Larry W4CSC wrote:
Jeff wrote in :


I'm confused. Perhaps this is why I left hardware engineering for
software. I thought the purpose of a diode was to pass current one
way, and block it the other. Did this fail because the passed current
was too high, or because the blocked voltage was too high?


When he hooked it up backwards, the diode was forward biased straight
across the battery's terminals. Current went through the roof, welding the
junction to a short which, microseconds later, blew the fuse. Once welded
to a short, unless you can really melt the mounting, it stays a
short....across the coil, not in series with it. It's a short both ways at
that point.

Thanks. The aspect that eluded me was that the dead short to ground
would pass "infinite" current until something popped, like the
breakers. Most circuits with diodes, such as rectifiers, have some
resistance implied elsewhere.




This diode's function is to short the relay's coil when the coil's field
collapses, producing inductive kick which forward biases the diode for a
few milliseconds each time power is removed from the coil. Otherwise, the
inductive kick, which could be several hundred volts for a few microseconds
like the spark from an ignition coil, could destroy the control electronics
or switch contacts which arc each time it is de-energized. With the diode
in place, inductive kick produces .6v pulses, the forward bias voltage of
the diode as it sinks the kick's power current pulse.

Yup. Subtle things like that I tend to remember. Its only the
obvious that I forget.

....
Why can't everything just be digital?


It is digital...(c; 1 = relay on.....0 = relay off...but it's not
hexidecimal..making it harder for a software man to comprehend...hee hee.

I beg your pardon. I'm software from the "old school," as in front
panel lights and flipping switches to install bootstraps and debug. I
still have the front panel from the first computer I owned - a Data
General Nova 1200.
http://ed-thelen.org/comp-hist/dg-nova.html

And since I came from the "mini" world, I learned Octal long before Hex.



While I'm on the subject, since I have twin engines, some of my
instruments, such as the fuel gauge are fed by the key switch from one
engine. Is there a way to hook up 2 diodes to allow either feed to
work/ Radio Shack part numbers please ;-)



Any 1 amp, 50V piv or higher rectifier will work.....for this OR
circuit...(c;

Hook the anodes of the diodes, one to each power switch on each engine.
Hook the cathodes (banded end) together to the common power + lead to each
fuel guage. If either engine is turned on, its diode will conduct turning
on both fuel guages. (classic OR gate - DDL logic!)

engine 1----------------||---------\
\
\
engine 2----------------||------------\-----------fuel guage +

Circuit may be too simple for mainframe engineers...(c;

Thanks. It looks too obvious for me.

Ken mentioned that the voltage drop could affect the gauges. I can
see how the cheap fuel gauge might have a problem but it made me
wonder if the water temp gauges would be affected. Not that I would
need the diodes for them, but the alternator output can vary from 12
something (turned off) to 14.5, depending on what's going on in the
regulator's little mind. I have to believe that such fluctuations
don't make much of a difference in the temp readout, but I couldn't
find a spec on the Teleflex site that indicated they were stable over
a reasonable voltage range. Any thoughts?

Roger,
You can do that if you want to, but diodes work better. You may still
get a spike on the line with the RC setup, but that won't matter to
anything but the electronics. The RC set does what it does by thying to
limit the rate of rise when the switch opens, but the diode makes the
power from the widing stay in the winding.

That is why little DIN (automotive default these days) relays are marked
87 for signal and 31 for ground.

Newer cars have a lot of this stuff going on.

Matt Colie

Roger Long wrote:
Nope. It was a dead short. I'm putting in the 10 ohm / .01 mfd
suppressors instead.

Interesting point. I can wire the diodes on the outside of my
controller just as easily as the snubber. If I do see any evidence of
spikes, I'll do that.

For future reference, do you know which diodes to use? I foolishly
tossed the ones I cut out of the sockets.

It's unlikely I'll see a problem though. The water will probably only
get to the high switch in our boat if the stuffing box hose splits.
If the boat is unattended, the minimal electronics we have will be
off. Mostly, this system will be operated by picking up one of the
float switches when checking the bilge. The most used feature of it
will be not having to hang down there holding the switch until the
bilge empties. Just being able to flip it up and forget it will be
worth all the effort I put into it. Operating in that mode, the relay
never engages.

--

Roger Long



"Matt Colie" wrote in message
...
Roger,
You can do that if you want to, but diodes work better. You may
still get a spike on the line with the RC setup, but that won't
matter to anything but the electronics. The RC set does what it
does by thying to limit the rate of rise when the switch opens, but
the diode makes the power from the widing stay in the winding.

That is why little DIN (automotive default these days) relays are
marked 87 for signal and 31 for ground.

Newer cars have a lot of this stuff going on.

Matt Colie

Roger Long wrote:
Nope. It was a dead short. I'm putting in the 10 ohm / .01 mfd
suppressors instead.

Comment below:
"Jeff" wrote in message
...
Larry W4CSC wrote:
Jeff wrote in :

chunks of stuff snipped for brevity

Ken mentioned that the voltage drop could affect the gauges. I can
see how the cheap fuel gauge might have a problem but it made me
wonder if the water temp gauges would be affected. Not that I would
need the diodes for them, but the alternator output can vary from 12
something (turned off) to 14.5, depending on what's going on in the
regulator's little mind. I have to believe that such fluctuations
don't make much of a difference in the temp readout, but I couldn't
find a spec on the Teleflex site that indicated they were stable over
a reasonable voltage range. Any thoughts?

I haven't seen a schematic for boat instrument systems but will make a
comment on how automotive instrument systems used to work. (They may still
work this way, but I haven't been around this stuff for years) Automotive
instrument systems (used to) receive their 12 V. + feed from a simple
voltage regulator mounted in the dashboard wiring system. This prevented
the varying voltage coming from the alternator from causing erratic readings
on the instruments. Automotive gauges at the time were thermal in operation
(because they were much cheaper to build that way), and so were sensitive to
voltage variation. You could tell good instruments (usually aftermarket)
from these thermal types when you turned the ignition switch on. Good ones
would snap to the indicated reading, the thermal ones would take a few
seconds to rise up to the correct indication. Gas gauges would often not
give a true reading of tank level until you started the motor and the
alternator kicked in. Again, I'm not sure if boat gauges are wired this
way.
--
Ken Heaton, Cape Breton Island, Canada
kenheaton AT ess wye dee DOT eastlink DOT ca

"Ken Heaton" wrote in
news:J3Mle.13142$HI.6104@edtnps84:

Gas gauges would often not
give a true reading of tank level until you started the motor and the
alternator kicked in. Again, I'm not sure if boat gauges are wired this
way.


Of course, we all know how DEAD ACCURATE those wonderful BOAT gas guages
monitor the fuel supply that can strand you miles from nowhere.

When my Sea Ray read 1/2 full, it meant you used 6.5 of the 25 gallon tank.
When it read empty, you had used 9.5 gallons of the 25 gallon tank. When
it read below empty, you simply opened the "trunk" and leaned down into the
bilge so you could see the fuel level in the stern end of the cheap
polyethelene tank supported by two little plastic brackets digging into the
milk jug material 25 gallons of fuel were waiting in to blow you all to
hell if it leaked. It's nice-looking Sea Ray gas guage was damned near
useless....

The tach seemed to work....