PSU/current question - probably stupid.

[lion]

It's obvious that a PSU has to deliver the current/amperes needed for a given circuit - but can too much PSU capacity be bad or even damage a circuit/components. As an example - running a circuit drawing 200mA on a 5A supply?

Erik

[Basstyra]

No....

Because a psu is a voltage supply, it supplies a voltage, 9v for example. It's the circuit that "takes" the current needed, in NO WAY the psu can impose the current.

If the psu is a current supply, there is a problem... but you won't easily find current supply, working on stompboxes !

[lion]

Basstyra - thanks!
Just curious - what is a currentsupply - and how do you tell the diff between the two.

Erik

[niftydog]

There is a difference between a "voltage source" and a "current source". In both cases, ideally, the source remains constant regardless of load.

So, a voltage supply keeps it's voltage steady regardless of the demands placed on the supply.

A current supply keeps it's current steady regardless of the demands placed on the supply.

Current supplys or current sources are usually implemented as a part of a larger design and you wouldn't normally find one in a similar form to a voltage source - like a wall wart.

[lion]

Thanks niftydog.

Erik

[Basstyra]

Yep.

You found current supply in op-amp for example. It's made with a few transistors.

[Paul Perry (Frostwave)]

While it is true that a fx box will take as much current as it needs... for a normal unregulated wallwart, the output voltage DOES vary somewhat with varying current load.
If a wart is rated at say "9v 250ma" then, you will get 9v when 250ma is drawn from it, and somewhat more voltage at a lower current. It can be instructive to measure the loaded voltage.
Normally, it doesn't matter much, as there is not much difference.. but, it IS one more thing to take into account.
It is interesting to check warts wiht a multimeter under no load, you will often see 12V or so for a nominally 9v wart. And if you put a super low current fx on the wart, you might very well get something like that!

[Basstyra]

Yes, but this is false for regulated power supply such as those we use for effects. They have a regulated voltage of 9v no matter the load (in theory, in fact it can varie a little, but in not really "usable" proportion, for instance 10 or 100mV). If we need regulated power supply, it is for that very reason !  :wink:

[Paul Marossy]

Here is a good reason to use caps rated for twice your operating voltage. If you had 10V electrolytic caps in a 9V circuit with a current draw of 20mA and you power that with a 300mA 9VDC wall wart, you're going to blow up your caps because your circuit will be seeing quite a bit more than 9V due to such a small current draw!  :wink: