Re: Best method for dropping supply voltage a volt or two for low power device?


"Fred Bloggs" <nospam@xxxxxxxxxx> wrote in message
news:480E3DF7.8020807@xxxxxxxxxxxxx


Jim Thompson wrote:
On Tue, 22 Apr 2008 15:12:33 -0400, Fred Bloggs <nospam@xxxxxxxxxx>
wrote:



Paul E. Schoen wrote:

"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:v13s0452scpm9p95f6p53mm2jj2a0i2jpb@xxxxxxxxxx


On Tue, 22 Apr 2008 10:13:30 -0600, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:



What is the best method for dropping the voltage of a device that has
a
range of 8 to 12V powered by a car battery(so max, say 14V) to bring
it
within spec? The device uses from 100uA to 3mA depending on state.

---
View in Courier:

.9-14VDC>------+
. |
. [300]
. |
. +------>8-12V
. |
. [1N4742]
. |
.GND>----------+------>GND


The load has a maximum draw of 3 mA, so the 300 ohms will drop less
than 1 volt under worst conditions, for 8 volts out with battery at 9
volts (pretty much a dead battery). At the other end, with 14 volts
input, the zener clamps the output at 12 VDC maximum, with no more than
13 mW wasted in the resistor. It's a good idea to add a capacitor
across the zener.

Paul


I think 8V is the cranking voltage spec for most apps- definitely not
9V-has nothing to do with a dead battery.


A century ago, when I was designing ignition systems, "cold crank" was
4V ;-)

...Jim Thompson

The 1908 models? Had they even discovered lead-acid or was it carbon zinc
back then?

That must have been a 6 volt system, or possibly the voltage at the starter
terminals. Of course, in 1908 many (most/all?) cars had a manual crank, and
probably a magneto ignition, with no battery.

The lead-acid battery was invented in 1859
http://en.wikipedia.org/wiki/Lead-acid_battery. The first practical
electric starter was invented in 1911
http://en.wikipedia.org/wiki/Electric_starter. The first battery powered
ignition was introduced in 1910
http://en.wikipedia.org/wiki/Ignition_system.

A simple way to avoid the effect of low voltage during cranking, in this
case, would be to add a diode and a fairly large storage capacitor. 10,000
uF should give you at least 10 seconds before the capacitor voltage dropped
below 8 volts (starting from 12 V).

Paul


.

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