las wrote:
>
> Shawn Lin wrote:Probably not enough mA, the original one was 5V@700mA I think.
>
> That's one of the major pieces of knowledge that I learned from being on this list
>for years, Although you would think that too much
> mA would burn your unit up and worst case, less just wouldn't work, less can
>actually cause damage.
>
> Also, the electronic gadgets that we are always talking about on this list,
>apparently can handle some fluctuation in voltage
> without causing damage (though as you discovered, you can have a Sharp unit that
>will work just fine on 4.5 volts-even their car
> adapter is 4.5 volts-yet 5 volts are required to charge the battery).
>
> AA batteries are rated from 1200 mA up, depending upon the type of battery. The
>only effect that more mA seems to have when it
> comes to batteries is playing time. But the voltage of batteries range from 1.2 to
>1.5 volts each, yet the units do just fine with
> the lower voltage batteries, why is that?
The current (Amps) rating of an electrical device can be thought of as
"amount of electricity consumed". Voltage can be thought of as
electrical pressure. So an MD player rated at 5V@700mA means it
"consumes" 700mA worth of 5V electricity.
Power supplies and batteries are rated differently than a load device.
The current rating of a power supply is a measure of maximum AVAILABLE
current. This is because with no load on the supply, there is 0mA of
power consumption. If you use a [EMAIL PROTECTED] (2500mA) power supply on an MD
player that only needs 5V@700mA, then the MD player will only consume
700mA but the power supply can safely deliver up to 2500mA. If you used
a device on the same 2500mA power supply that consumes 3000mA, then this
would overload the power supply causing it to overheat and deliver less
voltage as well. This is why it's OK (in fact, beneficial) to get a
power supply that can supply greater current than your device requires.
As for voltage, too LITTLE never hurts anything. Think of voltage like
water pressure... not enough water pressure in your pipes doesn't hurt,
but also doesn't work properly, like you're in the shower and the water
pressure is so low it's just dribbling out, you can't shower
successfully. In the case of MD players, not enough voltage causes
improper function or non-functioning. Too much water pressure and the
pipes could burst, valves could break, and if you were in a shower with
the water pressure of a fire hydrant, you could get seriously hurt.
Voltage works similarly. Too much and electrical components will start
to burn up. This is why you have to make sure the voltage of your power
supply matches the voltage required by your device.
Batteries are different. They're rated in mAh. "milliAmp hours". This
means a 1200mAh battery will last 1 hour with a load that consumes
1200mA. Most devices that require 1.5V batteries will work fine on 1.2V
batteries because most electrical devices have an acceptable voltage
range for proper functioning. Say you have a device that requires 6V (4
1.5V batteries). It probably has a proper operating range of 4.5-7V, so
4 1.2V batteries at 4.8V will work just fine. Going back to the water
pressure analogy again, it's kind of like within a certain range of
water pressure, you can successfully take a comfortable shower.
As for the Sharp specifically, why it needs 5V to charge seems to be due
to some voltage checking in the charge circuit. I noticed when I hooked
it up to my variable bench power supply, it will actually charge
batteries just fine at 4.5V. It just won't START charging until it
detects at least 4.7V. I couldn't get it to charge with the Radio Shack
4.5V supply because it's regulated. No matter how much the current
draw, it stays 4.5V. A non-regulated power supply probably would have
worked, because a 4.5V@700mA unregulated power supply would be 4.5V if
the device is drawing 700mA, but at 20mA for example, it could be
higher, perhaps 6V or so, it depends on the power supply. I was
actually able to trick the Sharp's charging circuit by starting my
variable supply at 5V, and then after charging started I could drop it
down to 4.5V and it would still keep charging with no problems.
Hope my explanations and silly analogies make sense. :)
Shawn
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