Input Buffer Circuit

[Dan N]

Can someone give me a hint on the best way to implement a buffer circuit
between the ignition switch on a car and a 2.8 volt input? Should I use a
voltage divider to get closer to 2.8 volts and then clip it with a 2.7
zener?

Thanks

Dan

 

[Phil Allison]

"Dan N"

Can someone give me a hint on the best way to implement a buffer circuit
between the ignition switch on a car and a 2.8 volt input? Should I use a
voltage divider to get closer to 2.8 volts and then clip it with a 2.7
zener?

** Definitely not !!!

2.7 volt zeners are soggy as hell.

Use an LM317 regulator IC.

Connect a 120 ohm resistor from the output to the "adjust" terminal and a
150 ohms resistor from there to ground or negative.

If the current is more than 100mA, be sure to use a heatsink for the IC.



....... Phil

 

[Dan N]

On Wed, 23 Jan 2008 14:53:46 +1100, Phil Allison wrote:

** Definitely not !!!

2.7 volt zeners are soggy as hell.

Thanks, makes sense, I knew there would be a better way.

Dan

 

[dmm]

On 23 Jan 2008 03:31:03 GMT, Dan N <dan@localhost.net> wrote:

Can someone give me a hint on the best way to implement a buffer circuit
between the ignition switch on a car and a 2.8 volt input? Should I use a
voltage divider to get closer to 2.8 volts and then clip it with a 2.7
zener?

Thanks

Dan

I'm assuming the 2.8V input is a logic level input.
Use a voltage divider with a sufficient input resistance such that it will adequately
current limit any spikes from the ignition. This is intended to limit the current through the
protection diodes on the logic input. Also use two series capacitors (say 10nF/50V or 100V)
in parallel with the resistor to ground. This will provide some filtering for spikes, and will
enable the circuit to continue filtering in case one of the capacitors goes short circuit.
If you are really worried about protecting the input from high energy spikes, then
use a transient voltage suppressor, not a zener. You should also be aware that the
voltage in a car can vary quite a bit, from 9V to 16V, due to the varying loads on
the supply, so your input must take this into account.

Vbat = 13.2V
Input resistor = 39k
resistor to gnd = 10k

dmm