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



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.

.

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