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General question about linear regulators and transformer VA

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queennikki1972:
I understand that a linear regulator like the LM317T is a max output current of 1.5 according to the specs?

I have a transformer of 24V 40VA for example, which I assume is 1.6 amps.

Does this mean the transformer is to large and would overload the regulator? Or does it simply mean it will only put out 1.5 amps on the output of the regulator?

Any links to explain something probably so simple are appreciated. I just cant seem to get a clear understanding..

Totalsolutions:
Voltage is something you have. Current is what you take. Quick answer.

bson:
A handy rule is that current in equals current out.  So if you put a 0.5A load on the regulator its current out is 0.5A, its current in is 0.5A, the rectifier's current out is 0.5A, its input is 0.5A, and finally the transformer's secondary current is 0.5A both on input and output (it will have at least two terminals).  Only the transformer primary will differ (it will have equal power to the secondary, minus some marginal efficiency loss).  Current is determined by the load.

Your transformer, however, is a bit too hot for the LM317T; it has a maximum input voltage of 30V.  24V RMS means the peak secondary voltage is just shy of 34V.  Even with a little voltage drop in the rectifier this means the 30V maximum will be exceeded.

not1xor1:

--- Quote from: queennikki1972 on August 13, 2019, 02:03:58 pm ---I understand that a linear regulator like the LM317T is a max output current of 1.5 according to the specs?

I have a transformer of 24V 40VA for example, which I assume is 1.6 amps.

Does this mean the transformer is to large and would overload the regulator? Or does it simply mean it will only put out 1.5 amps on the output of the regulator?

Any links to explain something probably so simple are appreciated. I just cant seem to get a clear understanding..

--- End quote ---

1.6A is the maximum current you can draw. The amount you actually draw depends on the load.
It is like a car. If the tachymeter range goes up to 200mph that doesn't mean you can only drive at that speed.

In this very case, a transformer, the maximum specified current applies only to resistive laods.
If you use a diode bridge and a levelling capacitor you must derate the maximum current value.
With a 24V 40VA you cannot usually take more than about 1ADC (i.e. about 60% of maximum resistive load current).

Chriss:
Voltage is always what you can measure on a circuit under power.

Current is measurable on a circuit when the circuit is under any load.

The load determines how much current you can measure.

So, in your case we can say:
The transformer secondary side gives you a voltage of 24VAC but no current even if it is rated to give out 1.6A.

If you connect anything to the transformer secondary winding lets say the rectifier diode bridge than you can measure a minimum current what is taken by the component.

If you put a bulb 24VAC/24W then you would measure a current of 1A on the secondary side of the transformer and almos 0.6A will left unused and unmetered.

The same story is for the linear regulator too, but dont try to connect you LMxx to the transformer cos you will kill the IC cos your input voltage is higher then the max Uin for the regulator.

My best regards.

Sent from my SM-J500F using Tapatalk

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