voltage conversion puzzle.

Simple, you can individual diodes with power dissipation of a couple
of watts. I.E. you diode string could probably handle a couple
amperes. If you are operating a device with wide ranging current
requirements over time (such as an audio amplifier), you may need to
dissipate more power than a single zener can handle. In addition you
can get any regulated voltage you want in increments of about .5 volts
if you use 'hot carrier' diodes, otherwise, it is increments of about
..7 volts. You can however mix and match...

However I'll concede that a 3 terminal regulator with a couple caps
and heat sink is the no brainer solutions, and you can get 3T
regulators that handle several amperes.

 

Circa Wed, 23 May 2007 16:27:33 -0400 recorded as
<> looks like Fred W

Stable voltage drop under varying load current. If the load current is
constant, then it doesn't matter.

 

But still ... what automotive electrical system has a constant voltage?
And to use diodes to subtract volts from a source seems an odd way to do
it. I can think of better things to do with a bunch of high-current
diodes.

As for the OP, I'd check the device's input voltage requirements. For
instance, the Garmin III+ was designed for marine and automotive
applications, so its input voltage range is wider than I'd ever expect
to see on a working car's electrical system.

 

Circa Thu, 24 May 2007 15:14:21 GMT recorded as
<hZh5i.13080$> looks like Rick

I'm confused. Why does four volts at 50 milliamps require more than a
one-half watt device?

 

Circa Fri, 25 May 2007 05:09:25 GMT recorded as
<9cu5i.9978$> looks like Rick

That info was in the original post. I saw a lot of discussion about high
power stuff, and I wondered why. I guess when the ideas start flying fast
and furiously, details get lost.

"The camera uses 50 milliamps during operation."

A half-amp for a portable 9V device is a pretty hefty requirement. The
typical snap-in 9V battery is rated at around 500 mAh.

 

Which tells us almost nothing about how much current a 9V battery
can be reasonably expected to supply, although it tells us roughly
how many joules it can supply.

There are rules of thumb which are commonly used to estimate what
sort of current in mA that you can draw from typical primary
consumer-style cells given their capacity in mAh, but the rule of
thumb for a hefty sized D cell might not work very well for a 9V
battery composed of six much smaller cells despite them both having
the same chemistry.

I'm not sure anyone brought up the fact that the device in question
may average 50mA at 9V but very possibly draws more current on
startup. It's a good idea when doing a power supply design (which
is what this really is) to make it robust so it works over all
normal conditions expected of the load, plus at least a little bit
of margin outside the expected area of operation. In the case of
this little camera, the three terminal regulator approach is a true
no-brainer because it really does cover these requirements well for
very little money, and it's reliable and simple. Is it overkill?
Probably... using diode drops would certainly work and probably
wouldn't hurt the camera at all, but the variable supply voltage
due to lack of regulation *might* cause unwanted changes in the
video signal output that you could pretty much guarantee wouldn't
happen with a regulated supply.

 

zeners go to 25 watts or more.
they are TO-3 case.
four volts at 50 ma is not too much for a 1/2 watt device.
it comes to two-tenths of a watt.
but, a bunch of cheap diodes MAY be more convenient.
a cheap and dirty solution. not elegant. but it works.