Sorry Colin, yet again I respectfully disagree :-).
Diesel engines often use this method because they don't have coils. They are
common on marine installations and many trucks and go for years without
problem. Even the Thielert diesel in the demonstration Cessnas use them.
They rely on the mass of the steel in each tooth to disturb a magnetic
field....they are not optical, dirt is irrelevant (especially in our
application) and air gap is not critical although the smaller, within
limits, the better. I think you might be confusing them with Hall Effect
sensors that rely on a magnet being set in the flywheel to create a current
in the sender and which are often used on modern cars for electronic timing
purposes. A short in your tacho can take your primary to earth irrespective
of the existence of diodes and I am sure there are automotive examples that
you may have not observed. Tiny Tachs are notoriously unreliable, difficult
to calibrate, do not read accurately from suppression leads and can cause
radio interference by transporting a sample of the ignition voltage through
the firewall. Check the archives on this subject. Finally WW recommends
tooth counters and that is good enough for me. I wouldn't bank on a lifetime
of flying on a couple of two dollar diodes.
John Martindale
29 Jane Circuit
TOORMINA NSW 2452
AUSTRALIA
phone: 61 2 66584767 (H)
61 2 66869075 (W)
mobile: 0403 049990
email: johnja...@optusnet.com.au
web: www.members.optusnet.com.au/johnjanet/Martindale.htm
----- Original Message -----
From: "Colin Rainey" <brokerpi...@bellsouth.net>
To: "KRnet" <kr...@mylist.net>
Sent: Thursday, October 12, 2006 1:19 AM
Subject: KR> Tach drives
> John said:
> "use a tooth counter sender off the ring gear"
>
>>From observations that is the hardest style to have as a primary or
> secondary due to the bracketing etc... to keep the air gap correct. Engine
> vibration tends to knock that one out of reading. Volvo and Chrysler both
> have used that method as an engine speed sensor, and wear on the ring gear
> from starters, dirt and such effect it. The best solution if not the coil
> attached tach, is the Tiny Tach that uses an inductive pickup off of one
> plug wire. We used these pickups with our Sun Scopes for years with no
> trouble. It clips around the plugwire and reads the pulses to a digital
> display. Very simple and reliable. In practice I have not seen any tach
> failures, and those reported that did fail, did not cause a total ignition
> failure, just quit reading. Diodes are a couple of bucks each, so applying
> 2
> in each line insures against total failure, and should last a lifetime of
> flying anyhow.
>
> Colin Rainey
> Independent Loan Officer
> Branch 2375
> Apex Mortgage Company
> 386.615.3388 Home Office
> 407.739.0834 Cell
> 407.557.3260 Fax
> brokerpi...@bellsouth.net
>
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