[Noisebridge-discuss] How close do I need to get with amperage?

[Andy Isaacson]

On Sat, Dec 29, 2012 at 10:03:09PM +1000, Steve Castellotti wrote:
> >>.  I scored a power supply that supplies 200 mA but am wondering if it's
> >>safe to use or if it might fry the amp?
> >With off-the-shelf power supplies you are fine up to at least 2x.  The
> >reason is, the power supply is designed to reliably run very close to
> >the rated voltage.  The current is variable depending on the load to
> >preserve the accuracy of the voltage level.
> >
> >There is an alternate kind of power supply known as "constant current"
> >which varies the voltage instead.  The nice "bench" power supply at
> >Noisebridge can operate in this mode, and there are some kinds of
> >circuits that need it, for example somebody told me that EL wire
> >transformers are constant current.
> >
> >Any commercial power supply that says "9 volts DC, 200 mA" is a constant
> >voltage supply.
> >
> >If your load is *really* low, like if you try to use a 2000 mA (2 amp)
> >supply to power a device that only uses 10 mA, then you might see
> >problems (voltage variations mostly).  A sufficiently fragile device
> >might be damaged that way.
> 
>     Late reply to the thread but just wanted to state that a year or
> so ago I blew through several batteries for a radio-controlled
> helicopter by using an AC/DC adapter which had too high of an
> amperage. I don't have the bits handy at the moment but I'm pretty
> sure it was less than 50% over what the charger was rated for.
> 
>     It was the kind which would use 4 AA batteries to power the
> charger, or there was a AC/DC power source optional. I had the
> voltage right for certain but didn't know anything was amiss until
> my helicopters wouldn't fly and I noticed the batteries were getting
> swollen.

Interesting, I'd like to see this charger!  Most rechargable batteries
need a "constant current" charging circuit ... it sounds like the
charger manufacturer depended on the DC power source having a limited
amperage to deliver the constant current feature.  It would be better
(but more expensive by a few cents) to have the charger circuit
explicitly implement the constant current requirement.

I'm sorry your batteries got destroyed, and thanks for the cautionary
tale!  I'll be more careful about recommending over-amperage supplies in
the future...

-andy