using TIP42 PNP paralleled

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TheEquineFencer

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I'm really new at working with elecronics so please bear with me. This is my first project and I need some help. I've built an O2 monitor that I want to drive a set of solenoids,(injectors) from it. I want to use individual TIP42 transistors for the switching. I have the thing working now driving one injector but want to drive a total of 8 from the same source. I'm using a 4N25Optoisolator driven from the LED on the monitor to turn on/off the TIP42. How do you parallel the 8 TIP42 Transistors so they will all work? Do they need some sort of isolation? At the present time I have the Terminal #5 on the 4N25 from the Voltage source,12VDC, also on the TIP42 Emiiter, going into the 4n25, coming out Terminal #4 going to the base of the TIP42, ahead of the resistor, to switch it off when the 4N25 is turned on from the O2 monitor. I have a 1663 Ohm resistor from the Base on the TIP42 to ground. The injector is connected to the Collector and to ground. Basicily I have the load on all the time and the Opto cuts it off by making the base more (+) than (-). Thanks, Floyd.
 
hi Floyd,
The way you describe the TIP42 connections to the solenoids suggests that the TIP42 are not actually in 'parallel'

Does each TIP42 drive its own solenoid?

Is it the TIP42 'base' connections you want to drive from a single source?

Do you have a drawing you could post?

Eric
 
Shoot me a e-mail at TheEquineFencer@AOL.com and I can send you a Word Doc in rtf format and you can see what I have now, I've not figured out how to post pics in here yet. but what I want to do is use the same voltage that is applied to the Base to turn the TIP42 off, to each of the (8) Tip42's, all will have the same source supplying the emitter, and all will share the same common ground.
 

hi,
Got your PM. >> Yes, each TIP42 will have it's on solenoid, all driven from the input voltage from the Opto,4n25.

As you are using TIP medium power transistors I expect that the solenoids draw upwards of 300mA ??
[whats the spec of the solenoids and the voltage etc??]

Based on that assumption I would say the opto will not be able to supply enough base current to switch 8 TIPs/Solenoids,
it will need an additional transistor between the opto and the 8 TIP bases.

The way to post a pic:
Scroll down the posting page, look for the grey 'Manage Attachments' button.
Left click on it.
A dialog box should appear, use the Browse button to locate the pic on your PC, then click the Upload button,
wait for the upload to finish.
Click 'close this window' [top right corner]

The dialog box lists the file types and sizes, I prefer a GIF, PDF or JPG type file.

Hope this helps, give it a go, posting a pic will enable us to offer better help.

Eric
 
I hope this works, I printed the diagram, then scanned to get it into a jpg. format. The injector coils are 14 Ohms. I'm thinking the bulk of the load will be on the transistor. I see what you mean about the Opto not being able to supply enough for the TIP42, I thought that might be a problem, I'm really new at this though.
 

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hi,
Will you confirm that the attached pic is how you have the solenoid driver connected?

The voltages on your pic suggest that you are not fully switching the TIP, are the solenoids a proportional type??

Which pin of the LM3914 is LED11 connected to?

Eric


EDIT: Drawing corrected, deleted +5V connection, added 10K
 
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seems like a lot of transistors if they're all being driven from the same source.
why not just have the tip drive one higher power transistor connected to all 8 relays? or did I miss something?
edit: oops, I see 14ohms.. 6+amps from one transistor. as he said above I don't think the opto will drive 8 transistors, you'll have to drive them from the transistor your using there..

as to your original question, since the collectors will each be going to a seperate load you do not need any isolation. If they were all truly parallel, say 5 transistors capable of 1 amp each running a 4 amp load you would need a small value resistor on each forcing them to share the load equally. if current was higher in one transistor the drop across that resistor would be greater lowering the current yadda yadda lol. trick used in power supplies. such as this:
**broken link removed**
 
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I don't see a protection diode across the injector solenoid to stop the high voltage it generates when it turns off from destroying the TIP42 transistor.

I am surprised that a '12V" injector works when it gets only 5.4V.
The TIP42 must get hot since it is not turning on completely.

The TIP42 is not turning on completely because the 1.6k resistor's value is too high. But if its value is lower then the opto might not work. The resistor should be about 270 ohms.
A TIP125 darlington PNP would work fine with the 1.6k ohm resistor to turn it on completely.
With darlington transistors, the 1.6k resistor sould be changed to 12k so that one opto can drive 8 of them in parallel.
 
Tip 42

I'm in class right now so this is from memory. In your drawing it is connected as you show, but when I checked the voltage going to the Opto from the LM3914 side it has 12VDC not 4.7VDC as in your drawing. As for if it's proportional,I have no idea what you mean. I connected my Fluke 87 Multi meter to the solenoid in series and tested it on a 12VDC battery reading ampereage. The result was the meter initaliy went OL then came back to .400mA and continued to drop to around .014 when it was saturated and started heating up.
 


