how to choose the proper adapter : current, voltage

[vr]

Hi,
my application is a PIC microcontroller programmer. it needs 12 to 14v dc as input. the spec also says 20ma current.
what would be a good spec to choose for the adapter?(with so many in the market)
how does one go about deciding from the adapter spec?
Thank you.

 

[Russlk]

Do you mean wall-wart? Just pick one that is 12 VDC and more than 20 Ma.

 

[vr]

yes, any wallwart.
called up Digikey. the tech person at Digikey said, they have an adapter which was:
15v (or maybe 12v) with a current rating 500mA. she said this is pretty much steady current
rating (10%). she said therefore it is not a suitable one, as it would provide a huge current always and would frey the pic chip.
but you said anything more than 20ma should be ok.
could you please explain? it is not clear if my underatanding is right.
Thank you.

 

[checkmate]

The current rating on adaptors are the MAX current rating. The voltage ratings are the FIXED voltage ratings, although they are usually very much higher than speced to allow the drop-out required by most voltage regulators. So a 500mA adaptor should work fine. Obviously, the techie at Digikey knows nuts!
Lastly, chances are that the programmer will consume more than 20mA, which is the current consumed by a single LED.

 

[mahinda jayasinghe]

That is right. Buy an adaptor with more currents than you required (at least 50%).

No matter it is 50 ma or 500 ma. Your circuit draws the current what is it reqire ie 20 ma.


In case, if your circuit draws more current, say 500ma, the power supply ( adaptor) will burns not your circuir.

 

[vr]

hi,
my understanding was also the same as your answer:
whatever o/p rating for voltage was given, would appear at the adapter output - constant.
current rating gives you the max which can be drawn by a load- so if the current rating for the device was less than the current supplied by the adapter, it must be ok - only the required current will be drawn.

another piece of info got from a friend:
if a circuit draws more than the adapter's rated max current, then the adapter's voltage cannot be guaranteed to remain the same as its rated value. it would start decreasing.

my theory is:
the adapter would be destroyed if it draws more than its rated max current.

anyway, the reply and explanation given by the digikey tech, regarding pumping constant 500mA current, irrespective of the load, made me confused.
Thank you all for the explanations you gave.

 

[vr]

Hi,
more information on choosing a "wallwart":
**broken link removed**
Thank you.

 

[Styx]

always buy an adaptor with a higher current rating than you need. You have to think about power-up where you have all the caps (even the ones you don't think about) suddenly charging up. it is not uncommon for a board with a quescent current of 300mA quite happily drawing 3A .

And if you have any FPGA's on the board make sure you have an extreamly beefy powersupply, they need a contiuous supply of 1A (at 3.3V) for the first few microseconds as it configures. If it doesn't get that power it gets locked in its program cycle and contiously draw 1A !!!!!!!!!! not good


If you have an adaptor that is only just rated and you try an power-up, you board power-up sequence will be thrown to shot as part's of your board does nto power-up correctly.

 

[vr]

it makes sense a lot.
Thank you.