AC Adapters

[Mark Lloyd]

On Sat, 04 Nov 2006 04:03:41 GMT, Tony Hwang <dragon40@shaw.ca> wrote:

Mark Lloyd wrote:

On Fri, 03 Nov 2006 21:09:20 -0000, clewis@nortelnetworks.com (Chris
Lewis) wrote:


According to Mark Lloyd <mlloyd@xmail.com10.invalid>:

I have one 12VDC 1A power supply that is connected to 2 separate
cameras. This is a regulated supply, which was needed to reduce noise
in the video output of the cameras.

If one of them needed a different voltage, it would be possible to use
diodes (consider current here too, diodes have current as well as
voltage ratings) to drop the voltage (2 diodes in series give about
1.5V drop). If the devices have a common ground (as those cameras do),
that needs to be considered when powering multiple devices with 1
adapter.

Diodes in series is a reasonable regulator, but it might not
be good enough if the current through them varies a lot.



2 diodes in series is exactly what you have with a full-wave bridge
rectifier. I haven't heard of voltage drop varying with current.
Perhaps you could explain.
Hi,
Usual Si diode has a typical voltage drop of 0.2V across it's junction.

I have ALWAYS heard .7V for Si. Bridge rectifiers drop 1.4V (2 diodes
in series). I have even measured this in real circuits.

If you put them in series it'll be multiple of 0.2. If current
increases, you do the math acoding to the Ohm's law.

That applies to resistors (including the series resistor used with a
zener diode or LED). A diode is a voltage regulator, keeping a
constant voltage across it.

It you have a wall-wart DC supply, with a diode in series with the
output (correct polarity). The output voltage will always be .7V lower
than the output of the wall wart. The diode does have a current limit
(1A with the common 1N4001 diodes) that needs to be observed.

BTW, I've also used diodes on AC to make a bulb light with half
brightness. Maybe you can figure out why 2 diodes DON'T give you
quarter brightness. The diode is a 1N4004 (1A, 200PIV).

LEDs are diodes (although with higher forward voltages). A LED would
destroy itself trying to maintain that voltage, without a series
resistor.

Also regulator
typically comes in two different flavors. Series or shunt type.

Yes. A shunt regulator would normally be used only for a small load,
because of the inefficiency of it.

One time I needed to supply 12V @ 1A and 6V @ 50mA to a circuit. I
used a series regulator (7812) for the 12V and a small shunt regulator
(zener diode) for the 6V.
--
51 days until the winter solstice celebration

Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

 

[jasen]

On 2006-11-04, Mark Lloyd <mlloyd@xmail.com10.invalid> wrote:

On Sat, 04 Nov 2006 04:03:41 GMT, Tony Hwang <dragon40@shaw.ca> wrote:

2 diodes in series is exactly what you have with a full-wave bridge
rectifier. I haven't heard of voltage drop varying with current.
Perhaps you could explain.
Hi,
Usual Si diode has a typical voltage drop of 0.2V across it's junction.

I have ALWAYS heard .7V for Si. Bridge rectifiers drop 1.4V (2 diodes
in series). I have even measured this in real circuits.

I've measured 0.6 to 0.8 dependant on load etc...

0.2 is for germnanium diodes.
0.3 for schottky

--

Bye.
Jasen

 

[krw]

In article <eijd1e$ro8$1@jasen.is-a-geek.org>, jasen@free.net.nz
says...

On 2006-11-04, Mark Lloyd <mlloyd@xmail.com10.invalid> wrote:
On Sat, 04 Nov 2006 04:03:41 GMT, Tony Hwang <dragon40@shaw.ca> wrote:

2 diodes in series is exactly what you have with a full-wave bridge
rectifier. I haven't heard of voltage drop varying with current.
Perhaps you could explain.
Hi,
Usual Si diode has a typical voltage drop of 0.2V across it's junction.

I have ALWAYS heard .7V for Si. Bridge rectifiers drop 1.4V (2 diodes
in series). I have even measured this in real circuits.

I've measured 0.6 to 0.8 dependant on load etc...

Of course, which means they do _not_ regulate in any way.

0.2 is for germnanium diodes.
0.3 for schottky

Depends on current. Germanium diodes have a weaker knee. We
always used .3V as a rule-of-thumb for Germanium.

--
Keith