This morning around 4am EST, I tried to lower the resistance of the
1663 Ohm, to raise the voltage level turning on the TIP42 with loadon, the voltage did go up but when I applied the voltage from the OPTO by adjusting the O2 level to where the last LED,#10 came on the thing started buzzing, like the injector was turning on and dropping out. My guess is the resistance was wrong for the TIP42 to stay turned on were wrong for the TIP42. What I did find when I unhooked the load and ran the collector open, just with my meter connected, the transistor was either on all the way, OPTO LED off, or off all the way when the OPTO LED on.
 
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hi Floyd,

>> What I did find when I unhooked the load and ran the collector open, just with my meter connected, the transistor was either on all the way, OPTO LED off, or off all the way when the OPTO LED on.

This is what you would expect to happen, basically the collector current thru the TIP is transistor is the base current multiplied by the transistor gain.

So when you removed the low resistance solenoid and inserted the high resistance meter, the lower current thru the TIP would switch off or saturate the TIP.

Example for explanation,
IF. the base current is say 10mA and the transistor gain is 40 then the collector current would be about 0.01*40= 400mA,
with a load resistance value of 14R, the voltage drop across the load would be about 0.4 * 14 = 5,6V [ which you measured]

If now you replace the 14R load with a higher resistor value of say 30R then you would get a voltage of 0.4 * 30 = 12V.
across the load [solenoid]

In order to get the same 12V across the 14R solenoid you will have to increase the base current to about 25mA. 0.25 * 40 = 1Amp.

Its important to note, that as the collector current is increased the transistor gain will fall.

Did you design the circuit?

Eric
 
The only part of it I designed was the part where I added the OptoIsolaters and everything past it,the lower section. The upper section that converts the O2 signal to power the LEDs is from somewhere else. I tried the best I could to draw it as it is connected on the PCB. One of the guys at school gave me a program for drawing the stuff and actually running a simulation but It does not have a LM3914 in the componet section and I do not know how to add one. There's a lot about running the simulations I do know how to do, I get a lot of error messages and have no idea what to do to correct them. My only other experiance into doing actual electronics is the simple AM radio we built for a lab project at school last semester. I thought about this for a while before attempting it, then did a little "research" into how things work and settled on this idea. I figured with the 4N25's if I screwed something up it would not take out the main PCB as well as give me on/off signal instead of trying to figure out how to drive them from the LM3914 and still have the LEDs work as normal. I figured I'd try this before trying to build a complete EFI system like on the MegaSquirt.com site. The way I'm hoping it works is as a normal O2 monitor and also function to enrich my fuel system if I get the Primary fuel metering wrong. A freind of mine killed two $1200 sets of pistons trying to get his $8-9000 EFI Fuel system right, leaning out a turbocharged engine under boost is like counting to five with a four second gernade, you loose parts. If you've got a better way to do what I'm trying to do I'm open to suggestions.
 
TheEquineFencer said:
If you've got a better way to do what I'm trying to do I'm open to suggestions.
Click to expand...
Get a car with a bigger engine. Nothing beats lots of displacement. A big engine doesn't blow up like a little turbo'd engine.

I drove a turbo'd car for years. Lots of fun. My son boosted the turbos on his cars. He also had lots of fun.
Now I go with the flow and save a fortune on gasoline.
 

I like this response.. I drive a turbo'd car. Agreed. Lots of fun.

I wanna know what car the OP is modding.
 
I'm turbocharging a 455 CID/7.4 liter Buick! There is no replacement for displacement! Here's the link to my old ProStreet car, **broken link removed** My lawyer said I had to sell it and give my Ex-wife 1/2 the money about 15 years ago, I sold it to the guy that helped me build all the cars we raced and got out of cars all togather until now. I was driving it on the street with a stock engine and sold him the car and all the parts for $1000USD. I paid over $1200USD for just the two tires and rims on the back. he still has it, it was in primer when he got it. It has a tube frame from the firewall back and a roll cage complete with 5 point harnesses for seat belts. I had Power steering,A/C, tilt whell and was adding cruise control when we split up. I also was starting on a twin turbo 455/7.4L engine for it then. I'm presently working on doing the same to a 1970 Buick Electra 225, basicaly what I call a land barge. I plan to install 21.5 x 33 tires on the rear like my old car and drive it for an everyday street car.
 
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Optikon said:
I wanna know what car the OP is modding.
Click to expand...
Does Suzuki still make a tiny car with a 3 cylinder turbo'd engine?
Does the SMART "car" (I call it a toy car) have a turbo on its tiny diesel engine?
 
We have the tiny SMART car in Canada now. It sells for the same price as a "normal small (much bigger) car". I laugh when I see one.
 
audioguru said:
Does the SMART "car" (I call it a toy car) have a turbo on its tiny diesel engine?
Click to expand...

Yes, all the Smart Fortwo cars are 3-cylinder turbos (petrol and diesel variants).
 
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