[Diy_efi] Re: Diy_efi Digest, Vol 2, Issue 30

[Bill Washington]

J,
    Schematic 4 has the advantage that you can adjust the trigger 
voltage, but note the points repeated below.

1. You reversed Diode D1, it was correct, and is now wrong. You should 
have reversed Diode D2, which remains wrong.
Diode D2 is essential because when you switch off the current to the 
coil the driven side (connected to the transistor) will try to go 
negative in voltage, the function of  the diode is to clamp it at 0V. 
Diode D1 is not essential.

2. The circuit will work opposite to description IE:
Condition A: #1>~#2 => #3 = 8V => Relay contacts closed
Condition B: #1<~#2 => #3 = 0V => Relay contacts open

3. R3 adds hysteresis, therefore the switching point will not be when #1=#2.

4. General description of operation #3 will switch to 8V (ie relay energised, contacts closed) when #1 rises 
above (1.01 x voltage at U1 pin 4)and remain there until #1 drops below (8V - 1.01 x(8V - voltage at U1 pin 4), at which time #3 will switch back to 0V (ie relay denergised, contacts open.

5. To invert the sense of the operation, IE:
Condition A: #1>~#2 => Relay contacts open
Condition B: #1<~#2 => Relay contacts closed
You must change to a PNP transistor or P Channel FET, BUT then ensure that the 12V supply is switched off with the ignition! 

Regards
Bill

>
>------------------------------
>
>Date: Tue, 18 Feb 2003 23:15:16 -0500
From: "Toyota Supra" <turbosupramk3 at hotmail.com>
>To: diy_efi at diy-efi.org
>Subject: Re: [Diy_efi] schematic critique
>Message-ID: <BAY2-F86ihe3lDpuGTM000313dd at hotmail.com>
>Content-Type: text/plain; format=flowed
>MIME-Version: 1.0
>Precedence: list
>Reply-To: List for general do-it-yourself EFI talk <diy_efi at diy-efi.org>
>Message: 7
>
>hi bill!
>
>ok, i deleted R4 and C3, i ran the collector of the transistor to 12vDC, (i 
>don't know if it'll require the 8v or the 12v, but i chose 12v because it 
>will be driving a 30amp bosch automotive relay, hopefully that was the right 
>decision) . i changed R5 to 1k
>
>now, here is where i am not sure i understand, i have D1 and D2, one allows 
>negative to flow to positive, and the other, positive to flow to negative. 
>i'm not sure which type of kickback the relay will have, but they are on the 
>positive side of the coil? i am guessing i only need 1 diode instead of two, 
>but let me know.
>
>i also have two renditions, links seen here
>
>with the 47k potentiometer
>http://68.7.7.84:8080/schematics/schematic4.jpg
>http://68.7.7.84:8080/schematics/schematic4.psd
>
>without the 47k potentiometer
>http://68.7.7.84:8080/schematics/schematic5.jpg
>http://68.7.7.84:8080/schematics/schematic5.psd
>
>
>it sounds to me, as if you would like me to stick with the "schematic5" 
>rendition, but i wanted to put up both options and get your opinion for 
>sure, number 4 might be used to take a voltage signal and drop it to the 
>desired 3.2vDC reference signal???? .... thanks soooo much for the guidance 
>throughout this
>
>-j
>
>
>
>
>
>
>  
>



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