Re: What is the real voltage from an AC/DC adaptor ?
redbelly98_at_yahoo.com
Date: 12/21/04
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Date: 20 Dec 2004 16:27:43 -0800
> If I have two devices that draw slightly different Voltages say 5.5
> 250mA and 6 300ma. (Assuming that the 5.5 device is rugged enough to
take a
> little larger voltage/amperage, many are I think) And say you use an
AC/DC
> adaptor that is rated as 6v DC and 300mA. Would the voltage if i
tested it
> where it goes into the device be the same when you test it with a
volt meter
> for both devices. (Hopefully what the AC/DC adaptor is rated at ?)
Okay, let me back up a little. This is somewhat lengthy,
so bear with me.
For the AC/DC adapter, (or any DC power supply), one
could make a graph of the output voltage vs. the current
while it is operating, for different load conditions.
The load could be a simple resistor, which is changed from
zero ohms, to several non-zero values, to infinite ohms (open
circuit). The graph will trace out current and voltage
outputs from that supply, and might look something like you'll
see at the following URL:
See Figure 8 on p. 6 of:
http://64.224.241.100/products/data/pdf/cmpwr330.pdf
If a power supply is rated for 6 V and 300 mA, that refers to
just one possible operating point on the voltage-current curve.
As Robert said, it usually means that you'll get at least 6 V
whenever the current is 300 mA or less.
Similarly, for a device that is to be powered by the DC supply,
one could make another graph of voltage vs. current. If the
device is a simple resistor, this graph will be a straight line,
given by the well-known equation V = I R. This will intersect
with the current-voltage curve of the power supply **somewhere**.
That intersection point gives the voltage and current when this
particular device is connected to this particular power supply.
In other words:
The device **must** operate somewhere on it's current-voltage
curve, and the power supply must operate somewhere on **it's**
current-voltage curve. The intersection point of the two
curves represents the only current and voltage that makes both
of those statements true.
Now, to your question about two devices that draw slightly
different currents at different voltages:
First, it is possible that the same device could both draw
250 mA when powered at 5.5 V, **and** draw 300 mA when it is
powered by 6 V. Or, a device that draws 250 mA at 5.5 V might
draw something other than 300 mA at 6 V. So, it is impossible
to say, from the information you give, whether the two devices
would give the same voltage reading when driven by that power
supply.
> It is my understanding that the real voltage that it gives
> out can only be seen when the adaptor is under load ?
Yes. However, there is not a single unique "real voltage" that
it gives out no matter what the load resistance is. For dif-
ferent loads, it would be different voltages (in general).
Hope this helps.
-- Mark
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