All material on this page is Copyright, all rights reserved. Unauthorised
commercial use prohibited. (Personal use is OK). All these ideas are unproven. Any design should be verified
by a qualified, competent engineer.
If you don't understand 100% how these things work you should not be using
them on your car!!
Rev Limiter Circuit:
In 1992 I designed & built a rev limiter
that can be connected to any conventional Kettering ignition (but has only
ever been tested on a points ignition).
It's based on an LM2917 and an SCR, which has the very
nice side effect of being impossible to generate any mis-timed sparks,
a highly desirable attribute for high output rotary engines. When
the limit is reached by accelerating to the rev limit, the transition
is smooth and causes no backfiring. (BUT it has the potential to damage the exhaust
from a backfire if the engine is downshifted and consequently over-revs; due to
unburned air/fuel mixture coming from the engine that may be ignited by
combustion when the RPMs fall below the limit).
The heart of the circuit is a National Semiconductor LM2917
Tachometer IC which detects the engine RPM and when it exceeds a given
level the 'load' SCRs are turned on which reduces/eliminates
the spark energy and hence the engine power.
The input signal comes via the 10k, 22k resistors and C1 (.02u). This forms
a signal shaping network that protects the IC input and filters out any
high frequency signals (eg coil ringing). This filtered input is compared
against a 0.6volt reference formed by D1 and it's 10k resistor. This further
serves to minimise false triggering.
Now there is a signal which pretty much represents a cleaned up version
of what's happening at the points. This passes into a "Charge pump"
with C2, C3 and the 100k resistor. This converts the frequency of the input
signal (i.e. the engine RPM) into a voltage level.
Up until this point everything is pretty much how a standard tachometer
From the charge pump, this voltage level is compared with the preset voltage
level (the 50k "Set rev limit" potentiometer - the 15k preset
associated with this is optional, and sets the absolute maximum rev limit.
This is handy if the rev limit is changed depending on the situation).
When the tachometer voltage exceeds the limit voltage (i.e. the ENGINE RPM
IS OVER THE LIMIT), the transistor in the tachometer IC turns on.
When this happens, the 2N2905 is turned on (the 9k, 220ohm and 18k resistors
simply limit the current in the 2905), which in turn operates the C122E
When the C122E turns on, it effectively "shorts out"
the points. The 4.7ohm resistor IS REQUIRED. Basically what happens is
when the points open and the SCR is turned on the 4.7 ohm resistor is across
the points. This causes the magnetic field inside the coil to collapse
only marginally, which in turn generates a much weaker spark than usual.
However, there is still enough signal for the IC to be able to sense the
As a bonus because of the way SCRs work, when the SCR
control signal is removed, the SCR will remain turned on until the points
Because it can't turn off by itself, it cannot generate any mis-timed sparks.
This is a very important advantage.
Furthermore the SCR can be turned on at any time because sparks are only
made when the points open, not close.
Please note when using this circuit that it was only ever
built as a prototype, which does work properly, however use at your own risk.
It did not seem to be able to repeatably restrict RPM (eg when used on
different days) to an accuracy of better than 500rpm. I have not investigated
exactly why this happens or any possible fixes - and as my electronics
knowledge has slipped over the last few years do not intend to.
I think it may be due to the wildly varying voltages in the operation of
the ignition system and the relative difficulty of making exact measurements
(for the triggering of the timing circuit), there is probably also some thermal
This design was an original creation based on application
notes for the LM2917 and my own ideas.
Further reading and acknowledgements:
* The best reference for further reading is the Bosch automotive
electrical handbook, which goes into detail about how ignition systems work.
* National Semiconductor LM2917 datasheet
Other relevant reading at Craig's Rotary Page (Please go via the INDEX
* 30 LED air-fuel meter
Other relevant sites on the Internet (Please go via the LINKS
* National Semiconductor/Bosch websites; Possibly Amazon for the Bosch book
(You'll have to find these things; I have no links to these).
[TOP OF THIS PAGE][TOP
PAGE OF SITE]
This page last updated 17/3/2001
24/3/2001 - Renamed page from PG01.HTM to PG13.HTM, all images renamed to
17/3/2001 - Changed more information section
16/3/2001 - Converted all text to new standard (Headings as Heading1, Some
sub-headings (e.g. tables) as 14 point normal bold italic, Most text as Normal,
Internal page links at top not all uppercase)
11/3/2001 - Minor content update (Spelling mistakes, tidy up). Removed Word
document version of file to avoid virus risk. PG01_02B.GIF edited to change
author to Craig's Rotary Page. Changed from Netscape to
FrontPage. Background image
changed to PG00_02B.JPG
27/11/1997 - Previous known update (May have been some before this)
This site is online at tripod (The large images on many pages don't
work due 20MB site limit)
Please do not 'rip' the site (due to bandwith restrictions)
I have a number of other sites. Visit the main
index page Please note this information is not definitive check the disclaimer