Reading the LM338T datasheet for current limits
So to a certain degree, when the transformer is sitting there without a load, it kind of just sits. The current that the circuit after the transformer pulls will affect how much current goes from the wall into the transformer and therefore into the circuit.
So the LM338T is rated for...*figures out how to find that value in a datasheet* anddd thanks to @mgburr:
So this is the datasheet for the LM338 and you see a parameter called "Current Limit" where it looks like the typical current draw is 8amps. But if output is 30v then 1Amp. 12v can be handled at a surge for 500 milliseconds.
Now, the chip will require a minimum current to be able to function as a regulator. Here we see that it's 3.5mA. The circuit has to draw at least the minimum otherwise the chip won't turn on.
So this is nice and all but now I have some uber awesome newbie questions.
So in "current limit" you have a max of 8-12A which is fine. But then you also have a limit minimum of 5-7A. Okay but now there's also a minimum load current parameter.
In "minimum load current" you have the minimum of 3.5mA but a maximum-minimum of 5mA.
What's the point of saying "MIN" 5-7? We're talking limits here, you have the minimum load current which should be the MIN and the current limit should be the MAX right? So why isn't the minimum current 3.5mA and the maximum 12A?
@mgburr smartly said: "Input current draw is not the same as output current load."
------Minimum Load Current----[LM338T]----Current Limit------
When you use a regulator, it switches the input at a frequency to make an output based on pulse width modulation. If you don't have a load on the output, then the PWM will not work correctly and cause it to work improperly and possibly fail.
The minimum load enables the regulator to continue proper operation without upsetting the PWM balance in that it generates normal pulses which prevent the circuitry from going into runaway or current overload. In our case it'd be the 3.5-5mA or "minimum load current". This could be considered the input current draw as well.
Now the output current is the maximum current that can be passed. In our case static 5-8A with pulses of 7-12A. The pulses have a rise and fall that the circuit has problems keeping up with, which is why the 7 is lower than the 8. And this could be considered the output current load.
And when you do step up or down, the currents are usually different between input and output. For example. With a 9V 3A source voltage, you can provide 27w of power. If you step the voltage down to 5V, your current will change.
Kinda confusing until you get a Mike to explain it to you!