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

jim jim at systemateka.com
Sat Dec 29 18:58:20 UTC 2012




    battery charging is different from equipment loads. 
    Equipment (electrical-electronic circuitry) generally 
works on voltage; excess current capacity of power 
supplies is pretty much a plus. 
    Battery cells are batches of chemical goo that cannot 
sustain high current without generating too much heat 
(hence the swelling). Good battery chargers manage the 
current. Most power supplies intended for equipment do 
not manage current. 
    The rule of 2x seems solid to me, and I'd not worry 
about higher current capacities unless my device is 
expensive or rare (caveat: I take chances). 


On Sat, 2012-12-29 at 22:03 +1000, Steve Castellotti wrote:
> On 12/21/2012 10:42 AM, Andy Isaacson wrote:
> > On Thu, Dec 20, 2012 at 07:32:37PM -0500, rachel lyra hospodar wrote:
> >> I have a piece of audio gear (roland micro cube) that requires 9v & 185 mA
> >> .  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.
> 
>      I ended up sticking to only charging from the AA's and have been 
> fine since.
> 
> 
> sc
> 
> 
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