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AC Adapter tolerance

Chris Robichaud wrote:
> If I give it more volts or amps than is recommended, am I risking a
> fire? How much of a tolerance do these things usually have?

Most of what's been said in the responses is accurate, so I'll just 
supplement by saying...the tolerance can vary by quite a bit. A typical 
"wall-wart" is unregulated, and its open circuit voltage (with nothing 
attached to it) is usually several volts higher than what's listed on 
the label. If you measured it, that 12 VDC supply might actually read as 
something like 15 or 16 VDC. These rely on the voltage dropping some 
when under load, electronics being tolerant, or the device having an 
on-board regulator. The last option is quite common.

These days it is not unusual to find more sophisticated wall adapters 
that use switch-mode power supplies, which uses the same technology as 
the power supply in your computer. You can usually spot them as they are 
much smaller and lighter than typical adapters for the equivalent 
wattage. They're inherently regulated, and the devices that use them are 
likely less tolerant of voltage variations.

So to answer your question, while you probably can supply 16 VDC to your 
device without problems, switching from a 12 V labeled adapter to a 16 V 
labeled adapter is going to feed something like 18 V to your device, 
which could overheat the regulator, depending on how well the circuit is 
designed and heatsinked.

Bill Ricker wrote:
> But if they've shaved design pennies, the device may rely upon the 
> supply to limit draw...

Can you cite a known example of this? Sure, that's something you might 
have seen decades ago on analog circuits, but I'd be surprised if such a 
situation exists in any modern devices. Unless the original wall adapter 
was designed to limit current, it would likely overheat.


Tom Metro
Venture Logic, Newton, MA, USA
"Enterprise solutions through open source."
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