power supply

[Darrell Norquay]

At 03:35 PM 7/16/96 +0200, Krister wrote:

>I have made a simple one like this (hope it is readable):
> 
>  --L---D-------[7805]----- +5V
>      |   |  |         |  |
>      C   Z  EC        C  Z
>      |   |  |         |  |
>      -   -  -         -  -
>
>The first L and C should filter most high frequencies, the diode and the
>large EC capasitor prevents sudden drops in voltage, and the first zener 
>( I have 22V) should ease up voltage spikes. The last zener is just in case, 
>and it actually sits in the CPU board along with some tantal. capacitors. 

This circuit may suffer from several problems.  First of all, you have no
means to limit the current into the first Zener.  If a spike comes along
with sufficient voltage to cause the zener to conduct,, it will likely self
destruct immediately.  Zeners usually fail shorted, and since you have no
fuse or other device to protect the circuit it will likely make big smoke!
Use a low value wirewound resistor in series with the input, along with a
fuse.  The resistor value should be chosen to drop around 3V at full supply
current, and it should have a power rating around double it's dissipation at
full operating current.  This Zener should also be a 5W device.  You could
also use a MOV (Metal Oxide Varistor) or Tranzorb type device, they are
usually rated at higher current than standard Zener diodes.

If you use a resistor in place of the inductor, you will still have good
filtering, and a resistor is a lot cheaper than an inductor.  It may also be
a good idea to place a reverse biased diode from + to ground to prevent
negative spikes from getting through, although the Zener will also do this
to some extent.

The second zener is OK for a Fail-Safe, don't limit it's current as you want
it to fail in the shorted mode.  This may protect your expensive processor
chip from damage in case of a catastrophic failure.  Use at least a 5.6 V
unit, though, to prevent drawing excessive current at normal operating voltage.

Even with the power supply filtered as you have shown, you may still have
problems with ignition spikes coming in on the power leads, the ground lead,
and sensor inputs.  This may cause random processor crashes.  The solution
to this is to run all input and output wires through feedthrough capacitors,
grounded directly to a full metal case.  Mount the processor away from the
engine, and run power and ground leads directly back to the battery.


regards
dn
dnorquay at awinc.com