"John Larkin" wrote in message
...
On Wed, 6 Jun 2012 16:37:47 +0100, "Ian Field"
wrote:


"John Larkin" wrote in
message
. ..
On Tue, 5 Jun 2012 20:35:23 +0100, "Ian Field"
wrote:


"John Larkin" wrote in message
m...
On Tue, 5 Jun 2012 19:00:00 +0100, "Ian Field"
wrote:


"John Larkin" wrote in message
news:6c7qs797vd7f771hct3n65emcf93iqaktb@4ax. com...
On Mon, 4 Jun 2012 18:41:30 +0100, "Ian Field"
wrote:

Its a 2-terminal relay for a motorcycle, after adding an earth lead
so
I
could fit a car type; I then discovered that the intermittent
operation
was
due to a corroded connector block hidden under the tank.

After breaking up the epoxy potting I found the old relay was
blameless.

The circuit could probably be adapted for 6V with a logic level
MOSFET -
but
would need 2 in parallel to handle twice the current (don't forget
filament
cold resistance surge!).


Now *that* is an application for a 555!


Whoever designed that may have had reservations about putting a 555
into
the
automotive envirenment - a quick google shows the bipolar 555 as
having
a
16V Vcc limit & 18V for the 7555, the BC547B is good for about 45V and
the
60V MOSFET is spec'ed for the automotive environment according to the
datasheet.

Note however the 1kV rating of the 1N4007 they've used to isolate the
reservoir from the MOSFET is somewhat superfluous, it only has to
withstand
the reservoir voltage as any externally applied reverse voltage would
be
clamped by the MOSFET body diode (maybe they got a better price break
on
quantity 4007s).


The fet gate is only rated for 20 volts, and the diode/cap will peak
detect and store any load dump spikes. So some sort of transzorb thing
(with a depletion mosfet?!) might be prudent.


I'm curious to see a schematic of a depletion MOSFET in that
application?

At first blush I'm thinking maybe an oscillator driving a negative O/P
diode
pump to generate the pinch off bias.

Wouldn't do much for production costs though!



No, I meant that you could use a depletion fet in the load-dump
protection part, something like this:

https://dl.dropbox.com/u/53724080/Circuits/Blinker.JPG


The bean-counters at the firm that made it would freak out if the
engineers
showed them that!


Yeah, it makes better financial sense to sell something that's a few
cents cheaper, and let the buyers deal with the blown fet gates.


The relay was date stamped 98 - which is older than the bike I took it off.
So it had already been salvaged from elsewhere and fitted to the bike as
replacement for whatever went brfore it.

As I stated in my original post, I wrongly blamed the relay for intermittent
operation which turned out to be a corroded connector hidden under the tank
(in my defence; it was much easier to swap the relay than take the tank
off).

At the end of the day: the astable they've used is cheaper and more reliable
than a 555 and all the extra gubbins to keep it safe in the automotive
environment.