Do you know of any compatible transistors to the ztx688? The only reason I ask is because my local electronic parts suppliers have an absolutely huge supply of transistors of seemingly all values but just not that one. I'm wondering if there is anything that would work in it's place. This has been typical with the MS. It seems to use many fairly rare parts that always need to be ordered from Digikey.

 

rg,

There's a multitude of transistors that would work in this application. You are looking for a high-gain transistor (hfe > 250), with a voltage rating above 15V, and a continuous current high enough for the idle valve you are using (2A?). A common choice is the TIP 120 series (TIP120, TIP121, TIP122 will all work fine) darlington transistor. They are rated for 60, 80, or 100V, and a continuous current of 5A, pulsed current of 8A, and should be readily available at your local electronics supplier for a buck apiece.

James has some instructions on using this transistor on his msnsextra site:

http://megasquirt.sourceforge.net/extra/pwmidle.html

 

I went out and looked for those numbers with no luck. Everyone around here sells parts from NTE electronics that use their own numbering system. That makes things hard sometimes. Fortunately I found out what their equivalent is. They have only 1 part that is supposed to be a replacement for all 3 numbers and it is 100v. The part number for anyone interested is NTE261

 

NTE-11(NPN ,TO92 case) should work if you want the replacement to be mounted on the board,it's rated at 5amps continuous/40V and has a dc gain(Hfe) of 340mA minimum. The other one you listed will have to be mounted on the case ,as it's in a different case package( but it is a more hefty transistor).

 

I'm mounting it on the end of the case and running wires to the board. I did figure out that it was too hefty and difficult to mount directly on it.

 

Can a 560 ohm resistor be used as a replacement for R19 on a V3 board? My local shops are out of 510's, 560 is the closest they have. Thanks.
-John

 

I've used 560 and 490, both without a problem.

 

lol, 2 minutes. Thanks Muy!

 

Im having trouble with this ckt and hoping someone can give advice/troubleshooting tips. heres exactly what I did.... its a V3 board. I removed Q20 and jumper R39 with a old component lead. I replaced R19 with a 510 ohm resistor but its a 1/2 watt size its all I could find locally..maybe thats a problem? I removed Q4 and replaced it with the ZTX688. I removed D8. I added a diode pn 1N4001 to pin 3 the collector of the ZTX688 and ran that to a 12 volt source. I have the fidle output on the DB37 going to 1 pin on the BAC and the other pin on the BAC goes to +12 volts switched with ignition. Ive double checked everything and theres no bad solder joints or anything obviously wrong. Any ideas would be greatly appreciated. Thanks in advance and gave a good holiday.

 

when you soldered the collector of the ztx688 to the 1n4001, did you make sure to leave that pin in the hole on the board? Also, which way is the band on the 1n4001 pointing... it should be on the +12v side of the diode, and not on the transistor side.

The 1/2 watt resistor won't cause a problem, so the problem is going to be either the transistor itself, or in the way you built the circuit, or the way the software is set up.

When my ztx689's come in (I'm going to try using those since they're rated for a higher voltage) I'll modify my v3 board the same way you did, and see what happens. IF you modded that circuit correctly, it'll be exactly the same as the 2.2 board's circuit, which works fine.

 

The collector or pin 3 is still in the board. The band from the diode is pointing toward the 12 volt source. Also noticed the ZTX688 has 2 flat sides I soldered it in with the slightly larger flat side the same orientation as the silk screen on the board. The other flat side is smaller but has the ZTX688 label on it. Maybe its backwards? Only thing I can think of. On the software Im lost can you help instruct me on what setting I need? Thanks again.

 

Ohh had the car running for like 5 seconds last night. Just need to get this ckt working to control air flow for a better idle and I think everything else is ready finally.

 

I think you have the hardware set up right, I'll post some software settings (if I didn't already in the FAQ) this weekend.

 

Thank you

 

I actually posted the settings I'm using in the FAQ already:

 

Ok thanks for posting that Muy its exactly how mine is currently set up. I have been thinking about this. I have the ckt built correctly and the software set correctly but it dosnt work...yet. In order for any DC ckt to work it needs power and ground. One pin from the BAC goes to a switch 12 volt power source (switched on with ignition). The other goes to the Fidle pin on the Db37. So the wire going to the fidle pin has to be grounded. How can the MS control how far this BAC opens when it only supplies ground?

 

It helps me to understand more than the basics,,thats why Im asking. Thanks,have a good Christmas

 

that's what PWM does... it switches the ground on and off really quickly, and varies how long the ground is on/off to vary the amt of current going to the bac valve.

 

Ok was hoping it stood for pulse width modulation. So that means I can set my DMM to continuity and stick the probes in the connector crank the engine and listen for it to beep beep beep real fast. I was just looking for a way to test it as everything is built wired and set properly. Thanks.

 

haha, it won't be able to keep up with it if you let it beep really fast...

However, if you have a DMM it should have a mode where it can measure duty cycle... so you could use that to see if it's doing what it should.

PWM DOES stand for Pulse-width-modulation, I was just describing how it works.