T o d d P a t t i s t wrote in
:

Marc Ramsey wrote:

The 302 has an internal capacitor across the power supply lines, the
Volkslogger does not. Volkslogger owners can solve this problem by
adding an external capacitor, as is suggested in the manual...

IIRC, the Volkslogger recommended capacitor size was 22,000
microfarads. That's a fairly large cap (physically and
electrically), and it holds the VL on for only a second or
so. It's enough to switch batteries, but don't delay. It
also can cause problems on initial power-on when it first
charges up if your fuse is not large enough. The inrush
current is pretty high.

The inrush current issue can probably be solved by a circuit thus:


-------batteries---------------------------------logger---
| -ve and +ve | |
| switches | |
| and fuse ------------- |
| | | |
| ---- _ |
| /\ | | |
| Diode / \ | | Resistor |
| ---- |_| R |
| | | |
| ------------- |
| | |
| Capacitor --- +ve |
| 22mF --- -ve |
| | |
----------------------------------------------------------


When the switch is first closed the voltage across the capacitor
is zero and the diode is reverse biassed. Hence there is negligible
current through the diode and current flow into the capacitor
is limited to a peak of V/R amps.

After the capacitor is fully charged and the switch is opened,
current will flow through the forward biassed diode into the
logger with a voltage drop of ~0.6V (silicon diode) or ~0.3V
(schottkydiode).

Choose a diode with:
- reverse voltage greater than the battery voltage
- forward current greater than the peak logger current

Choose the resistor to limit the inrush peak current flow.