On 25/01/13 18:38, Mark Rogers wrote:
On 25 January 2013 18:06, Anthony Anson tony.anson@girolle.co.uk wrote:
Read the spec.
That only works if you believe the spec.
The problem I have now is that I have a mixture of chargers, most of them "branded", some of which it is become clear are likely fakes, but without a way to know which are which I don't know which to trust. (Eg I have a mixture of Sony Ericsson chargers, a least one of which is genuine, but if any are fakes I can't tell them apart.)
Plug them in and test the voltage across the (low voltage) supply. That will tell you which ones cut the mustard.
It's the voltage which determines the speed of charging, not the rated amps
- but if the amperage at the required voltage isn't sufficient to cope,
either the delivered voltage will drop, or the supply will lose all its magic smoke.
So puting it another way: if a PSU says it can deliver 1A @5V, then I should be able to stick a 5ohm (R=V/I) resister across the terminals and the voltage shouldn't drop substantially below 5V?
Theoretically. Remember that the resistance of a component is often temperature sensitive, and a resistor's value is probably measured at expected working temperature. While it's unlikely to vary a lot over the range, if the PSU is working at its limit, it could be put at risk.
Better to measure the supplied potential difference with the device connected. If it's within sensible limits (the spec of the device should tell you that if you dig deeply enough), that's good enough.
If the measured voltage is too low, you risk magic smoke from the PSU. If it's too high,look for the smoke to come from your device as it chars.
And provided I pick a suitable 5ohm resistor (5W? P=VI?) then I shouldn't get any loud bangs, burning smells, or tripped power?
There is a difference between resistance and impedance - not always numericlly. I'd forget about using a resistor.
Not really. Just choose the supply which delivers enough and then a bit.
The problem is I have picked supplies which *claim to* deliver enough but I don't believe them. Not least I'd like to prove they don't and get me money back!
If they don't deliver enough power the potential difference between the + and - lines will be below the stated voltage figure. This will not harm your device, but may destroy your charger - given time.