Probably because the EC3 already have a 1K resistor on the board to ground. So with the 18K in series you have a 18:1 attenuation.
I questioned Tracy about adding the 1K resistor (in addition to the two zeners) but he told me to do it. That's now 500 ohms across the output from the CAS sensor.
There's already a pretty elaborate signal conditioning circuit on the EC3 board.
I guess in the end it's all about getting a reliable clean timing signal to the CPU interrupt input. I suspect the CAS output voltage varies significantly with RPM.
Undoubtedly there are many different circuits that could accomplish that. Some better than others for specific non-factory installations.
Finn
On 6/23/2020 7:16 AM, Bobby J. Hughes bhughes@qnsi.net wrote:
Dennis H. and I used that circuit with the EC2. It’s external in the wiring near the EC2 connector. It’s not compatible with the EC3. Engine would not start.
Bobby
Sent from my iPad
> On Jun 22, 2020, at 9:00 PM, Stephen Izett stephen.izett@gmail.com <flyrotary@lancaironline.net> wrote:
>
> I had this circuit from somewhere
> <Renesis CAS Zener Circuit.pdf>
>
> Steve Izett
>
>> On 23 Jun 2020, at 9:22 am, Finn Lassen finn.lassen@verizon.net <flyrotary@lancaironline.net> wrote:
>>
>> Just for the record.
>> <ommkelgblomchfkc.jpg>
>> The 1K resistor and the two 2.7V zener diodes are the CAS input clamping circuit.
>>
>> The rest is for redundant +5V to control panel (mixture voltage being the critical one).
>> Vcc3 is J2 pin1 (+5V to control panel).
>> I just used BAX16 diodes because I had them. Any 100mA (16V reverse voltage or more) diode will do.
>> LM78L05 flat side down. Left pin is output. Center pin is ground and right pin is input.
>>
>> <jjfkkebmdbnhneko.jpg>
>> If you want to secure components against vibration make sure you use Natural Cure Sealant (non -acidic)!
>> I'm using Dowsil 737 RTV sealant.
>> The auto parts store RTV is acidic and you don't want your circuit board damaged (etched) by that.
>>
>> Finn
>>
>>> On 6/17/2020 6:57 PM, Finn Lassen finn.lassen@verizon.net wrote:
>>> In addition to the CAS input clamping circuit, Tracy also recommends shorting pins 1 and 3 of D18. The voltage drop across the diode to the PCM module causes slightly lower mixture and program voltage than the software was programmed for.
>>>
>>> However, if you are powering A and B controllers separately (e.g. separate fuses or busses), I have a better idea.
>>> 1) Remove the D18 diodes (just below C45 which they feed).
>>> 2) Add a LM78L05 (or similar) voltage regulator. After each (A and B) of the power (L1 and L2) input coils (+ on the big C20 and C21 capacitors) feed the input of the LM78L05 with a diode. Output of it goes to J2 pin 2 (or + on C45).
>>>
>>> Same principle as powering the PCM from either of the 5V regulators on the EC3 board, but the 0.7V diode drop now happens before the regulator, not after the 5V.
>>>
>>> CAS input clamping circuit:
>>> 1) A 1K resistor between J2-5 and J2-15 (CAS input and ground).
>>> 2) Two 2.7V zener diodes in series, arrows or striped ends (anodes) connected together. The non-striped ends (cathodes) to J2-5 and J2-15 respectively (CAS input and ground). (Basically in parallel with the 1K resistor). If you got the polarity of the zener diodes correct you should see the signal from the CAS sensor clamped to no more than 0.7 + 2.7 = +-3.4 volts on an oscilloscope.
>>>
>>> I now have two bad chips (does not respond to reading or programming attempts -- do "work" in the EC3). The chip pins used to program the chip go to the Mode switch on the PCM. J2 pins 22 and 24. One might consider protection diodes for those pins. Oddly the pins continue working for Mode switch input, just not for re-programming the chip.
>>>
>>> The EC3 board was designed for in-circuit chip re-programming (set mode switch to zero or disconnect J2). Vpp/MasterClear are on J2 pin 11 (for A) and pin 1 (for B). PGC on pin 22 and PGD on pin 24. However, Tracy found that the combined capacitance of the programming pins of two chips made in circuit programming unreliable. I wonder if it might work if one could control the speed with which it communicates with the chip?
>>>
>>> Finn
>>>
>>>
>>> --
>>> Homepage: http://www.flyrotary.com/
>>> Archive and UnSub: http://mail.lancaironline.net:81/lists/flyrotary/List.html
>>>
>>
>>
>
> --
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