Rev Limiter Circuit

Posted on Feb 5, 2014

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 fro

Rev Limiter Circuit
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m 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. 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...

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