Transformer with higher amp rating than schematic calls for?

[TheLaw]

I would first like to start off by saying I am a complete newbie to electronics so please bear with me.

I've worked on some battery based projects in the past but I'd like to step it up to the big leagues. That is power supplies and such.

So I am looking at this schematic: http://ecelab.com/circuit-pwr-sup-multi-1.htm

..and liking what I am seeing. But I haven't been exposed to much of this stuff yet, nor have I ever taken official classes on this sort of stuff. But I am learning...slowly.

So my problem is finding transformers that are going to match the exact specification as the schematic calls for. It wants one with a secondary of 18V with 3A and one with a secondary of 12V with 2A.

So my question is. If I can't find transformers that are exactly that specification, that is regarding amperage, can I have it be able to put out more amps? If it wants one rated for 3A, can I install a transformer rated for 3.4A? I would of course keep the voltages the same (18V and 12V). But on the amp spectrum, is that a legal move?

Thanks very much in advance. I appreciate it.

 

[sghosh]

Unless you have space constraints, you can use as big a one as you can get. Bigger is better for transformers.

 

[TheLaw]

Re: Transformer with higher amp rating than schematic calls

Unless you have space constraints, you can use as big a one as you can get. Bigger is better for transformers.

No space limitations really, but I can have an output of as many amps as I want? What effect does this have on the rest of the power delivery circuit? Will I need to upgrade the ICs to handle more amps or am I just sounding stupid?

Also, in regards to voltage ratings on caps. It's never bad to have a voltage rating higher than what the circuit is intended to use, right? For example, if the circuit is supposed to be 12V, can I use a 25V rated cap?

Thanks for your answer sghosh. Much appreciated.

 

[Audioguru]

The voltage regulator ICs and the size of their heatsinks determine if they supply their max rated current. The datasheets for the 78xx and 79xx regulators show a max current spec'd at 1A but can typically go to 1.5A.
If the transformer can supply thousands of Amps then the regulator's 1A and 1.5A ratings do not change.

A 25V rated capacitor will last a long time in a 12V circuit. A 12V rated capacitor might fail tomorrow.

 

[TheLaw]

Re: Transformer with higher amp rating than schematic calls

The voltage regulator ICs and the size of their heatsinks determine if they supply their max rated current. The datasheets for the 78xx and 79xx regulators show a max current spec'd at 1A but can typically go to 1.5A.
If the transformer can supply thousands of Amps then the regulator's 1A and 1.5A ratings do not change.

A 25V rated capacitor will last a long time in a 12V circuit. A 12V rated capacitor might fail tomorrow.

Okay, you have just posted one of the most helpful posts I have ever recieved in forum history, anywhere. It was the simplicity.

Even so, I'd just like to clarify one last thing. I can put a larger transformer than spec'd in the schematic without doing any harm to the 7812, 7912, 7805 and 7905s?

And when I mean larger, I am referring to amperage. If it outputs more volts than recommended by the schematic does it matter if all of my components are rated for the higher voltages.

You see, I am much more used to working with PC hardware. The most complicated it can get is choosing whether you want your memory to run at 1.5V or 1.8V. This stuff is so new. Can't get it straight.


Yikes, I still got some work to do.

Thanks though.

 

[Audioguru]

Re: Transformer with higher amp rating than schematic calls

I can put a larger transformer than spec'd in the schematic without doing any harm to the 7812, 7912, 7805 and 7905s?

Yes.
Look at the datasheet for a 78xx or 79xx regulator IC.
It limits the peak current to 2.2A and 2.1A. A circuit takes only as much current that it needs.
If it gets too hot then it shuts down, cools then starts working again. It is not recommended to let it over-heat, cool, over-heat and cool again too many times because the thermal stress might break it.
Its short circuit current is reduced to a lower amount depending on its voltage rating.

If it (the rectified transformer) outputs more volts than recommended by the schematic does it matter if all of my components are rated for the higher voltages.

The passive components must have a suitable voltage rating.
The regulator IC might over-heat if the input voltage is too high since heat is caused by the current times the input to output voltage.

 

[TheLaw]

Re: Transformer with higher amp rating than schematic calls

Alright. That helped a lot, just trying to figure out things in general. I will take a look at the data sheets and I will probably be fixating some good heatsinks to the volt regs.

Thanks.