On Sep 6, 1:14*pm, "Dave Liquorice"
wrote:
On Sun, 06 Sep 2009 04:36:03 +0100, John Rumm wrote:
One solution you could try:

* * * * * * * * R1 * *LED1
* * * * * * * -\/\/\--|^---
* * * * * * *| * * * * * * *|
* * * * * * *| *D1 *D2 *D3 *| * */
Live in *------|--|--|------o o---------- to lamp
* * * * * * *| * * * * * * *| * sw1
* * * * * * *| * * *D4 * * *|
* * * * * * * -----|-------

Interesting idea. An improvement might be to use back to back zeners
in place of D1 to 4.

On a quick search I can only find 5v1 ones at 5W dissipation so the
upper limit on the lamp load would be 230W (ish) and you'd have 10W
of heat to get rid of at 230W load. Now if there are some 2v zeners
out there at 2W dissipation...

filament lamps eat around 8x rated current at switch on, but diodes
dont tolerate these overloads. So your real life lamp limit for a 5v
5w 1A zener will be 1/8A apx, or around 30w.


NT