get a higher VA rated transformer. Anyone know of any good sources for variable linear power supplies i.e books, sites etc?

#### dl324

Joined Mar 30, 2015
13,122
Anyone know of any good sources for variable linear power supplies i.e books, sites etc?
Datasheets usually have application suggestions.

The National Semiconductor Voltage Regulator handbook has a wealth of information. Including how your choice of rectification/filtering will impact transformer current requirement:

#### hunterage2000

Joined May 2, 2010
487
Nice one dl324, ive googled it and found it. I'll have a read through it.

#### hunterage2000

Joined May 2, 2010
487
Cool, another good source to read.

#### crutschow

Joined Mar 14, 2008
27,732
you could only get a maximum load current of 250mA DC before the output voltage drops?
The VA rating is not where the voltage drops, since there is typically a 10-15% voltage drop from no-load to full-load due to winding resistance.

The VA rating is determined by the heating of the transformer.
Beyond that VA rating the transformer can overheat.