7805 V-Regulator mA Question

Stupid question of the week! :smiley:

If i have a power adapter that supplies 12v & 700mA but the specific 7805 voltage regulator i am using is rated as being able to supply 300mA and i plug it into my board will,

A) Terrible horrible things happen.

or

B) The regulator will drop it down to 300mA.

If the answer is B) are there any side affects like over heating, need for more caps, etc that i should concern myself with?

Thanks

You seem to be mixing up voltage and current.

You will only get up to 300mA from the regulator, it’s the limiting factor. The current capability of the supply is that, only a capability, it doesn’t produce that current unless a suitable load is placed across it.

However if you actually draw 300mA from the regulator you have to look at the data sheet to see what the power dissipation is. Without a heat sink it is likely that you won’t even get this before it overheats. Most regulators have a thermal cut out to prevent damage.

Your power adapter is capable of 700mA; that doesn’t mean it will forcefully shove 700mA down the wire when you turn it on (well, unless it’s a current source, but that’s a different animal), it just means it can supply at most 700mA.

-j

So as long as I don’t attempt to draw more currant then the regulator is rated for I’m good, and if the power adapter is only rated to < or = to the regulators max current rating it’s foolproof?

Not quite - you want the adapter rated for at least the max you will draw through the 7805.

-j