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

On Tue, 22 Apr 2008 12:28:07 -0700, 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

Y'know, I'd bookmark a link to a page or site that gives tips on how to
build power supplies that'll work from typical auto voltage, and protect
the small stuff on the output side from any mess on the input side.

Anyone got a url?
.

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  • Re: Best method for dropping supply voltage a volt or two for low power device?
    ... Paul E. Schoen wrote: ... for 8 volts out with battery at 9 volts ... I think 8V is the cranking voltage spec for most apps- definitely not ...
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  • Re: Best method for dropping supply voltage a volt or two for low power device?
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